Identification of NEK3 interacting proteins and functional characterization of its signaling mechanisms

NEKs ( N IMA-r e lated k inases) are a group of kinases that share high amino acid sequence identity to NIMA ( N ever i n m itosis gene A ), which exists as a single member in the fungi Aspergillus nidulans and is functionally involved in the initiation of mitosis. NEK3 is a 506 amino acid serine/threonine kinase, localizes both to the nucleus and cytoplasm, and its gene localizes to 13q14.2 chromosome. It has an N-terminal catalytic domain and a C-terminal regulatory domain, which contains Thr475 in its PEST domain, which gets phosphorylated upon activation. Expression of mutants without Thr475 or PEST domain causes changes in cellular morphology and polarity of neuronal cells. NEK3 is also involved in cell motility and invasiveness of breast cancer tumor cells through interaction with regulators of the Rho GTPases Rac1 and RhoA, mediated by prolactin induced association of NEK3 to the human Prolactin Receptor (PRLR). Using the Matchmaker Gold Yeast Two-Hybrid system, a screening for interaction partners wasperformed and 65 clones were obtained, which cDNAs encode 27 different proteins. The identified candidate interacting proteins are functionally involved in sumoylation, ubiquitinylation, transcriptional regulation, DNA repair, RNA processing, and the regulation of cell proliferation, invasiveness and metastasis.Some interaction partners for NEK3 are located in the nucleus and plasma membrane but most of them localize to the cytoplasm. One of the cytoplasmatic interactors for NEK3, RhoGDI2, is a regulator of RhoGTPases and inhibits Rac1 and RhoA activation levels. In our pull down assay, NEK3 overexpression increases Rac1-GTP while concomittant overexpression of RhoGDI2 reducesit to non-detectable levels. Our data suggests that NEK3 interaction with RhoGDI2 is a important new regulatory elements in Rac1 signaling pathways.