IDENTIFICATION OF RLK, A NOVEL PROTEIN-TYROSINE KINASE WITH PREDOMINANT EXPRESSION IN THE T-CELL LINEAGE

The control of phosphorylation by protein tyrosine kinases represents an important regulatory mechanism in T cell growth, function, and diff erentiation. We have identified a 62-kDa murine protein tyrosine kinas e predominantly expressed within the T cell lineage, which we have ter med Rlk (for Resting lymphocyte kinase), rlk mRNA was found to be expr essed in the fetal thymus as early as day 13 of embryonic development as well as in adult thymus and mature resting peripheral T cells, The sequence of rlk showed that it is most closely related to the subfamil y of cytoplasmic tyrosine kinases that includes the Btk, Itk, and Tec proteins. However, Rlk differs from these kinases by virtue of its uni que aminoterminal domain, which lacks a region of pleckstrin homology common to the other members of this protein subfamily. Examination of rlk abundance within different T cell subpopulations revealed preferen tial expression in Th1 relative to Th2 T cell clones, suggesting a pos sible role in signal transduction pathways that selectively regulate c ytokine production in mature CD4(+) T cell subsets. Rlk thus represent s a novel cytoplasmic tyrosine kinase with potential functions in intr athymic T cell development and mature T cell signaling.