Molecular Cloning and Characterization of the G Protein γ Subunit of Cone Photoreceptors

Abstract

The phototransduction process in cones has been proposed to involve a G protein that couples the signal from light-activated visual pigment to the effector cyclic GMP phosphodiesterase. Previously, we have identified and purified a Gβγ complex composed of a Gβ3isoform and an immunochemically distinct Gγ subunit (Gγ8) from bovine retinal cones (Fung, B. K.-K., Lieberman, B. S., and Lee, R. H. (1992) J. Biol. Chem. 267, 24782-24788; Lee, R. H., Lieberman, B. S., Yamane, H. K., Bok, D., and Fung, B. K.-K. (1992a) J. Biol. Chem. 267, 24776-24781). Based on the partial amino acid sequence of this cone Gγ8, we screened a bovine retinal cDNA library and isolated a cDNA clone encoding Gγ8. The cDNA insert of this clone includes an open reading frame of 207 bases encoding a 69-amino acid protein. The predicted protein sequence of Gγ8shares a high degree of sequence identity (68%) with the Gγ (Gγ1) subunit of rod transducin. Similar to rod Gγ1, it terminates in a CIIS motif that is the site for post-translational modification by farnesylation. Messenger RNA for Gγ8is present at a high level in the retina and at a very low level in the lung, but is undetectable in other tissues. Immunostaining of bovine retinal sections with an antipeptide antibody against the N-terminal region of Gγ8further shows a differential localization of Gγ8to cones with a pattern indistinguishable from that of Gβ3. This finding suggests that Gβ3γ8is a component of cone transducin involved in cone phototransduction and color vision. The phototransduction process in cones has been proposed to involve a G protein that couples the signal from light-activated visual pigment to the effector cyclic GMP phosphodiesterase. Previously, we have identified and purified a Gβγ complex composed of a Gβ3isoform and an immunochemically distinct Gγ subunit (Gγ8) from bovine retinal cones (Fung, B. K.-K., Lieberman, B. S., and Lee, R. H. (1992) J. Biol. Chem. 267, 24782-24788; Lee, R. H., Lieberman, B. S., Yamane, H. K., Bok, D., and Fung, B. K.-K. (1992a) J. Biol. Chem. 267, 24776-24781). Based on the partial amino acid sequence of this cone Gγ8, we screened a bovine retinal cDNA library and isolated a cDNA clone encoding Gγ8. The cDNA insert of this clone includes an open reading frame of 207 bases encoding a 69-amino acid protein. The predicted protein sequence of Gγ8shares a high degree of sequence identity (68%) with the Gγ (Gγ1) subunit of rod transducin. Similar to rod Gγ1, it terminates in a CIIS motif that is the site for post-translational modification by farnesylation. Messenger RNA for Gγ8is present at a high level in the retina and at a very low level in the lung, but is undetectable in other tissues. Immunostaining of bovine retinal sections with an antipeptide antibody against the N-terminal region of Gγ8further shows a differential localization of Gγ8to cones with a pattern indistinguishable from that of Gβ3. This finding suggests that Gβ3γ8is a component of cone transducin involved in cone phototransduction and color vision. The family of structurally homologous G proteins plays an essential role in transducing extracellular signals from cell-surface receptors to intracellular effectors (Stryer and Bourne, 1986Stryer L. Bourne H.R. Annu. Rev. Cell Biol. 1986; 2: 391-419Crossref PubMed Scopus (627) Google Scholar; Gilman, 1987Gilman A.G. Annu. Rev. Biochem. 1987; 56: 615-649Crossref PubMed Scopus (4728) Google Scholar; Birnbaumer, 1990Birnbaumer L. Annu. Rev. Pharmacol. Toxicol. 1990; 30: 675-705Crossref PubMed Google Scholar). Members of this group of proteins are heterotrimers composed of Gα, Gβ, and Gγ subunits. In most biological systems, the Gα subunits are generally recognized as the signal carrier that dictates the specificity of signaling pathways. The Gβ and Gγ subunits, which form a tightly associated Gβγ complex, usually play a more passive role by promoting interactions between the Gα subunit and the receptor (Fung, 1983Fung B.K.-K. J. Biol. Chem. 1983; 258: 10495-10502Abstract Full Text PDF PubMed Google Scholar; Florio and Sternweis, 1985Florio V.A. Sternweis P.C. J. Biol. Chem. 1985; 260: 3477-3483Abstract Full Text PDF PubMed Google Scholar). However, there is now a growing body of evidence demonstrating that Gβγ also participates in a wide range of other G protein functions in some systems. These include the promotion of cholera toxin- and pertussis toxin-catalyzed ADP-ribosylation of the Gα subunit (Yamane and Fung, 1993Yamane H.K. Fung B.K.-K. Annu. Rev. Pharmacol. Toxicol. 1993; 32: 201-241Crossref Scopus (56) Google Scholar), interaction with phosducin (Lee et al., 1987Lee R.H. Lieberman B.S. Lolley R.N. Biochemistry. 1987; 26: 3983-3990Crossref PubMed Scopus (154) Google Scholar, Lee et al., 1992bLee R.H. Ting T.D. Lieberman B.S. Tobias B.E. Lolley R.N. Ho Y.K. J. Biol. Chem. 1992; 267: 25104-25112Abstract Full Text PDF PubMed Google Scholar) and receptor kinase (Haga and Haga, 1992Haga K. Haga T. J. Biol. Chem. 1992; 267: 2222-2227Abstract Full Text PDF PubMed Google Scholar; Inglese et al., 1992Inglese J. Koch W.J. Caron M.G. Lefkowitz R.J. Nature. 1992; 359: 147-150Crossref PubMed Scopus (234) Google Scholar; Pitcher et al., 1992Pitcher J.A. Inglese J. Higgins J.B. Arriza J.L. Casey P.J. Kim C. Benovic J.L. Kwatra M.M. Caron M.G. Lefkowitz R.J. Science. 1992; 257: 1264-1267Crossref PubMed Scopus (573) Google Scholar), and regulation of the activities of effectors such as adenylate cyclase types II and IV, phospholipase A2, phospholipase C, and cardiac K+channels (Clapham and Neer, 1993Clapham D.E. Neer E.J. Nature. 1993; 365: 403-406Crossref PubMed Scopus (590) Google Scholar). The molecular mechanism by which the Gβγ complex regulates a diversity of signaling processes is still not fully understood. To date, five forms of Gβ (Watson et al., 1994Watson A.J. Katz A. Simon M.I. J. Biol. Chem. 1994; 269: 22150-22156Abstract Full Text PDF PubMed Google Scholar) and multiple forms of Gγ subunits have been identified by biochemical, immunological, and molecular cloning studies (Simon et al., 1991Simon M.I. Strathmann M.P. Gautam N. Science. 1991; 252: 802-808Crossref PubMed Scopus (1591) Google Scholar). At the amino acid level, the Gβ subunits are highly conserved. In contrast, the Gγ subunits are more divergent. Because of the diversity of the Gγ sequences, it is generally believed that the Gγ subunit determines the functional specificity of the Gβγ complex. However, it is unclear what combinations of Gβ and Gγ subunits occur physiologically to account for the differences in their functions. The Gβγ complexes isolated from most tissue preparations are heterogeneous mixtures containing different forms of Gβ and Gγ subunits (Gautam et al., 1990Gautam N. Northup J. Tamir H. Simon M.I. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: 7973-7977Crossref PubMed Scopus (97) Google Scholar). An exception to this is the cone Gβγ complex, which is composed of Gβ3and a novel Gγ subunit (Fung et al., 1992Fung B.K.-K. Lieberman B.S. Lee R.H. J. Biol. Chem. 1992; 267: 24782-24788Abstract Full Text PDF PubMed Google Scholar; Lee et al., 1992aLee R.H. Lieberman B.S. Yamane H.K. Bok D. Fung B.K.-K. J. Biol. Chem. 1992; 267: 24776-24781Abstract Full Text PDF PubMed Google Scholar). In this study, we report the isolation and expression of the cDNA of this Gγ subunit. We further show that this Gγ subunit is highly homologous to the Gγ1subunit of rod transducin (Hurley et al., 1984Hurley J.B. Fong H.K. Teplow D.B. Dreyer W.J. Simon M.I. Proc. Natl. Acad. Sci. U. S. A. 1984; 81: 6948-6952Crossref PubMed Scopus (118) Google Scholar; Yatsunami et al., 1985Yatsunami K. Pandya B.V. Oprian D.D. Khorana H.G. Proc. Natl. Acad. Sci. U. S. A. 1985; 82: 1936-1940Crossref PubMed Scopus (54) Google Scholar) and is localized in cone photoreceptors of the retina. Following the convention of naming the G protein subunits numerically, we will refer to this newly identified cone Gγ subunit as Gγ8in this paper. We speculate that Gγ8may serve an important function in the regulation of phototransduction in cones. Retinal G protein (transducin) Gβγ complex was isolated from bovine retinas purchased from J. A. Lawson (Lincoln, NE) (Fung, 1983Fung B.K.-K. J. Biol. Chem. 1983; 258: 10495-10502Abstract Full Text PDF PubMed Google Scholar). The cone-specific Gβ3γ8complex was separated from rod-specific Gβ1γ1by subtractive chromatography using an affinity column of immobilized monoclonal antibodies against rod Gγ1as reported previously (Fung et al., 1992Fung B.K.-K. Lieberman B.S. Lee R.H. J. Biol. Chem. 1992; 267: 24782-24788Abstract Full Text PDF PubMed Google Scholar). The bovine retinal cDNA library in Uni-Zap XR was a gift from Dr. Wolfgang Baehr (Baylor College of Medicine), and Ha-Ras cDNA in the pTrcA vector (Invitrogen) was obtained from Dr. Vincent Jung (Cold Spring Harbor). Rac2 cDNA was amplified from the bovine retinal cDNA library using PCR. 1The abbreviations used are:PCRpolymerase chain reactionTricineN-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycinePAGEpolyacrylamide gel electrophoresis. Recombinant Ha-Ras and Rac2 fusion proteins containing a histidine-rich N-terminal sequence were expressed in Escherichia coli, and the proteins were purified according to the procedure as described for the expression of Gγ8and stored in 40% glycerol at −20°C. polymerase chain reaction N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine polyacrylamide gel electrophoresis. Peptide CMAQELSEKELLKME, corresponding to residues 1-14 of the deduced amino acid sequence of Gγ8plus an N-terminal cysteine for coupling purpose, was synthesized by Immuno-Dynamics, Inc. (La Jolla, CA). A polyclonal antiserum was generated by immunization of rabbits with the synthetic peptide coupled to keyhole limpet hemocyanin. The IgG fraction of the antiserum was purified by chromatography on a DEAE-Affi-Gel blue column (Bio-Rad). The purified Gβ3γ8com-plex was separated on high resolution Tricine-polyacrylamide gels (Schägger and von Jagow, 1987Schägger H. von Jagow G. Anal. Biochem. 1987; 166: 368-379Crossref PubMed Scopus (10505) Google Scholar), transblotted onto Immobilon-P membranes (Millipore Corp.), and visualized by staining with Coomassie Blue (Matsudaira, 1987Matsudaira P. J. Biol. Chem. 1987; 262: 10035-10038Abstract Full Text PDF PubMed Google Scholar). The region of the membranes containing the Gγ8subunit was excised, cleaved with cyanogen bromide, and subjected to gas-phase protein sequence analysis at the Protein Sequencing Facility, University of California, Los Angeles. Degenerate PCR oligonucleotide primers corresponding to residues KKEVKN (5′-primer) and KGIPED (3′-primer) were designed from the partial amino acid sequence of Gγ8. PCR mixtures were prepared in 50 μl containing 1 × PCR buffer (10 m M Tris, pH 8.3, 50 m M KCl, 1.5 m M MgCl2, 0.001% gelatin), 1 μM degenerate primers, 50 μM dNTPs, and 0.5 μl (2.5 × 105plaque-forming units) of bovine retinal cDNA library in Uni-Zap XR. Reactions were performed for five cycles with melting, annealing, and extension at temperatures of 94, 37, and 72°C, by cycles with an of A PCR of was and the amplified cDNA was from an and to the cloning vector The isolated cDNA clone was to that it to the sequence predicted from the partial amino acid sequence of Gγ8. A was synthesized from the vector and used to × at The containing the insert were from the according to the A of from to in 1 was amplified by PCR and the blue vector The cDNA insert was at the and the expression vector and expressed as a fusion protein containing a histidine-rich N-terminal sequence and a and of the protein were according to the procedure by the The was at × for and to a column of protein was from the column with m M Tris, pH 0.5 M m M The protein were for against 50 m M Tris, pH m M 1 m M and with for at at a of further purified on a The Gγ8is structurally to the for the of N-terminal residues with In of Gγ8, Ha-Ras protein was at in a of μl of buffer m M Tris, pH m M KCl, m M MgCl2, 0.5 m M The reaction for 1.5 μM purified μM μM and μl of was the that the and were with and 1 of the reaction was by the of an of × and the proteins were separated on high resolution Tricine-polyacrylamide To the of of the gel containing Gγ8, Ha-Ras were in and by RNA from bovine retina was isolated by acid and P. N. Anal. Biochem. 1987; PubMed Scopus Google Scholar). RNA from other bovine was prepared by and et al., G. B. J.A. 1983; 2: PubMed Scopus Google Scholar), and was further purified by to The RNA were on gels containing M and to with bovine and was for at in buffer containing × 50 m M pH × and 50 The of the membranes was performed in a containing × and at for was by to for at using an bovine were obtained from a and the and body were The was at for by in prepared from of retina to the were from the and for in The were in in and stored at sections of the were at on a to with and using the complex according to by the staining was performed at in a and were the The and sections were for in purified IgG at a of A for in and for in complex. To the the sections were for 1 in were in a and with Protein were by Coomassie Blue M.M. Anal. Biochem. PubMed Scopus Google Scholar) using as a of proteins was performed by the of and von Jagow, 1987Schägger H. von Jagow G. Anal. Biochem. 1987; 166: 368-379Crossref PubMed Scopus (10505) Google gel in of proteins on Immobilon-P membranes was according to a procedure of et al., H. T. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google of the membranes with bovine and IgG the were by with the We have previously reported the of a cone Gβγ complex composed of Gβ3and an immunochemically distinct Gγ subunit (Fung et al., 1992Fung B.K.-K. Lieberman B.S. Lee R.H. J. Biol. Chem. 1992; 267: 24782-24788Abstract Full Text PDF PubMed Google Scholar; Lee et al., 1992aLee R.H. Lieberman B.S. Yamane H.K. Bok D. Fung B.K.-K. J. Biol. Chem. 1992; 267: 24776-24781Abstract Full Text PDF PubMed Google Scholar). analysis of this cone Gγ peptide obtained by cyanogen an amino acid sequence that from other Gγ subunits. To the sequence of cone we amplified from a bovine retinal cDNA library a of this cDNA by PCR using a of degenerate oligonucleotide primers from amino acid sequence We used this PCR as a to the cDNA library and isolated clone containing the region of this cone analysis of the cDNA insert of the clone a open reading frame encoding a 69-amino acid protein with a molecular of The deduced amino acid sequence with the sequence of the cyanogen that the cDNA cone This newly identified Gγ was Gγ8. A of the deduced amino acid sequence of Gγ8and other of Gγ subunits is in for a cone G protein Gγ8shares the sequence identity (68%) with retinal rod transducin. The differences between forms are at the N-terminal region and in the region between residues and of Gγ8. In contrast, a degree of sequence identity to other forms of Gγ subunits, from to other of the the Gγ8subunit terminates with a motif cysteine and amino which has been in et al., T. H. T. T. Nature. 1990; PubMed Scopus Google Scholar; et al., D. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar; Fung et al., B.K.-K. Yamane H.K. 1994; PubMed Scopus Google Scholar) and Gγ et al., Casey P.J. A.G. S. Sternweis P.C. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar; Yamane et al., H.K. Fung S. J.A. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar) to the signal sequence for multiple post-translational and To the tissue of the we analysis of RNA from bovine retina and RNA from other bovine tissues. in the recognized a which is expressed in the and a which is present at a very low level in the other not show a level of the was to the from as a et al., S. Fong H.K. Proc. Natl. Acad. Sci. U. S. A. 1991; PubMed Scopus Google Scholar), a of was in preparations of tissue This that of Gγ8is expressed in The of a CIIS motif at the of that it by To this we the of Gγ8to Recombinant a histidine-rich N-terminal sequence was expressed as a fusion protein and purified by affinity The of the fusion protein obtained was as by Coomassie Blue staining of the purified proteins separated by This purified fusion protein was with to the N-terminal and from the was separated from by gel on a column In of in in the of Recombinant Ha-Ras and Rac2 previously to and et al., P.J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar; et al., J. Biol. Chem. 1991; Full Text PDF PubMed Google Scholar), were used as to the with in the of in the of and of Gγ8and The in of to Ha-Ras to of the Gγ8subunit in the of the and In contrast, the of Rac2 protein with a motif is in the of with that Gγ8is by farnesylation. is that Gγ1, which is highly homologous to Gγ8, is also et al., T. H. T. T. Nature. 1990; PubMed Scopus Google Scholar; et al., D. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar; Fung et al., B.K.-K. Yamane H.K. 1994; PubMed Scopus Google Scholar). the other has been to occur in a homologous form in the et al., Casey P.J. A.G. S. Sternweis P.C. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar; Yamane et al., H.K. Fung S. J.A. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar). on studies that the specificity of the is by the of the sequence et al., Proc. Natl. Acad. Sci. U. S. A. 1991; PubMed Scopus Google Scholar; et al., J.L. Proc. Natl. Acad. Sci. U. S. A. 1991; PubMed Scopus Google Scholar). In with Gγ8, which are in a at the a is present at the of other forms of Gγ of of antiserum was generated against the synthetic peptide corresponding to residues 1-14 of the deduced amino acid sequence of Gγ8. To the specificity of the we purified bovine retinal rod-specific and cone-specific and analysis of proteins with the antipeptide in the in the antiserum the Gγ subunit from the but not the Gγ subunit from the purified complex The antiserum also in coli, which was used in the in the in the containing the N-terminal histidine-rich residues as as are with the antipeptide The antiserum is also to a with an molecular of present in the this peptide with and the antiserum not with not the is most a of the of the residues were To Gγ8is localized to cone bovine retinal sections were with antiserum against the N-terminal peptide of Gγ8. in the was in cones. The bovine retinal sections with the antiserum show staining in the cone the region of cone and The most staining was in the In the retinal sections with the staining was in These that Gγ8is localized in cone photoreceptors the retina and the that it is the Gγ subunit of cone transducin involved in phototransduction and color vision. 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Sci. 1986; Scholar). At the molecular level, the cone-specific Gα subunit et al., D.E. J.B. Science. 1986; PubMed Scopus Google Scholar), and J.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar; et al., H. H. P.J. J.A. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar; et al., K. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar), and the et al., A. 1993; Full Text PDF PubMed Scopus Google Scholar) have been and a Gβγ complex composed of Gβ3and an immunochemically distinct Gγ subunit has also been purified from bovine retinal cones (Fung et al., 1992Fung B.K.-K. Lieberman B.S. Lee R.H. J. Biol. Chem. 1992; 267: 24782-24788Abstract Full Text PDF PubMed Google Scholar; Lee et al., 1992aLee R.H. Lieberman B.S. Yamane H.K. Bok D. Fung B.K.-K. J. Biol. Chem. 1992; 267: 24776-24781Abstract Full Text PDF PubMed Google Scholar). The deduced amino acid of cone-specific proteins are highly homologous to of their corresponding rod-specific that are the of the cone phototransduction In an to the Gγ subunit that with retinal we screened a bovine retinal cDNA library and isolated a cDNA clone encoding Gγ8. The predicted protein sequence for that it a of the Gγ subunit further that Gγ8shares a degree of with sequence with other of Gγ subunits from to The in between Gγ8and Gγ8may play a role in cone To the of Gγ8is localized to of in the we generated a antipeptide antibody against the N-terminal region of Gγ8and performed on the sections of bovine retina. The shows that Gγ8, molecular is present in cones. also show a distinct of Gγ8in the and with the staining in the This pattern of Gγ8in cones is indistinguishable from the staining pattern obtained by using a antipeptide antibody (Lee et al., 1992aLee R.H. Lieberman B.S. Yamane H.K. Bok D. Fung B.K.-K. J. Biol. Chem. 1992; 267: 24776-24781Abstract Full Text PDF PubMed Google Scholar), that Gβ3and a physiologically functional complex in of in cones. et al., Proc. Natl. Acad. Sci. U. S. A. 1992; PubMed Scopus Google have previously reported that antipeptide antibodies against corresponding to residues and of the cone that also localized in cones. A of the deduced amino acid of Gγ8and the that antibodies with Gγ8. in the cone (Lee et al., 1992aLee R.H. Lieberman B.S. Yamane H.K. Bok D. Fung B.K.-K. J. Biol. Chem. 1992; 267: 24776-24781Abstract Full Text PDF PubMed Google Scholar; et al., Proc. Natl. Acad. Sci. U. S. A. 1992; PubMed Scopus Google Scholar) and Gβ3and not form a complex in and Gautam N. Proc. Natl. Acad. Sci. U. S. A. 1992; PubMed Scopus Google Scholar), it that associated with other forms of Gβ subunits et al., Strathmann M.P. Simon M.I. Biochem. 1992; PubMed Scopus Google Scholar; et al., 1994Watson A.J. Katz A. Simon M.I. J. Biol. Chem. 1994; 269: 22150-22156Abstract Full Text PDF PubMed Google Scholar). Similar to the Gγ1subunit of rod Gγ8is to by farnesylation. This is in to the Gγ subunits of the G which are by et al., Casey P.J. A.G. S. Sternweis P.C. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar; Yamane et al., H.K. Fung S. J.A. Proc. Natl. Acad. Sci. U. S. A. 1990; 87: PubMed Scopus Google Scholar). the are Gγ8is to with the with this the complex from cone membranes at low (Lee et al., 1992aLee R.H. Lieberman B.S. Yamane H.K. Bok D. Fung B.K.-K. J. Biol. Chem. 1992; 267: 24776-24781Abstract Full Text PDF PubMed Google Scholar), and the of the protein was in the In contrast, Gβγ protein containing a more group in the of et al., B. P. G. C. P. A. Biochem. 1987; PubMed Scopus Google Scholar). In to the group of the Gγ subunit have other biological functions. In the rod phototransduction the of a group to II and to interaction et al., Gautam N. J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar). of the cysteine in the interaction of transducin with et al., T. K. T. T. T. J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar). other Gγ subunits in photoreceptors are predicted to by we speculate that the of Gγ8may a functional for the phototransduction in cones. the cloning of the Gγ8subunit reported we that the protein of the cone phototransduction are now in to the protein of the in a expression to the cone phototransduction in and to the of component in The is to the phototransduction process and to an for the and differences between and cones. We Lieberman and for and for the We also Dr. and for and reading of the