Isolation of Candidate Nek2@iterating Protein Genes (NIPs)

Characterization of NIP2/centrobin, a novel substrate of Nek2, and its potential role in microtubule stabilization

Nek2 is a mitotic kinase whose activity varies during the cell cycle. It is well known that Nek2 is involved in centrosome splitting, and a number of studies have indicated that Nek2 is crucial for maintaining the integrity of centrosomal structure and microtubule nucleation activity. In the present study, we report that NIP2, previously identified as centrobin, is a novel substrate of Nek2. NIP2 was daughter-centriole-specific, but was also found in association with a stable microtubule network of cytoplasm. Ectopic NIP2 formed aggregates but was dissolved by Nek2 into small pieces and eventually associated with microtubules. Knockdown of NIP2 showed significant reduction of microtubule organizing activity, cell shrinkage, defects in spindle assembly and abnormal nuclear morphology. Based on our results, we propose that NIP2 has a role in stabilizing the microtubule structure. Phosphorylation may be crucial for mobilization of the protein to a new microtubule and stabilizing it.

NIP1/XB51/NECAB3 is a potential substrate of Nek2, suggesting specific roles of Nek2 in Golgi

Nek2 is a mammalian protein kinase structurally homologous to Aspergillus NIMA. We previously observed that the Nek2 protein was localized in multiple sites within a cell in a cell cycle stage-specific manner. Such dynamic behavior of Nek2 allowed us to propose that Nek2 may be a mitotic regulator that is involved in diverse cell cycle events. To better understand the cellular processes in which Nek2 participates, we carried out yeast two-hybrid screening and isolated Nek2-Interacting Protein 1 (NIP1), which has been also named as XB51 and NECAB3. Physical interactions of Nek2 with NIP1 were confirmed. In fact, Nek2 can phosphorylate NIP1 in vivo. Immunostaining experiments revealed that NIP1 is a Golgi protein. These results propose a possible involvement of Nek2 in biological processes of the Golgi body, perhaps in relation to the inheritance of Golgi during mitosis or to cell cycle stage-specific regulation of exocytosis.