rlk/TXK encodes two forms of a novel cysteine string tyrosine kinase activated by Src family kinases

Rlk/Txk is a member of the BTK/Tec family of tyrosine kinases and is primar ily expressed in T lymphocytes. Unlike other members of this kinase family, Rlk lacks a pleckstrin homology (PH) domain near the amino terminus and in stead contains a distinctive cysteine string motif We demonstrate here that Rlk protein consists of two isoforms that arise by alternative initiation of translation from the same cDNA. The shorter, internally initiated protei n species lacks the cysteine string motif and is located in the nucleus whe n expressed in the absence of the larger form. In contrast, the larger form is cytoplasmic We show that the larger form is palmitoylated and that muta tion of its cysteine string motif both abolishes palmitoylation and allows the protein to migrate to the nucleus, The cysteine string, therefore, is a critical determinant of both fatty acid modification and protein localizat ion for the larger isoform of Rlk, suggesting that Rlk regulation is distin ct from the other Btk family kinases, We further show that Rlk is phosphory lated and changes localization in response to T-cell-receptor (TCP) activat ion and, like the other Btk family kinases, can be phosphorylated and activ ated by Src family kinases, However, unlike the other Btk family members, R lk is activated independently of the activity of phosphatidylinositol 3-kin ase, consistent with its lack of a PA domain. Thus, Rlk has tao distinct is oforms, each of which may have unique properties in signaling downstream fr om the TCR.