The alternative-splice isoforms of the PDGF A-chain differ in their ability to associate with the extracellular matrix and to bind heparin in vitro.
Platelet-derived growth factor (PDGF) consists of disulfide-linked homo- or heterodimers of A and B chains. mRNA encoding the A chain (PDGF-A) occurs in two versions that differ by the presence or absence of a single short exon. These alternatively-spliced mRNAs encode polypeptides that differ in length by fifteen amino acids. The longer isoform (PDGF-AL) possesses a highly basic carboxy-terminal extension that is responsible for retaining PDGF-AL homodomers at the cell surface after secretion, while homodimers of the shorter isoform (PDGF-AS) are released into the extracellular medium. We have investigated the mechanism by which PDGF-AL remains in association with the cells that produce it. We expressed epitope-tagged versions of PDGF-AL and PDGF-AS in Cos cells and compared their intra- and extracellular distributions by immunofluorescence microscopy. PDGF-AL, but not PDGF-AS, was detected on and around cells in a diffuse pattern suggesting associated with the extracellular matrix (ECM). Metabolically radiolabelled PDGF-AL, but not PDGF-AS, could be eluted from ECM preparations by washing in high salt. Moreover, PDGF-AL bound reversibly to heparin-Sepharose in vitro at physiological salt concentrations, eluting at a salt concentration around 0.5 M. PDGF-AS did not bind to heparin under the same conditions. Thus, PDGF dimers that contain PDGF-AL may remain immobilized near the cells that secrete them by virtue of binding to heparin-like constituents of the ECM.
Pollock
R A
RA
Department of Biology, University College London, England.
Richardson
W D
WD
D016475:3T3 Cells / Q000166:cytology; D000818:Animals; D002455:Cell Division; D002460:Cell Line; D002522:Chlorocebus aethiops; D005109:Extracellular Matrix / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D015870:Gene Expression; D006493:Heparin / Q000378:metabolism*; D007163:Immunosorbent Techniques; D007668:Kidney; D051379:Mice; D008856:Microscopy, Fluorescence; D010982:Platelet-Derived Growth Factor / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012326:RNA Splicing; D012333:RNA, Messenger / Q000235:genetics; D014162:Transfection
D010982:Platelet-Derived Growth Factor; D012333:RNA, Messenger; D006493:Heparin