The Alternative-Splice Isoforms of the PDGF A-Chain Differ in their Ability to Associate with the Extracellular Matrix and to Bind HeparinIn 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.