Adhesion induced expression of the serine/threonine kinase Fnk in human macrophages

Members of the polo subfamily of protein kinases play crucial roles in cell proliferation. To study the function of this family in more detail,,ve iso lated the cDNA of human Fnk (FGF-inducible kinase) which codes for a serine /threonine kinase of 646 aa, Despite the homology to the proliferation-asso ciated polo-like kinase (Plk), tissue distribution of Fnk transcripts and e xpression kinetics differed clearly, In contrast to Pile no correlation bet ween cell proliferation and Fnk gene expression was found. Instead high lev els of Fnk mRNA were detectable in blood cells undergoing adhesion. The tra nsition of monocytes from peripheral blood to matrix bound macrophages was accompanied by increasing levels of Fnk with time in culture. Neither treat ment of monocytes with inducers of differentiation nor withdrawal of serum did influence Fnk mRNA levels significantly, suggesting that cell attachmen t triggers the onset of Fnk gene transcription, The idea that Fnk is part o f the signalling network controlling cellular adhesion was supported by the analysis of the cytoplasmic distribution of the Fnk protein and the influe nce of its overexpression on the cellular architecture. Fnk as fusion prote in with GFP localized at the cellular membrane in COS cells. Dysregulated F nk gene expression disrupted the cellular f-actin network and induced a sph erical morphology. Furthermore, Fnk binds to the Ca2+/integrin-binding prot ein Cib in two-hybrid-analyses and co-immunoprecipitation in assays. Moreov er, both proteins were shown to co-localize in mammalian cells, The homolog y of Cib with calmodulin and with calcineurin B suggests that Cib might be a regulatory subunit of polo-like kinases.