NOVEL PHOSPHORYLATION AT A MITOTIC SITE, SERINE-75, IN HUMAN PP60(C-SRC) FROM UNSYNCHRONIZED HUMAN TUMOR-CELLS HAVING A SPHERICAL MORPHOLOGY

Endogenous pp60(c-src) from Y79 unsynchronized human retinoblastoma cells is phosphorylated at an additional N-terminal serine residue relative to unsynchronized fibroblast pp60(c-src). We confirmed that the novel phosphorylation site is Ser 75, which is the same as that phosphorylated in overexpressed human pp60(c-src) during NIH3T3 cell mitosis. We also showed that the Ser 75 phosphorylation pattern correlates with cell rounding in 14 various unsynchronized human tumor cell lines. Especially, pp60(c-src) from Lu135 having a spherical morphology similar to that of Y79 is phosphorylated as high as Y79 pp60(c-src). Mitotic spherical cells of the epithelial-like HepG2 express pp60(c-src) phosphorylated on Ser 75. On the other hand, Y79 pp60(c-src) is phosphorylated on Ser 75 throughout the cell cycle. These data suggest that this phosphorylation may be important in spherical cell morphology. (C) 1995 Academic Press, Inc.