Two novel human serine/threonine kinases with homologies to the cell cycle regulating Xenopus MO15, and NIMA kinases: cloning and characterization of their expression pattern.

Using polymerase chain reaction (PCR)-based methods, we have isolated cDNA clones of two new members of serine/threonine kinases, STK1 and STK2, from a cDNA library constructed from the BT-20 human breast cancer cell line. STK1 is transcribed as a 1.4 kilobase (kb) mRNA encoding for a protein of 346 amino acids. Based on amino acid sequence analysis, STK1 is 86% identical to the Xenopus p40mo15, a cdc2-related serine/threonine kinase recently found to be the activating kinase for p34cdc2 and p33cdk2. Thus, STK1 is most likely the human homologue of MO15. An alternatively spliced STK1 message expressed variably in cell lines and in primary carcinomas generates a predicted 58 amino acid protein that lacks the kinase domain. STK2 is transcribed into a 4.0 kb mRNA encoding for an 841 residue protein which exhibits 50% identity in the kinase domain with the mouse nek1 gene product, the relative of the fungal G2-M regulator, nimA. STK1 and STK2 display a variable pattern of expression among a series of primary carcinomas as well as cancer cell lines. Both STK1 and STK2 were expressed at the highest levels in the heart but were also detected in all other organs tested. In embryonal tissues, lower levels of expression were noted. Using cell cycle inhibitors, we have shown that both STK1 and STK2 mRNA levels remain relatively invariant through the cell cycle. Chromosomal assignment has localized STK1 on chromosome 2pcen-2p15, a region implicated in hereditary non-polyposis colorectal carcinoma, and STK2 on chromosome 3p21.1, a region frequently showing chromosomal alterations in renal cells carcinomas.