KIS Is a Protein Kinase with an RNA Recognition Motif

Abstract

Protein phosphorylation is involved at multiple steps of RNA processing and in the regulation of protein expression. We present here the first identification of a serine/threonine kinase that possesses an RNP-type RNA recognition motif: KIS. We originally isolated KIS in a two-hybrid screen through its interaction with stathmin, a small phosphoprotein proposed to play a general role in the relay and integration of diverse intracellular signaling pathways. Determination of the primary sequence of KIS shows that it is formed by the juxtaposition of a kinase core with little homology to known kinases and a C-terminal domain that contains a characteristic RNA recognition motif with an intriguing homology to the C-terminal motif of the splicing factor U2AF. KIS produced in bacteria has an autophosphorylating activity and phosphorylates stathmin on serine residues. It also phosphorylates in vitro other classical substrates such as myelin basic protein and synapsin but not histones that inhibit its autophosphorylating activity. Immunofluorescence and biochemical analyses indicate that KIS overexpressed in HEK293 fibroblastic cells is partly targetted to the nucleus. Altogether, these results suggest the implication of KIS in the control of trafficking and/or splicing of RNAs probably through phosphorylation of associated factors.Evidence is growing that controls of RNA processing and translation are major mechanisms associated with the regulation of gene expression in eukaryotes. These controls involve numerous RNA-protein interactions mediated by protein domains that in most cases are characterized by specific sequence features (for reviews see Refs. 1-3). Of these, RNA recognition motifs (RRMs) 1 are the best characterized. RRM domains are regions of about 80 amino acids containing several well conserved residues, some of which cluster into two short submotifs, RNP-1 (octamer) and RNP-2 (hexamer). One or more RRMs are found in a variety of RNA-binding proteins, and the identification within a sequence of an RRM is a strong indication that the function of this protein involves binding to RNA (2).Protein phosphorylation is another important aspect of the regulation of RNA processing; phosphorylation seems to be involved at multiple steps of RNA processing, most probably through various mechanisms. Spliceosome assembly has been shown to be inhibited by phosphatase 1 (4). Following assembly the spliceosome activation requires dephosphorylation events (5, 6). Different kinases have been proposed to be involved in these processes (see references in Ref. 7). In the cytoplasm phosphorylation of eIF-2␣ (eukaryotic initiation factor 2 ␣-subunit) inhibits translational initiation of the majority of mRNAs (reviewed in Ref. 8), whereas phosphorylation of eIF-4E (the smallest subunit of the cap-binding factor eIF-4F) correlates with enhanced translation (for review see Ref. 9). In this context of growing evidence of the importance of phosphorylation in RNA processing, we present here the first complete identification and analysi...

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“…In addition, the position of mouse introns was found to coincide in one case with a SmSLP intron and in two other cases with the start and the end, respectively, of the SmSLP coding sequence (data not shown). A further element of similarity with stathmins was found at the biochemical level, because stathmins are known to be particularly heat-soluble (2,12,13), and SmSLP retained its solubility after 10 min of boiling, as shown in Fig. 1B, lane 4.…”

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