Functional regulation of centrosome and Golgi apparatus by signal-anchoring proteins.

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CG-NAP (centrosome and Golgi localized PKN-associated protein) is a coiled-coil protein identified as a binding protein for the regulatory domain of PKN that has a catalytic domain highly homologous to PKC in the carboxyl-terminal region and a unique regulatory domain in the amino-terminal region. In this study, we have analyzed the function of CG-NAP in the Golgi formation and centrosome splitting and obtained the results as follows.1. CG-NAP was found to interact with both microtubules and a cytoplasmic dynein subunit p150^<Glued> and localize to the Golgi apparatus in a microtubule-dependent manner. By examining the recovery process from the depolymerizing microtubules or inhibiting cytoplasmic dynein, it was revealed that CG-NAP is recruited to the minus ends of microtubules by interacting with cytoplasmic dynein, thereby localizes to the Golgi apparatus.2. The Golgi apparatus in mammalian cells forms a continuous ribbon of interconnected stacks of flat cisternae that are positione … More d close to the centrosome. Neither the molecular requirements for, nor the purpose of, Golgi ribbon formation are known. It was revealed that the Golgi apparatus is fragmented in the cells lacking CG-NAP at the Golgi. In these cells transport and glycosylation of membrane proteins occurred normally with some delay, indicating that these stacks are functional. These results suggest that CG-NAP is required for the lateral fusion of the Golgi stacks to form fully functional apparatus.3. The centrosome splitting occurs between duplicated centrosomes at late G2 phase, which is caused by phosphorylation of cohesion proteins between two centrosomes by a protein kinase Nek2A. The ratio of the cells with split centrosomes was increased when CG-NAP or kendrin was suppressed by siRNA. Further, these proteins were found to associate specifically with hyper-phosphorylated inactive Nek2A, suggesting their role in the suppression of Nek2A activity at centrosomes. Possible mechanisms of Nek2A inhibition, such as phosphorylation and binding, are being examined. Less