STIM1 (where STIM is stromal interaction molecule) is a candidate tumour suppressor gene that maps to human chromosome 11p15.5, a region implicated in a variety of cancers, particularly embryonal rhabdomyosarcoma. STIM1 codes for a transmembrane phosphoprotein whose structure is unrelated to that of any other known proteins. The precise pathway by which STIM1 regulates cell growth is not known. In the present study we screened gene databases for STIM1-related sequences, and have identified and characterized cDNA sequences representing a single gene in humans and other vertebrates, which we have called STIM2. We identified a single STIM homologue in Drosophila melanogaster (D-Stim) and Caenorhabditis elegans, but no homologues in yeast. STIM1, STIM2 and D-Stim have a conserved genomic organization, indicating that the vertebrate family of two STIM genes most probably arose from a single ancestral gene. The three STIM proteins each contain a single SAM (sterile alpha-motif) domain and an unpaired EF hand within the highly conserved extracellular region, and have coiled-coil domains that are conserved in structure and position within the cytoplasmic region. However, the STIM proteins diverge significantly within the C-terminal half of the cytoplasmic domain. Differential levels of phosphorylation appear to account for two molecular mass isoforms (105 and 115 kDa) of STIM2. We demonstrate by mutation analysis and protein sequencing that human STIM2 initiates translation exclusively from a non-AUG start site in vivo. STIM2 is expressed ubiquitously in cell lines, and co-precipitates with STIM1 from cell lysates. This association into oligomers in vivo indicates a possible functional interaction between STIM1 and STIM2. The structural similarities between STIM1, STIM2 and D-STIM suggest conserved biological functions.

Several findings suggest that signals from tyrosine kinases are transduced, at least in part, through ras proteins. These findings include (i) blociage of the transforming activity of constitutively active tyrosine kinases by inhibing ras function and (it) genetic screens in Caenorhabdis elegans and in Drosophila that identified ras genes as downstream effectors of tyrosine kinases. The recently isoated Drosophila gene Son of sevenless (Sos) is postulated to act as a positive regulatory link between tyrosine kinase and ras proteins by catalyzing exchange of GDP for GTP on ras protein. Such exchange proteins have been reported in extracts of mammalian cells but have not been previously characterized at a molecular level. As Sos appears to function in this role in Drosophila, we sought to isolate a vertebrate counterpart(s). We have characterized two widely expressed murine genes with a high degree of homology to Sos. Hybridization with human DNA and RNA indicates a high degree of conservation of these genes in other vertebrates.Protein tyrosine kinases (PTKs) are a superfamily of genes whose protein products regulate many aspects of cellular proliferation, survival, and differentiation (1). Many of the extracellular ligands that modulate the activity of individual PTKs have been identified. However, less is known of the intracellular factors that interact either directly with the catalytic domain of tyrosine kinases or form more distal parts of the tyrosine kinase signaling pathway. There is compelling evidence (for review, see ref. 1) to suggest that proteins such as phospholipase C-y and the c-rafl seine threonine kinase are phosphorylated, and their activity is modulated, as a result of the activation of tyrosine kinases. Ras proteins have also been identified as important components in PTK signal transduction in mammalian cells (2, 3), Caenorhabditis elegans (4), and Drosophila (5, 6). However, these proteins must represent only some of the components of the tyrosine kinase signaling pathway.We have previously attempted to study the mechanism of signaling by PTKs by conducting a systematic genetic screen in Drosophila, involving the PTK receptor sevenless (5). Sevenless is required for the differentiation of R7 photoreceptor cells in the Drosophila eye. By creating circumstances where the sevenless signaling pathway was barely adequate for R7 formation, we were able to uncover mutations in several genes probably involved in signal transduction by the sevenless protein. Genetic analyses suggested that these genes are involved in additional developmental pathways in Drosophila. In particular, at least four genes appear to be also involved in signal transduction by another tyrosine kinase, the epidermal growth factor receptor. Of the four genes likely to be general components in tyrosine kinase signaling two have been characterized at a molecular level (5, 6). These are the Drosophila Rasl gene (7) and Son of sevenless (Sos, ref. 8).A region of the Sos gene shows a high degree of homology with several ye...

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STIM1 (where STIM is stromal interaction molecule) is a candidate tumour suppressor gene that maps to human chromosome 11p15.5, a region implicated in a variety of cancers, particularly embryonal rhabdomyosarcoma. STIM1 codes for a transmembrane phosphoprotein whose structure is unrelated to that of any other known proteins. The precise pathway by which STIM1 regulates cell growth is not known. In the present study we screened gene databases for STIM1-related sequences, and have identified and characterized cDNA sequences representing a single gene in humans and other vertebrates, which we have called STIM2. We identified a single STIM homologue in Drosophila melanogaster (D-Stim) and Caenorhabditis elegans, but no homologues in yeast. STIM1, STIM2 and D-Stim have a conserved genomic organization, indicating that the vertebrate family of two STIM genes most probably arose from a single ancestral gene. The three STIM proteins each contain a single SAM (sterile α-motif) domain and an unpaired EF hand within the highly conserved extracellular region, and have coiled-coil domains that are conserved in structure and position within the cytoplasmic region. However, the STIM proteins diverge significantly within the C-terminal half of the cytoplasmic domain. Differential levels of phosphorylation appear to account for two molecular mass isoforms (105 and 115kDa) of STIM2. We demonstrate by mutation analysis and protein sequencing that human STIM2 initiates translation exclusively from a non-AUG start site in vivo. STIM2 is expressed ubiquitously in cell lines, and co-precipitates with STIM1 from cell lysates. This association into oligomers in vivo indicates a possible functional interaction between STIM1 and STIM2. The structural similarities between STIM1, STIM2 and D-STIM suggest conserved biological functions.