Growth-dependent subnuclear localization of a 66 kDa phosphoprotein in FER protein overexpressing cells

p94fer and p51ferT are two nuclear tyrosine kinases encoded by the FER locus in the mouse. While p94fer accumulates in somatic cells, p51ferT is found solely in meiotic spermatogenic cells. Ectopic expression of p94fer or p51ferT in CHO cells, led to tyrosine phosphorylation of cellular 66, 68 and 120 kDa proteins. A 120, 68 and 66 kDa phosphoproteins, coimmunoprecipitated with p94fer and p51ferT from extracts of transfected CHO cells. Subcellular fractionation analysis indicated that the 66 kDa tyrosine phosphorylated protein colocalizes with p51ferT to perinuclear and nuclear fractions in actively growing cells. However, in growth arrested cells, the 66 kDa phosphoprotein was associated mainly with chromatin while its level in the other nuclear compartments was significantly reduced. The 66 kDa phosphoprotein may thus mediate the nuclear function of the FER proteins and link it to cell growth.