A ROLE FOR THE EPITHELIAL-CELL-SPECIFIC TYROSINE KINASE SIK DURING KERATINOCYTE DIFFERENTIATION

Sik, the mouse homologue of the breast tumor kinase Brk, is expressed in differentiating cells of the gastrointestinal tract and skin. We ex amined expression and activity of Sik in primary mouse keratinocytes a nd a mouse embryonic keratinocyte cell line (EMK). Calcium-induced dif ferentiation of these cells has been shown to be accompanied by the ac tivation of tyrosine kinases and rapid phosphorylation of a 65-kDa GTP ase-activating protein (GAP)-associated protein (GAP-A.p65). We demons trate that Sik is activated within 2 min after calcium addition in pri mary keratinocytes and EMK cells. In EMK cells, Sik binds GAP-A.p65, a nd this interaction is mediated by the Sik Src homology 2 domain. Alth ough Sik directly complexes with GAP-A.p65, overexpression of wild-typ e or kinase defective Sik in EMK cells does not lead to detectable cha nges in GAP-A.p65 phosphorylation. These data suggest that Sik is not responsible for phosphorylation of GAP-A.p65. GAP-A.p65 may act as an adapter protein, bringing Sik into proximity of an unidentified substr ate. Overexpression of Sik in EMK cells results in increased expressio n of filaggrin during differentiation, supporting a role for Sik in di fferentiation.