MAFbx/Atrogin-1 Controls the Activity of the Initiation Factor eIF3-f in Skeletal Muscle Atrophy by Targeting Multiple C-terminal Lysines

Abstract

We recently presented evidence that the subunit eIF3-f of the eukaryotic initiation translation factor eIF3 that interacts with the E3-ligase Atrogin-1/muscle atrophy F-box (MAFbx) for polyubiquitination and proteasome-mediated degradation is a key target that accounts for MAFbx function during muscle atrophy. To understand this process, deletion analysis was used to identify the region of eIF3-f that is required for its proteolysis. Here, we report that the highly conserved C-terminal domain of eIF3-f is implicated for MAFbx-directed polyubiquitination and proteasomal degradation. Site-directed mutagenesis of eIF3-f revealed that the six lysine residues within this domain are required for full polyubiquitination and degradation by the proteasome. In addition, mutation of these six lysines (mutant K5–10R) displayed hypertrophic activity in cellulo and in vivo and was able to protect against starvation-induced muscle atrophy. Taken together, our data demonstrate that the C-terminal modifications, believed to be critical for proper eIF3-f regulation, are essential and contribute to a fine-tuning mechanism that plays an important role for eIF3-f function in skeletal muscle. We recently presented evidence that the subunit eIF3-f of the eukaryotic initiation translation factor eIF3 that interacts with the E3-ligase Atrogin-1/muscle atrophy F-box (MAFbx) for polyubiquitination and proteasome-mediated degradation is a key target that accounts for MAFbx function during muscle atrophy. To understand this process, deletion analysis was used to identify the region of eIF3-f that is required for its proteolysis. Here, we report that the highly conserved C-terminal domain of eIF3-f is implicated for MAFbx-directed polyubiquitination and proteasomal degradation. Site-directed mutagenesis of eIF3-f revealed that the six lysine residues within this domain are required for full polyubiquitination and degradation by the proteasome. In addition, mutation of these six lysines (mutant K5–10R) displayed hypertrophic activity in cellulo and in vivo and was able to protect against starvation-induced muscle atrophy. Taken together, our data demonstrate that the C-terminal modifications, believed to be critical for proper eIF3-f regulation, are essential and contribute to a fine-tuning mechanism that plays an important role for eIF3-f function in skeletal muscle. Skeletal muscle is a dynamic tissue that has the capacity to continuously regulate its size in response to a variety of external cues including mechanical load, neural activity, hormones/growth factors, stress, and nutritional status. In addition, skeletal muscle serves as the most significant repository for protein in the body, a source that is tapped to provide a pool of amino acids for tissue repair and gluconeogenesis under conditions of starvation and other stresses. The maintenance of muscle mass is controlled by a fine balance between catabolic and anabolic processes, which determine the level of muscle proteins and the diameter of muscle fibers. Muscle loss occurs as the result of a number of disparate conditions including cancer, diabetes, AIDS, sepsis, renal failure, aging, cachexia, and other systemic diseases (1Tisdale M.J. J. Support. Oncol. 2005; 3: 209-217PubMed Google Scholar). These diverse conditions result in reduced protein synthesis and increased protein breakdown. The process of atrophy is characterized by the activation of the ATP-dependent ubiquitin-proteasome proteolysis pathway (2Jagoe R.T. Goldberg A.L. Curr. Opin. Clin. Nutr. Metab. Care. 2001; 4: 183-190Crossref PubMed Scopus (331) Google Scholar). Proteins destined for degradation by the ubiquitin-proteasome pathway are marked by covalent linkage with a chain of ubiquitin molecules (Ub) 3The abbreviations used are: Ub, ubiquitin; MAFbx, muscle atrophy F-box; eIF3, eukaryotic initiation factor 3; eIF3-f, eukaryotic initiation factor 3 subunit f; eIF1, eukaryotic initiation factor 1; eIF4F, eukaryotic initiation factor 4F; eIF4E, eukaryotic initiation factor 4E; MyHC, myosin heavy chain; MCK, muscle creatine kinase; PBS, phosphate buffered saline; TA, tibialis anterior; PI3K, phosphatidylinositol-3-phosphate kinase; mTOR, mammalian target of rapamycin; S6K1, ribosomal S6 kinase 1; rpS6, ribosomal protein S6; MAPK, mitogen-activated protein kinase; ERK, extracellular signal-regulated protein kinase; MEK, MAPK/ERK kinase; 4E-BP1, eIF4E-binding protein 1; CHX, cycloheximide; E1, Ub-activating enzyme; E2, Ub-conjugating enzyme; E3, Ub ligase; GFP, green fluorescent protein; HA, hemagglutinin; WT, wild type. on lysine residue(s) for further degradation into short peptides by the 26 S proteasome. This process requires an Ub-activating enzyme (E1), an Ub-conjugating enzyme (E2), and an Ub ligase (E3) that acts as the last step of the cascade (3Ciechanover A. Schwartz A.L. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 2727-2730Crossref PubMed Scopus (348) Google Scholar). E3 proteins regulate the timing and the substrate specificity in protein degradation. In multiple models of skeletal muscle atrophy, the muscle-specific F-box protein MAFbx/Atrogin-1 (MAFbx) is up-regulated and appears to be essential for accelerated muscle protein loss (4Gomes M.D. Lecker S.H. Jagoe R.T. Navon A. Goldberg A.L. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 14440-14445Crossref PubMed Scopus (1370) Google Scholar, 5Bodine S.C. Latres E. Baumhueter S. Lai V.K. Nunez L. Clarke B.A. Poueymirou W.T. Panaro F.J. Na E. Dharmarajan K. Pan Z.Q. Valenzuela D.M. DeChiara T.M. Stitt T.N. Yancopoulos G.D. Glass D.J. Science. 2001; 294: 1704-1708Crossref PubMed Scopus (2665) Google Scholar, 6Sacheck J.M. Hyatt J.P. Raffaello A. Jagoe R.T. Roy R.R. Edgerton V.R. Lecker S.H. Goldberg A.L. FASEB J. 2007; 21: 140-155Crossref PubMed Scopus (451) Google Scholar). MAFbx mRNA increases 8–40-fold in all types of atrophy studied, and this increase precedes the onset of muscle weight loss. Moreover, knock-out animals lacking MAFbx show a reduced rate of muscle atrophy after denervation (5Bodine S.C. Latres E. Baumhueter S. Lai V.K. Nunez L. Clarke B.A. Poueymirou W.T. Panaro F.J. Na E. Dharmarajan K. Pan Z.Q. Valenzuela D.M. DeChiara T.M. Stitt T.N. Yancopoulos G.D. Glass D.J. Science. 2001; 294: 1704-1708Crossref PubMed Scopus (2665) Google Scholar). These observations suggest a major role for MAFbx in regulating muscle size via its participation in a muscle-specific ubiquitin ligase complex. Recently, we demonstrated that MAFbx interacts physically with the f-subunit of the eukaryotic initiation factor eIF3 (eIF3-f) for polyubiquitination and proteasome-mediated degradation during skeletal muscle atrophy (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). eIF3 is a multisubunit complex with an apparent molecular mass of ∼650 kDa. It plays an important role in translation by binding directly to the 40 S ribosomal subunit and promoting the formation of the 43 S preinitiation complex, consisting of the Met-tRNA-eIF2-GTP ternary complex, eIF1, eIF1A, and the 40 S ribosomal subunit (8Fracer C.S. Lee J.Y. Mayeur G.L. Bushell M. Doudna J.A. Hershey J.W. J. Biol. Chem. 2004; 279: 8946-8956Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar, 9Lamphear B.J. Kirchweger R. Skern T. Rhoads R.E. J. Biol. Chem. 1995; 270: 21975-21983Abstract Full Text Full Text PDF PubMed Scopus (472) Google Scholar). Furthermore, eIF3 interacts with the eIF4F-mRNA complex, thus facilitating the binding of mRNA to the 43 S ribosomal complex, thereby forming the 48 S complex. Mammalian eIF3 is composed of at least 13 non-identical subunits, among which is the regulatory subunit eIF3-f. eIF3-f is a member of the Mov34 family, containing an Mpr1/Pad N-terminal (MPN) motif, which is found in certain subunits in two macromolecular complexes, homologous to eIF3, the COP9 signalosome, and the lid of the 19 S proteasome (10Hoffmann K. Bucher P. Trends Biochem. Sci. 1998; 23: 204-205Abstract Full Text Full Text PDF PubMed Scopus (242) Google Scholar). eIF3-f function within the eIF3 complex remains to be defined. However, eIF3-f is essential for Schizosaccharomyces pombe viability, and its depletion markedly decreases the global protein synthesis in fission yeast (11Zhou C. Arslan F. Wee S. Krishnan S. Ivanov A.R. Oliva A. Leatherwood J. Wolf D.A. BMC Biol. 2005; 3: 14Crossref PubMed Scopus (110) Google Scholar). The same effect on protein translation was recently described in cells infected with the coronaviruses severe acute respiratory syndrome coronavirus (SARS coV) and coronavirus infectious bronchitis virus (12Xiao H. Xu L.H. Yamada Y. Liu D.X. PLoS ONE. 2008; 3: e1494Crossref PubMed Scopus (61) Google Scholar). In humans, down-regulation of eIF3-f is associated with several tumors (13Doldan A. Chandramouli A. Shanas R. Bhattacharyya A. Cunningham J.T. Nelson M.A. Shi J. Mol. Carcinog. 2008; 47: 235-244Crossref PubMed Scopus (54) Google Scholar, 14Doldan A. Chandramouli A. Shanas R. Bhattacharyya A. Leong S.P. Nelson M.A. Shi J. Mol. Carcinog. 2008; 47: 806-813Crossref PubMed Scopus (40) Google Scholar), and when overexpressed, eIF3-f negatively regulates cell growth by affecting translation efficiency and activation of apoptosis (15Shi J. Kahle A. Hershey J.W. Honchak B.M. Warneke J.A. Leong S.P. Nelson M.A. Oncogene. 2006; 25: 4923-4936Crossref PubMed Scopus (93) Google Scholar). In skeletal muscle, overexpression of eIF3-f induces a marked hypertrophy associated with an increase of sarcomeric proteins such as myosin heavy chain, troponin T, and desmin (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). Thus, the specific targeting of eIF3-f by the SCFMAFbx may account for the decreased protein synthesis observed in multiple types of skeletal muscle atrophy by specific translation inhibition of key muscle proteins expression (reviewed in Ref. 16Csibi A. Tintignac L.A. Leibovitch M.-P. Leibovitch S.A. Cell Cycle. 2008; 7: 1698-1701Crossref PubMed Scopus (24) Google Scholar). In the present work, we have mapped the regions of eIF3-f responsible for its MAFbx-mediated ubiquitination. We show that multiple lysine residues located in the C-terminal domain of eIF3-f are required for its polyubiquitination. Preventing polyubiquitination of eIF3-f by point mutations (K5–10R) leads to stabilization of the protein. (K5–10R indicates point mutations in which lysines are sequentially replaced by arginine (Lys → Arg) starting by lysine 218 (K5) to lysine 357 (K10).) We also demonstrate that the eIF3-f mutant K5–10R induces a strong hypertrophic phenotype in cellulo and in vivo. Finally, overexpression of eIF3-f K5–10R shows an enhanced ability in avoiding skeletal muscle atrophy. Taken together, these data suggest that the ubiquitination of eIF3-f on C-terminal lysine residues may two in protein degradation by the SCFMAFbx during skeletal muscle atrophy and in regulating the translation of specific pool of mRNA of key sarcomeric and and was and of have described (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). eIF3-f by mutagenesis a mutagenesis to the The mutations by Cell skeletal muscle cell was in with of and and was in cells by the with our animals in with and cells the of the at a of on in containing and with cells with and in with growth was in by the to for of cells with of in and with 3 of by level efficiency for was by a for after with a of to expression for the protein was as of expression level in cells was the and and and and cells with expression as after cells and in the number of for the to the and protein expression for proteasome stabilization of the C-terminal lysine residues polyubiquitination and eIF3-f. with expression wild eIF3-f wild cells with Cell in to with and by with and the cells with after to protein Cell at the and by cells with the expression eIF3-f the mutant K5–10R and with as described in and by of eIF3-f on of is The indicates a of and by the The was used to of at least in a region and the diameter was and and as described (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar, L.A. J. S. Leibovitch M.-P. Leibovitch S.A. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). of the of of and ribosomal S6 kinase as described A. A. C. Biochem. J. PubMed Scopus Google Scholar). to with and containing and at with heavy chain and and and The have described L.A. J. S. Leibovitch M.-P. Leibovitch S.A. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). and for with a The revealed by the enhanced to the with In and in ubiquitination as described L.A. J. S. Leibovitch M.-P. Leibovitch S.A. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). at and with containing The to and the in cellulo ubiquitination and into cells in the of expression MAFbx wild was to the the Cell in ubiquitination and and with proteins revealed on and for at in and with for at with and by with and the The of was used to the and the of was used to the in containing in and and as described Muscle and vivo as described (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). and by in and in with in a with for at with PBS, and with and for in used at and to the with and with and for our animals in data are as the by analysis of by significant of was C-terminal of eIF3-f MAFbx-directed of skeletal muscle atrophy, MAFbx the initiation factor eIF3-f for ubiquitination and degradation by the proteasome (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). To determine which region of eIF3-f is in the MAFbx-mediated we a of eIF3-f deletion The expression in and the of eIF3-f protein was after with a proteasome 48 The of the domain and the Mov34 after and with to eIF3-f In the C-terminal domain of eIF3-f acids was that is in the of the protein the conserved eIF3-f protein six lysines in the point mutations in which lysines are sequentially replaced by arginine (Lys → Arg) starting by lysine 218 (K5) to lysine 357 used in in and in cellulo The ability of these eIF3-f to in the of was in The of for demonstrated that all the eIF3-f proteins to proteasome degradation mutation of the six lysines to proteasome-mediated eIF3-f degradation. lysines such as and to be implicated in the polyubiquitination of eIF3-f, that of these residues is the ubiquitination In the mutation of the six lysines in (mutant K5–10R) the to these data suggest that all six lysine residues are in the proteasome-mediated degradation of eIF3-f. In addition, in an in ubiquitination SCFMAFbx in cells L.A. J. S. Leibovitch M.-P. Leibovitch S.A. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google the polyubiquitination of of the eIF3-f starvation increases the expression of MAFbx, to atrophy (5Bodine S.C. Latres E. Baumhueter S. Lai V.K. Nunez L. Clarke B.A. Poueymirou W.T. Panaro F.J. Na E. Dharmarajan K. Pan Z.Q. Valenzuela D.M. DeChiara T.M. Stitt T.N. Yancopoulos G.D. Glass D.J. Science. 2001; 294: 1704-1708Crossref PubMed Scopus (2665) Google Scholar, J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar), the of on eIF3-f ubiquitination. The polyubiquitination of of the six eIF3-f was increased after in when with of of the of the six mutant (K5–10R) was for MAFbx-mediated polyubiquitination of eIF3-f. The eIF3-f mutant K5–10R a reduced polyubiquitination by the SCFMAFbx complex and was when with Furthermore, MAFbx-mediated polyubiquitination of eIF3-f mutant K5–10R was in MAFbx the mutant MAFbx of the F-box domain required for with eIF3-f mutant eIF3-f K5–10R and into eIF3-f was with and with to eIF3-f eIF3-f of polyubiquitination in the of MAFbx in the of MAFbx mutant (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar), MAFbx the mutant the ubiquitination of the eIF3-f mutant K5–10R To the that the observed of mutant K5–10R to MAFbx-mediated polyubiquitination a to MAFbx, this was in In with mutant K5–10R Finally, to the of the C-terminal lysine polyubiquitination on eIF3-f we the of and mutant K5–10R in in mutagenesis of the lysines was to markedly eIF3-f To MAFbx the of the mutant eIF3-f K5–10R in we with the mutant K5–10R into cells and eIF3-f protein after overexpression of MAFbx eIF3-f K5–10R was enhanced of eIF3-f by its to revealed that all six lysines and the residues are conserved in eIF3-f to suggest that the MAFbx-mediated polyubiquitination of eIF3-f and degradation by the proteasome of six lysine residues to located in the C-terminal Thus, mutation of C-terminal lysine residues the polyubiquitination and eIF3-f. The eIF3-f K5–10R in Skeletal Muscle the the myosin heavy chain is a target of multiple muscle in cellulo and in vivo S. J. S. J. Clin. 2004; PubMed Scopus Google Scholar). This to the eIF3-f mutant K5–10R was able to the loss of in muscle cells with expression eIF3-f the mutant eIF3-f K5–10R under the of the in for in analysis revealed an increase in MAFbx expression in atrophy, and expression of the eIF3-f mutant K5–10R increased the of the in conditions loss in atrophy In addition, overexpression of the mutant K5–10R the starvation-induced atrophy by the of diameter in cells and in overexpression of eIF3-f an increase in diameter when with eIF3-f K5–10R a enhanced hypertrophic effect eIF3-f as observed by the increased diameter and Muscle hypertrophy is associated with increased protein synthesis M.A. Biochem. J. PubMed Scopus Google Scholar). This process is by the activation of the pathway S.C. Stitt T.N. M. G.L. R. E. A. Glass D.J. G.D. Cell Biol. 2001; 3: PubMed Scopus Google Scholar). mammalian target of regulates the by of its the J. C. K. K. Mol. Biol. 2001; PubMed Scopus Google and the protein initiation factor N. 2001; PubMed Scopus Google Scholar). Moreover, eIF3-f interacts directly with and to the of the translation preinitiation complex B.A. S.P. J. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar, A. J. Rhoads R.E. EMBO J. 2006; 25: PubMed Scopus Google Scholar). This to the hypertrophy in cellulo is with of of by an described A. A. C. Biochem. J. PubMed Scopus Google Scholar). of and in with the highly by this we found an increased of in eIF3-f and in the mutant eIF3-f K5–10R when with Furthermore, the of and also of in eIF3-f and mutant K5–10R and and of the was found in cells with the In addition, we the of the target of S6K1, the ribosomal protein S6 in overexpression of eIF3-f an increased of in when with and in cells the mutant eIF3-f Moreover, in atrophy, of the was observed in cells with an was present in eIF3-f and at a level with the mutant that the was during atrophy of eIF3-f K5–10R in against Muscle our in we expression eIF3-f the mutant eIF3-f K5–10R the of the into the muscle of the and the into muscle of the as after for 48 to size was and with the of reduced the by in with to observed in the analysis of reduced atrophy in with eIF3-f and to in eIF3-f K5–10R a key role for eIF3-f in of muscle atrophy. Moreover, activation of eIF3-f has recently to hypertrophy in and in skeletal muscle (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). mutation of the polyubiquitination and degradation of eIF3-f during muscle atrophy also we to determine these mutations also eIF3-f function in skeletal muscle. We the role of eIF3-f K5–10R in Muscle eIF3-f eIF3-f mutant K5–10R the same enhanced hypertrophic as that observed in cells We recently and evidence that in response to muscle atrophy, the Ub ligase MAFbx physically interacts with eIF3-f and to its ubiquitination and its degradation by the proteasome. We demonstrated that eIF3-f as a that increases the efficiency of the muscle protein to muscle hypertrophy in cellulo and in vivo. These data suggest that eIF3-f is a substrate of MAFbx during skeletal muscle atrophy, and is a key target that accounts for MAFbx function in atrophy by translation of proteins important for by the proteasome is the mechanism that all function of eIF3-f, and this is by the ubiquitin-proteasome pathway (2Jagoe R.T. Goldberg A.L. Curr. Opin. Clin. Nutr. Metab. Care. 2001; 4: 183-190Crossref PubMed Scopus (331) Google Scholar, J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). To identify lysine which are the of by a of deletion of the eIF3-f protein was and proteasome-mediated degradation was We show that the C-terminal which is highly conserved in the eIF3-f protein family, is the target for proteasomal degradation of eIF3-f. The C-terminal domain of the lysine residues found in eIF3-f. The of the ubiquitination in the degradation was by the lysine residues to arginine in the eIF3-f protein. We show that to protein that of the lysines is essential for substrate ubiquitination that a ubiquitination The six C-terminal lysines as ubiquitination and mutations the its This that the reduced ubiquitination of eIF3-f K5–10R may its decreased for Furthermore, is to that the of this eIF3-f mutant is able to MAFbx as by and to that this MAFbx function other such as This mechanism is under eIF3-f targeting and degradation may be to the decreased protein synthesis observed in several models of skeletal muscle atrophy by of the ubiquitin-proteasome pathway J.M. Hyatt J.P. Raffaello A. Jagoe R.T. Roy R.R. Edgerton V.R. Lecker S.H. Goldberg A.L. FASEB J. 2007; 21: 140-155Crossref PubMed Scopus (451) Google Scholar). The role of eIF3-f in the mammalian eIF3 translation complex as as its role in specific pool of suggest that muscle atrophy result in a specific translation inhibition of key muscle proteins we recently that in an protein expression activation of eIF3-f in a hypertrophy associated with an increase of sarcomeric proteins (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). that eIF3-f muscle atrophy by starvation is with in cellulo data that loss of eIF3-f in is to atrophy (7Lagirand-Cantaloube J. Offner N. Csibi A. Leibovitch M.-P. Batonnet-Pichon S. Tintignac L.A. Segura C.T. Leibovitch S.A. EMBO J. 2008; 27: 1266-1276Crossref PubMed Scopus (244) Google Scholar). In such a muscle atrophy occurs MAFbx we have found that when of eIF3-f are by of the eIF3-f K5–10R mutant into are to a atrophy by of lysines to the rate of eIF3-f degradation also are to be responsible for the hypertrophic activity in and in the tibialis in also suggest that in the of a capacity for C-terminal lysine modifications, eIF3-f K5–10R is eIF3-f these data suggest that these conserved C-terminal lysines have to a fine of eIF3-f activity in skeletal muscle. In skeletal muscle, growth factor is via two major the protein pathway and the The pathway is the major that regulates growth of skeletal muscle and protein synthesis by activation of the also by protein degradation as as the expression of M. C. A. C. E. A. K. S. Lecker S.H. Goldberg A.L. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar, T.N. Clarke B.A. Panaro F. Y. M. Yancopoulos G.D. Glass D.J. Mol. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar, E. A.R. J. F.J. Y. Yancopoulos G.D. Glass D.J. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google of the pathway induces hypertrophy by translation via of the kinase increases protein synthesis via activation of two important and the eukaryotic initiation factor and inhibition of the translation important is the that and directly with eIF3-f to the of the translation preinitiation complex a process and B.A. S.P. J. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar, A. J. Rhoads R.E. EMBO J. 2006; 25: PubMed Scopus Google Scholar). overexpression of eIF3-f and eIF3-f K5–10R increases of and and its substrate rpS6, the of the translation and the of the It as the function of eIF3-f in translation initiation as the of protein These provide important into the of and and the of protein Muscle mass which in an increase in hypertrophy to and hypertrophy by activation in response to and activation requires and and requires data show that activation of eIF3-f induces of eIF3-f in skeletal muscle induces atrophy and muscle These observations to the of eIF3-f in the of and activation in the of a preinitiation complex specific to mRNA proteins in muscle that the of physically with the Mov34 domain of eIF3-f. the same Mov34 domain is to MAFbx binding and ubiquitination of eIF3-f. Moreover, atrophy the of S6K1, that the MAFbx-mediated degradation of eIF3-f in during atrophy and the of MAFbx interacts with eIF3-f molecules with molecules to We C. the and for with