CLEAVAGE OF K-FGF PRODUCES A TRUNCATED MOLECULE WITH INCREASED BIOLOGICAL-ACTIVITY AND RECEPTOR-BINDING AFFINITY

The K-FGF/HST (FGF-4) growth factor is a member of the FGF family whic h is efficiently secreted and contains a single N-linked glycosylation signal. To study the role of glycosylation in the secretion of K-FGF, we mutated the human K-fgf CDNA to eliminate the glycosylation signal and the mutated cDNA was cloned into a mammalian expression vector. S tudies of immunoprecipitation from the conditioned medium of cells exp ressing this plasmid revealed that the lack of glycosylation did not i mpair secretion, however the unglycosylated protein was immediately cl eaved into two NH2-terminally truncated peptides of 13 and 15 kD, whic h appeared to be more biologically active than the wild-type protein. These two proteins also showed higher heparin binding affinity than th at of wt K-FGF. We have expressed in bacteria the larger of these two proteins (K140), in which the NH2-terminal 36 amino acids present in t he mature form of K-FGF have been deleted. Mitogenicity assays on seve ral cell lines showed that purified recombinant K140 had approximately five times higher biological activity than wild-type recombinant K-FG F. Studies of receptor binding showed that K140 had higher affinity th an wt K-FGF for two of the four members of FGF receptor's family, spec ifically for FGFR-1 (flg) and FGFR-2 (bek). K140 also had increased he parin binding ability, but this property does not appear to be respons ible for the increased affinity for FGF receptors. Thus removal of the NH2-terminal 36 amino acids from the mature K-FGF produces growth fac tor molecules with an altered conformation, resulting in higher hepari n affinity, and more efficient binding to FGF receptors. Although it i s not clear whether cleavage of K-FGF to generate K140 occurs in vivo, this could represent a novel mechanism of modulation of growth factor activity