EXPRESSION AND PHOSPHORYLATION OF FIBROBLAST-GROWTH-FACTOR-INDUCIBLE KINASE (FNK) DURING CELL-CYCLE PROGRESSION

Fnk is a member of the polo family of cell-cycle-regulated serine/thre onine kinases. We report here that it is present in serum-starved quie scent cells and that mitogenic stimulation of quiescent cells with cal f serum results in the modification of a significant fraction of the F nk pool. This modification results in a slower migrating form when ana lysed by SDS/PAGE. The modification is transient and by 9 h after stim ulation all of the Fnk is again present as the faster migrating form. We also show that the Fnk protein increases in abundance as cells prog ress from G(I) to mitosis and is post-translationally modified as cell s enter and exit mitosis. The Fnk modification is again manifested as a slower migrating species by SDS/PAGE and is due to phosphorylation o f the protein. The mitotic-specific phosphorylation of Fnk correlates with an increase in its kinase activity, and this activity is dramatic ally reduced by phosphatase treatment of mitotic Fnk immunoprecipitate s. During the later stages of mitosis, Fnk is dephosphorylated such th at, by the time the cells enter G(I), it is all present as the dephosp horylated form. These results suggest that Fnk has two functions, one during the entry of cells into the cell cycle and a second during mito sis of cycling cells.