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 Gosopodarowicz, 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 approximately 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.