JAK2 Tyrosine Kinase Phosphorylates and Is Negatively Regulated by Centrosomal Protein Ninein

Abstract

JAK2 is cytoplasmic tyrosine kinase critical for cytokine signaling. In this study, we have identified a novel centrosome-associated complex containing ninein and JAK2. We have found that active JAK2 localizes around the mother centrioles where it partly co-localizes with ninein, a protein involved in microtubule (MT) nucleation and anchoring. We demonstrated that JAK2 is an important regulator of centrosome function. Depletion of JAK2 or use of JAK2-null cells cause defects in MT anchoring and increased numbers of cells with mitotic defects, however, MT nucleation is unaffected. We showed that JAK2 directly phosphorylates the N-terminus of ninein while the C-terminus of ninein inhibits JAK2 kinase activity in vitro. Overexpressed WT or C-terminal (1179-1931) ninein inhibit JAK2. This ninein-dependent inhibition of JAK2 significantly decreases prolactin- and IFN γ-induced tyrosyl phosphorylation of STAT1 and STAT5. Down-regulation of ninein enhances JAK2 activation. These results indicate that JAK2 is a novel member of centrosome associated complex and that this localization regulates both centrosomal function and JAK2 kinase activity therefore controlling cytokine-activated molecular pathways.