Specific Ser-Pro phosphorylation by the RNA-recognition motif containing kinase KIS

We present here a first appraisal of the phosphorylation site specificity o f KIS (for 'kinase interacting with stathmin'), a novel mammalian kinase th at has the unique feature among kinases to possess an RNP type RNA-recognit ion motif (RRM). In vitro kinase assays using various standard substrates r evealed that KIS has a narrow specificity, with myelin basic protein (MBP) and synapsin I being the best in vitro substrates among those tested. Mass spectrometry and peptide sequencing allowed us to identify serine 164 of MB P as the unique site phosphorylated by KIS. Phosphorylation of synthetic pe ptides indicated the importance of the proline residue at position +1. We a lso identified a tryptic peptide of synapsin I phosphorylated by KIS and co ntaining a phosphorylatable Ser-Pro motif. Altogether, our results suggest that KIS preferentially phosphorylates proline directed residues but has a specificity different from that of MAP kinases and cdks.