Citron (CIT)

CRIK can be expressed as at least 2 isoforms, one of which encompasses the previously reported form of citron in almost its entirety. The long form of CRIK is a 240-kD protein in which the kinase domain is followed by the sequence of citron. The short form, CRIK-SK (short kinase), is an approximately 54-kD protein that consists mostly of the kinase domain. CRIK and CRIK-SK proteins are capable of phosphorylating exogenous substrates as well as of autophosphorylation, when tested by in vitro kinase assays after expression into COS-7 cells. CRIK kinase activity is increased several-fold by coexpression of constitutively active Rho, while active Rac has more limited effects. Kinase activity of the endogenous CRIK is indicated by in vitro kinase assays after immunoprecipitation with antibodies recognizing the citron moiety of the protein.