CDC2-related kinase PITALRE phosphorylates pRb exclusively on serine and is widely expressed in human tissues.

Mammalian cell cycle progression is regulated by sequential activation and inactivation of cyclin-dependent kinases (cdks). Recently, several new members of the cdk family were cloned, and some of these were shown to complex with different cyclins and to be active at discrete stages of the cell cycle. PITALRE, a new member of this family, was cloned by our laboratory and was shown to be able to phosphorylate pRb protein in vitro. In the current work, we found that PITALRE kinase activity phosphorylated pRb at sites similar to those phosphorylated by the CDC2 kinase, which itself is known to mimic, in vitro, the in vivo phosphorylation of pRb. Phosphorylation of pRb by the PITALRE-associated kinase activity was on Ser residues exclusively. Moreover, we investigated the expression pattern of PITALRE in normal human tissues, using immunohistochemical techniques so as to gain additional data on the characteristics of this new cdk family member. The protein was widely expressed, although a different tissue distribution and/or level of expression was found in various organs. Some specialized tissues such as blood, lymphoid tissue, ovarian cells, and the endocrine portion of the pancreas showed a high expression level of PITALRE. The specific expression pattern found suggests that PITALRE may be involved in specialized functions in certain cell types.