ABUNDANT EXISTENCE OF 40KD CDC2-RELATED PROTEIN IN RAT-THYROID CELLS

The cdc2 kinase family is known to be one of the important factors for cell proliferation in both yeast and mammalian cells. By using polycl onal antibodies against PSTAIRE region of cdc2HS, we studied the amoun t of cdc2-related kinases in the rat thyroid cell line, FRTL-5, during the cell cycle. The immunoreactive protein with molecular weight 34kD was hardly detectable, instead, 40kD protein exists constantly in amo unt through G(0) phase to S phase. Further, it did not decrease when t he cells were cultured in serum- and hormone-free medium. Moreover, we observed an increase of this protein in the nuclear fraction as the c ells enter S phase. On the contrary, we could not detect any immunorea ctive 40kD protein in primary cultures of rat thyroid cells. These res ults may indicate abundant existence of this protein might concern cel l cycle regulation of FRTL-5 or its immortal feature.