α2-Chimerin, an SH2-Containing GTPase-Activating Protein for the ras-Related Protein p21^rac Derived by Alternate Splicing of the Human n-Chimerin Gene, Is Selectively Expressed in Brain Regions and Testes

Abstract

n-Chimerin (α 1-chimerin) is a brain GTPase-activating protein (GAP) for the ras-related p21^rac. We now report the occurrence of another form of chimerin, termed α 2-chimerin. This is the product of an alternately spliced transcript of the human n-chimerin gene encoding an N-terminal SH2 (src homology 2) domain in addition to the phorbol ester receptor and GAP domains. α 1- and α 2-chimerin mRNAs were expressed differently. In the rat brain, only α 1-chimerin mRNA was expressed in cerebellar Purkinje cells, although both α 1- and α 2-chimerin mRNAs occurred in neurons in the cerebral cortex, hippocampus, and thalamus. Only α 2-chimerin RNA was expressed in rat testes, in early pachytene spermatocytes. A 45-kDa SH2-containing chimerin corresponding to the α 2 form was purified from rat brain. As with Escherichia coli 45-kDa recombinant α 2-chimerin, purified brain α 2-chimerin exhibited racGAP activity which was stimulated by phosphatidylserine. The recombinant SH2 domain bound several ³²P-labelled phosphoproteins of PC12 cells, whose phosphorylation increased in response to trophic factors, including nerve growth factor. To examine the relationships of α 1- and α 2-chimerin transcripts, human genomic DNA clones were characterized. In α 2-chimerin mRNA, a 3' splice acceptor site within exon 1 of α 1-chimerin mRNA was used, replacing its 5' untranslated region and N-terminal coding sequence. The single human n-chimerin gene was mapped to chromosome 2q31-q32.1, colocalizing with the CRE-BP1 transcription factor gene (2q32). It contained several splice junctions conserved with the sequence-related protein kinase C and bcr genes. α 2-Chimerin is only the second SH2-containing GAP and the first example of an SH2 domain generated by alternate splicing.