Brain fibroblast growth factor. Nonidentity with myelin basic protein fragments

Fibroblast growth factor (FGF) from bovine brain has been reported to be a family of three polypeptide fragments derived by limited proteolysis from myelin basic protein (MBP) (Westall, F.C., Lennon, V.A., and Gospodarowicz, D. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4675-4678). However, fragments of sequence similar to the proposed active ones, generated from bovine MBP) by acid proteases, are inactive in stimulating [ ³H]thymidine incorporation in BALB/c 3T3 cells. Further, the principal active component of the brain FGF preparation (Gospodarowicz, D., Bialecki, H., and Greenburg, G. (1978) J. Biol. Chem. 253, 3736-3743) which can be recovered in high yield from isoelectric focusing in sucrose has a pI between 4.8 and 5.8 in contradistinction to the MBP fragments (pI ~ 10) and is not retained on a column of chicken anti-bovine MBP-Sepharose. Therefore, although the reported preparation of brain FGF gives an increase in activity units/mg of protein of about 1000-fold over the crude brain extract, the main protein components, the MBP fragments, do not possess the mitogenic activity. Additional purification of as much as 50- to 100-fold may be required to obtain a homogeneous preparation of the real brain FGF.