Regulation of CD147 Cell Surface Expression

Abstract

CD147, also known as extracellular matrix metalloproteinase inducer, is a regulator of matrix metalloproteinase production and serves as a signaling receptor for extracellular cyclophilins. Here we demonstrate that the cell surface expression of CD147 is regulated by cyclophilins via the transmembrane domain of CD147. Solution binding experiments demonstrated that the transmembrane domain was both necessary and sufficient for CD147 binding to cyclophilin A (CypA). Treatment with cyclosporin A significantly reduced surface expression of CD147 and of CD8-CD147 fusion protein carrying the extracellular domain of CD8 fused to the transmembrane and cytoplasmic domains of CD147, but did not affect expression of CD8. Peptide binding studies demonstrated specific interaction between CypA and the proline-containing peptide from the CD147 transmembrane domain. Mutation of this proline residue reduced binding of CD147-derived peptides to CypA and also diminished transport of CD147 to the plasma membrane without reducing the total level of CD147 expression. These results suggest involvement of a cyclophilin-related protein in CD147 cell surface expression and provide molecular details for regulation of CD147 trafficking by cyclophilins. CD147, also known as extracellular matrix metalloproteinase inducer, is a regulator of matrix metalloproteinase production and serves as a signaling receptor for extracellular cyclophilins. Here we demonstrate that the cell surface expression of CD147 is regulated by cyclophilins via the transmembrane domain of CD147. Solution binding experiments demonstrated that the transmembrane domain was both necessary and sufficient for CD147 binding to cyclophilin A (CypA). Treatment with cyclosporin A significantly reduced surface expression of CD147 and of CD8-CD147 fusion protein carrying the extracellular domain of CD8 fused to the transmembrane and cytoplasmic domains of CD147, but did not affect expression of CD8. Peptide binding studies demonstrated specific interaction between CypA and the proline-containing peptide from the CD147 transmembrane domain. Mutation of this proline residue reduced binding of CD147-derived peptides to CypA and also diminished transport of CD147 to the plasma membrane without reducing the total level of CD147 expression. These results suggest involvement of a cyclophilin-related protein in CD147 cell surface expression and provide molecular details for regulation of CD147 trafficking by cyclophilins. CD147 is a type 1 integral membrane protein shown to regulate intercellular adhesion pathways through an intracellular signaling mediated mechanism (1Kasinrerk W. Tokrasinwit N. Phunpae P. Immunology. 1999; 96: 184-192Crossref PubMed Scopus (72) Google Scholar). It is also known as extracellular matrix metalloproteinase inducer (EMMPRIN), 1The abbreviations used are: EMMPRIN, extracellular matrix metalloproteinase inducer; CypA, cyclophilin A; PPIase, peptidyl-prolyl cis-trans-isomerase; CHO, Chinese hamster ovary; CsA, cyclosporin A; HPLC, high pressure liquid chromatography; PE, phycoerythrin; MIF, migration inhibitory factor; MALDI-TOF, matrix-assisted laser desorption ionization time-of-flight; FITC, fluorescein isothiocyanate; PBS, phosphate-buffered saline; GFP, green fluorescent protein; HEK, human embryonic kidney; CHO, Chinese hamster ovary; PerCP, peridinin chlorophyll protein. and studies by Biswas and co-workers (2Ellis S.M. Nabeshima K. Biswas C. Cancer Res. 1989; 49: 3385-3391PubMed Google Scholar, 3Biswas C. Zhang Y. DeCastro R. Guo H. Nakamura T. Kataoka H. Nabeshima K. Cancer Res. 1995; 55: 434-439PubMed Google Scholar) demonstrated that EMMPRIN expressed by cultured tumor cells stimulates fibroblasts to produce very high levels of collagenase activity, which likely facilitates tumor metastasis. To support its key role in the processes of tumorigenesis and metastasis, EMMPRIN was reported as one of the most constantly up-regulated mRNAs in metastatic cells (4Klein C.A. Seidl S. Petat-Dutter K. Offner S. Geigl J.B. Schmidt-Kittler O. Wendler N. Passlick B. Huber R.M. Schlimok G. Baeuerle P.A. Riethmuller G. Nat. Biotechnol. 2002; 20: 387-392Crossref PubMed Scopus (247) Google Scholar). Another physiologic role of CD147 relates to its activity as a receptor for extracellular cyclophilins (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar, 6Yurchenko V. O'Connor M. Dai W.W. Guo H. Toole B. Sherry B. Bukrinsky M. Biochem. Biophys. Res. Commun. 2001; 288: 786-788Crossref PubMed Scopus (120) Google Scholar). Because cyclophilins emerge as mediators of intercellular communication that may regulate chemotactic responses in many physiologic and pathologic processes, such as cell-mediated immunity and inflammation (7Bukrinsky M.I. Trends Immunol. 2002; 23: 323-325Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar), regulation of CD147 expression may determine the outcome of these events. The fact that CD147 interacts with extracellular cyclophilins in a cyclosporin-sensitive fashion (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar) suggests that such an interaction may also occur intracellularly, where cyclophilins are abundant. Cyclophilins are a family of proteins that share peptidyl-prolyl cis-trans-isomerase (PPIase) activity and serve as receptors for the immunosuppressive drug cyclosporin A (CsA). The PPIase activity of cyclophilins is the basis for their proposed role in protein folding (8Ivery M.T. Med. Res. Rev. 2000; 20: 452-484Crossref PubMed Scopus (116) Google Scholar). However, cyclophilins also bind folded proteins, raising the possibility that certain functions of mature proteins may be regulated by cyclophilins. An example of such activity is the inhibitory effect of cyclophilin A (CypA), a prototypic member of the cyclophilin family, on the tyrosine kinase Itk (9Brazin K.N. Mallis R.J. Fulton D.B. Andreotti A.H. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 1899-1904Crossref PubMed Scopus (240) Google Scholar), which is essential for the maintenance of proper CD4+ T cell responses (10Colgan J. Asmal M. Neagu M. Yu B. Schneidkraut J. Lee Y. Sokolskaja E. Andreotti A. Luban J. Immunity. 2004; 21: 189-201Abstract Full Text Full Text PDF PubMed Scopus (170) Google Scholar). Another established activity of cyclophilins is regulation of protein trafficking in cells. Cyclophilin A has been shown to be a component of the transport complexes that regulate surface expression of asialoglycoprotein receptor expression in human cells (11Huang T. Deng H. Wolkoff A.W. Stockert R.J. J. Biol. Chem. 2002; 277: 37798-37803Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar). In yeast, CypA was shown to mediate the import of fructose-1,6-bisphosphatase into intermediate transport vesicles for vacuole delivery (12Brown C.R. Cui D.Y. Hung G.G. Chiang H.L. J. Biol. Chem. 2001; 276: 48017-48026Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar) and to promote nuclear export of Zpr1, an essential zinc finger protein (13Ansari H. Greco G. Luban J. Mol. Cell. Biol. 2002; 22: 6993-7003Crossref PubMed Scopus (54) Google Scholar). The Drosophila cyclophilin homolog NinaA participates in trafficking of rhodopsin, the most abundant subclass of photoreceptors (14Colley N.J. Baker E.K. Stamnes M.A. Zuker C.S. Cell. 1991; 67: 255-263Abstract Full Text PDF PubMed Scopus (278) Google Scholar, 15Baker E.K. Colley N.J. Zuker C.S. EMBO J. 1994; 13: 4886-4895Crossref PubMed Scopus (276) Google Scholar, 16Ferreira P.A. Nakayama T.A. Pak W.L. Travis G.H. Nature. 1996; 383: 637-640Crossref PubMed Scopus (188) Google Scholar). NinaA and its mammalian counterparts are retina-expressed integral membrane proteins with the cyclophilin-homologous domain located in the lumen of the endoplasmic reticulum and intracellular transport vehicles (15Baker E.K. Colley N.J. Zuker C.S. EMBO J. 1994; 13: 4886-4895Crossref PubMed Scopus (276) Google Scholar, 17Ferreira P.A. Hom J.T. Pak W.L. J. Biol. Chem. 1995; 270: 23179-23188Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 18Stamnes M.A. Shieh B.H. Chuman L. Harris G.L. Zuker C.S. Cell. 1991; 65: 219-227Abstract Full Text PDF PubMed Scopus (221) Google Scholar). In all these examples, except interaction with Zpr1, cyclophilins were found to form a specific stable complex with the substrate, suggesting that they function as a chaperone escorting its protein substrate through the traffic pathway. Recent reports (19Shiraishi S. Yokoo H. Kobayashi H. Yanagita T. Uezono Y. Minami S. Takasaki M. Wada A. Neurosci. Lett. 2000; 293: 211-215Crossref PubMed Scopus (27) Google Scholar, 20Chklovskaia E. Nissen C. Landmann L. Rahner C. Pfister O. Wodnar-Filipowicz A. Blood. 2001; 97: 1027-1034Crossref PubMed Scopus (44) Google Scholar) suggest that cyclophilins may be involved in cell surface externalization of two other proteins, namely insulin receptor and Flt3 ligand. In both cases, treatment of the cells with CsA reduced surface expression of the proteins without altering their total cellular levels, suggesting a block at the level of transition from the endoplasmic reticulum to trans-Golgi network. The cyclophilin(s) involved in regulation of trafficking of these proteins have not yet been identified. The mechanisms by which cyclophilins regulate protein trafficking remain unknown. Molecular studies of Drosophila ninaA mutants have shown that CsA has only a minor effect on rhodopsin transport, and ninaA mutations need not be within the PPIase/CsA-binding domain to cause severe reductions in rhodopsin levels (21Ondek B. Hardy R.W. Baker E.K. Stamnes M.A. Shieh B.H. Zuker C.S. J. Biol. Chem. 1992; 267: 16460-16466Abstract Full Text PDF PubMed Google Scholar, 22Schneuwly S. Shortridge R.D. Larrivee D.C. Ono T. Ozaki M. Pak W.L. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 5390-5394Crossref PubMed Scopus (151) Google Scholar, 23Shieh B.H. Stamnes M.A. Seavello S. Harris G.L. Zuker C.S. Nature. 1989; 338: 67-70Crossref PubMed Scopus (215) Google Scholar). Cell surface expression of the Flt3 ligand and insulin receptor, on the other hand, was greatly reduced by CsA, suggesting involvement of the PPIase/CsA-binding domain of cyclophilin in the regulation of trafficking of these proteins (19Shiraishi S. Yokoo H. Kobayashi H. Yanagita T. Uezono Y. Minami S. Takasaki M. Wada A. Neurosci. Lett. 2000; 293: 211-215Crossref PubMed Scopus (27) Google Scholar, 20Chklovskaia E. Nissen C. Landmann L. Rahner C. Pfister O. Wodnar-Filipowicz A. Blood. 2001; 97: 1027-1034Crossref PubMed Scopus (44) Google Scholar). However, the cyclophilin-interacting domain within these target proteins has not been determined. Analysis of CD147 cell surface expression revealed its unexpected sensitivity to CsA, suggesting involvement of a cyclophilin in the regulation of CD147 intracellular trafficking. In this report, we present evidence that sensitivity to CsA is eliminated by mutating a single proline residue in the transmembrane domain of CD147. Mutation of this residue abrogates binding of cyclophilin to CD147 and also reduces the cell surface expression of CD147. These results suggest that intracellular trafficking of CD147/EMMPRIN is regulated by a cyclophilin. Antibodies and Reagents—FITC- or PE-conjugated anti-CD147 monoclonal antibodies were purchased from Ancell (Bayport, MN); PerCP-conjugated anti-CD4 monoclonal antibody was from BD Biosciences, and PE-labeled anti-CD71 monoclonal antibody was from Caltag (Burlingame, CA). Rabbit polyclonal anti-CypA antibody was from US Biological (Swampscott, MA). Cyclosporin A was purchased from Sigma. Plasmids and Transfection—Complementary DNAs for human CypA and CypB were cloned into the pET14b vector (Novagen, Madison, WI). Cyclophilins were expressed in Escherichia coli and purified by using the histidine-binding methodology as described (24Sherry B. Zybarth G. Alfano M. Dubrovsky L. Mitchell R. Rich D. Ulrich P. Bucala R. Cerami A. Bukrinsky M. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 1758-1763Crossref PubMed Scopus (93) Google Scholar). For expression of CD147, a DNA fragment encoding the full-length human CD147 was cloned into the pcDNA3.1 vector (Invitrogen). CD147-GFP construct (25Zucker S. Hymowitz M. Rollo R. J. D.C. Toole J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar) was by of mutations were using the and by DNA For protein expression in mammalian cells were using to the of of and was For binding DNA encoding the full-length only the or of CD147, CD8 and protein were cloned into the vector and expressed using the Madison, WI). Solution were by human CypA or CypB to to the was except that cyclophilins were by the migration inhibitory or the were using proteins for at in the binding proteins were by the with the binding and proteins were with the and revealed by and were with CypA in the binding at peptides were by through the that through the was on the and by the from to with for Analysis by were at in the or of CypA for at which the was through a to of the and the from the CypA and were in by through a to and on for was on the using as a were in the with Analysis of CD147 cell or embryonic cells were with for at were and in with or without CsA were for the the and were with or or PE-conjugated anti-CD147 or anti-CD71 antibodies and by cells cultured on were with the vector encoding the CD147-GFP fusion protein (25Zucker S. Hymowitz M. Rollo R. J. D.C. Toole J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). cells were and with in for at by a at in in with PBS, were with in for 1 at and with monoclonal antibody the matrix protein by were in an to a were with a were and using and Cyclosporin A Cell of studies (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar, 6Yurchenko V. O'Connor M. Dai W.W. Guo H. Toole B. Sherry B. Bukrinsky M. Biochem. Biophys. Res. Commun. 2001; 288: 786-788Crossref PubMed Scopus (120) Google Scholar) demonstrated that CD147 functions as a signaling receptor for extracellular cyclophilins A and in a cyclosporin the of a on CD147, which also bind cyclophilin reports (14Colley N.J. Baker E.K. Stamnes M.A. Zuker C.S. Cell. 1991; 67: 255-263Abstract Full Text PDF PubMed Scopus (278) Google Scholar, S. Yokoo H. Kobayashi H. Yanagita T. Uezono Y. Minami S. Takasaki M. Wada A. Neurosci. Lett. 2000; 293: 211-215Crossref PubMed Scopus (27) Google Scholar, 20Chklovskaia E. Nissen C. Landmann L. Rahner C. Pfister O. Wodnar-Filipowicz A. Blood. 2001; 97: 1027-1034Crossref PubMed Scopus (44) Google Scholar) cyclophilins in regulation of cell surface expression of transmembrane proteins, rhodopsin, insulin receptor, or Flt3 ligand. that surface expression of CD147 be regulated by a cyclophilin. To this we the effect of cyclosporin A on the cell surface of CD147. Because we to a human cell that not CD147, we used hamster cells and with a vector human CD147. shown in 1 in the a in CD147 expression was in the of In the expression of human CD8 in cells with a vector was not by CsA in the suggests that CD147 expression is regulated by cyclophilin and its PPIase/CsA-binding domain. effect be to diminished expression of the or transport to the plasma To between these two we the effect of CsA on expression of the CD147 construct carrying at the which has been shown (25Zucker S. Hymowitz M. Rollo R. J. D.C. Toole J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar) to be to the CD147. Analysis of CD147 expression demonstrated that CsA significantly reduced the surface expression of CD147-GFP without total expression the effect of CsA on CD147 expression in cells. To CsA surface expression of CD147 in human we a human cell with to cells of surface CD147, and we of CD147 on the cell surface in the or of These experiments demonstrated that CD147 surface expression was to levels treatment in to only a in CD147 expression in a in the of CD147 on the cell surface in the of CsA with CsA did not affect of treatment that the effect was not to of CsA and that was specific for CD147. using the we the effect of CsA on CD147 surface expression in human shown in CD147 surface expression on cells was greatly reduced by treatment with but was to the results with cells in the of CsA expression of CD147 treatment was reduced by as revealed by of the in the such was with the receptor transmembrane the of the CsA To intracellular of CD147, we cells with the vector and CD147 in and cells by fluorescent shown in cell surface of CD147-GFP was reduced in with results in A and B. as for the transmembrane CD147 with the matrix protein and this in cells in the results in this suggest that transport of CD147 from the to the plasma membrane is regulated by a cyclophilin and may the (PPIase) of the cyclophilin this mechanism in both and and human and cells. on CD147, the of into the mechanisms of regulation of CD147 we to the domain of CD147 for interaction with cyclophilins. Because CD147 interaction with cyclophilin to the PPIase domain is to and cyclophilins are in their M.A. Cell. Mol. Sci. 1999; 55: PubMed Scopus Google Scholar), we used CypA for these To the in CD147, we expressed of CD147 of the extracellular domain extracellular and transmembrane domains or extracellular and cytoplasmic domains fused and we binding using CypA and these binding was with the full-length CD147 and which expressed the extracellular and transmembrane domains binding was with the or which expressed the extracellular domain but did not the transmembrane domain of the protein. was that CD147 has been shown to function as a receptor for extracellular CypA (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar). results were using suggesting that interaction with the CD147 transmembrane domain may the to all cyclophilins. To the transmembrane domain of CD147 was both necessary and sufficient to bind we a fusion protein of the extracellular domain of the CD8 not bind CypA, fused to the transmembrane and cytoplasmic domains of CD147 expressed in this construct was to CsA to CD147 in the A specific binding interaction was between this construct and To the possibility of a binding to the we a with with the MIF, a with molecular to that of CypA J. T. Bucala R. J. Mol. Med. 1998; PubMed Scopus Google Scholar). binding was the of the interaction between CD147 and these results suggest that the transmembrane of CD147 is both necessary and sufficient for the binding interaction with CypA in To the within the CD147 transmembrane we peptides of the domain Because these peptides were very and be only in we of the peptides by two at the also peptides with mutations in peptide a to in the to of CD147, in peptide was by a To binding of peptides to CypA, we used were with CypA at and peptides were by through the that through the was on the using as a demonstrated that peptides and CypA, and did A example of binding and is shown in for peptides and To binding in a we on were with CypA as described The was through a the and CypA and peptide were by peptides used in this from other and from CypA by The of binding is shown in The of the peptides did not from their in with results of peptide 1 its form peptide but not peptides or The binding was to the residue in the peptide to of was by a single to of the in by the did not CypA binding to that the was not involved in this Because the greatly reduced the interaction between the CD147-derived peptide and cyclophilin we CD147 surface expression in cells with human CD147 with the fused at the or human carrying a proline to In both and of the cells expressed CD147 expression was by with PE-conjugated anti-CD147 antibody and on cells. shown in surface expression of was reduced with CD147 of from to is for the surface expression of CD147, one the cell surface expression of to be of cyclophilin and to the cell surface expression of the was reduced with the was not by CsA treatment in to the CD147. The experiments in this demonstrate that the cell surface expression of CD147 with the of its transmembrane domain to with cyclophilin. also as a in this interaction was by CsA or by mutating the proline transport of CD147 to the cell surface was effect is with the proposed role of cyclophilins in transport of the proteins from the endoplasmic reticulum to the plasma membrane (15Baker E.K. Colley N.J. Zuker C.S. EMBO J. 1994; 13: 4886-4895Crossref PubMed Scopus (276) Google Scholar, S. Yokoo H. Kobayashi H. Yanagita T. Uezono Y. Minami S. Takasaki M. Wada A. Neurosci. Lett. 2000; 293: 211-215Crossref PubMed Scopus (27) Google Scholar). Mutation of the binding to cyclophilin and the cell surface expression of CD147 suggesting that these two are However, possibility is that reduces CD147 cell surface expression by its and not cyclophilin of evidence this the effect of is very in to the effect of CsA expression of in cells to expression in cells in a be to occur protein were and CsA treatment did not the cell surface expression of that the is not regulated by cyclophilin. It that is for of interaction with and studies determine the effect of on CD147 An of CD147 is that this which has been shown to function as a receptor for extracellular CypA and CypB (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar, 6Yurchenko V. O'Connor M. Dai W.W. Guo H. Toole B. Sherry B. Bukrinsky M. Biochem. Biophys. Res. Commun. 2001; 288: 786-788Crossref PubMed Scopus (120) Google Scholar), with cyclophilins as a mature protein expressed on the plasma membrane and as an form its intracellular The interaction the proline residue located in in the of the CD147 extracellular domain (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar). It is that the also in this interaction with extracellular cyclophilins. a of to the transmembrane to K. W. Biol. Chem. Scholar, M. M. T. In Biol. 2002; Google Scholar), the is located between and to the other as a of the as G. 1994; Google Scholar) between and M. E. G. A. PubMed Google Scholar) A. B. G. J. Mol. Biol. 2001; PubMed Scopus Google Scholar) and B. P. R. Sci. 1996; PubMed Scopus Google Scholar) in all except the residue in most this proline residue is to the The of the is in the of The between the of such is Because the of for is be to is not the residue in the is likely that this proline residue is for interaction with cyclophilin in the lumen of the The of the cyclophilin for CD147 trafficking unknown. It that this cyclophilin is CypA, the most abundant cytoplasmic as experiments to CypA in CD147 T. W. W. B. and M. The intracellular interaction of CD147 with cyclophilin likely transport of CD147 from the to the plasma in CD147 extracellular of which has been shown to the receptor function of CD147 (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar), is also at this and may with cyclophilin. However, this interaction is not involved in the regulation of CD147 intracellular as of with not the surface expression of CD147, signaling to extracellular CypA (5Yurchenko V. Zybarth G. O'Connor M. Dai W.W. Franchin G. Hao T. Guo H. Hung H.C. Toole B. Gallay P. Sherry B. Bukrinsky M. J. Biol. Chem. 2002; 277: 22959-22965Abstract Full Text Full Text PDF PubMed Scopus (275) Google Scholar). and not to the binding for the intracellular interaction between CD147 and cyclophilin. Because proline is to the extracellular domain of CD147, the cyclophilin be on the of the membrane to this also CypA as a regulator of CD147 CypA binding to CD147 in CypA is a protein. In results demonstrate that plasma membrane expression of CD147 is regulated by cyclophilin and as the residue for this The of the cyclophilin involved to be determined. are to S. of for the of the CD147-GFP C. for the of and J. for the protein. R. for and for with the at the for and for and K. for of the are to A. Cerami for the of this