Functional analysis of cell cycle-related protein kinase in terminally differentiated neurons

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PCTAIREs are members of a subfamily of Cdc2-related kinases that have been shown to be preferentially expressed in post-mitotic cells. To examine the neural functions of PCTAIREs, rat cDNA clones encoding PCTAIRE 1, 2, and 3 (PCTAIREs) were isolated and their expression patterns in the brain were analyzed. Among the three PCTAIREs, only PCTAIRE 2 was found to be specifically expressed in the brain, furthermore its expression was transiently increased during brain development, peaking from 7 to 15 days after birth. Within the brain, PCTAIRE 2 was concentrated in the neuronal layers of the hippocampus and olfactory bulb, which mostly consist of post-mitotic neurons. In an immunocytochemical experiment, immunoreactivity for PCTAIRE 2 was detected in the cell bodies and extended neurites of neurons, but not in astrocytes. The PCTAIRE 2 protein was recovered in particulate fraction and resistant to solubilization with nonionic detergent, suggesting that PCTAIRE 2 might be present as a compo … More nent of a large protein complex. An immunoprecipitation assay revealed that the PCTAIRE 2 protein was associated with Ser/Thr phosphorylating activity for histone H1 and that its activity depended on association with some regulatory partner that can be released under high salt conditions. These findings suggest that PCTAIRE 2 is a Ser/Thr kinase that might play a unique role in terminally differentiated neurons. To elucidate the neural function of PCTAIRE 2, proteins that can associate with PCTAIRE 2 were surveyed by yeast two-hybrid screening. A positive clone was found to encode a novel protein that can interact with PCTAIRE 2 in vitro and in vivo, and was designated as Trap (Tudor Repeat Associator with PCTAIRE 2). Whole structure of Trap shows no significant homology to any proteins, but contains five repeated domains (tudor-like repeat) conserved in Drosophila tudor protein and A-kinase anchoring protein. Trap associates with N-terminal domain of PCTAIRE 2 through its C-terminal domain containing two tudor-like repeats. PCTAIRE 1, but not PCTAIRE 3, can also associate with Trap. Trap is predominantly expressed in brain and testis, and gradually increases during development in consistent with the expression patterns of PCTAIRE 2. Immunoreactivities for PCTAIRE 2 and Trap were colocalized on the cell body of neurons and on mitochondria in COS7 cells. Immunohistochemical analyses showed that PCTAIRE 2 and Trap were distributed in the same cell layer of the cerebral cortex and cerebellum. These findings suggest that Trap acts as a physiological partner of PCTAIRE 2 in the terminal differentiated neurons. Less