ABSTRACT
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.
Subject(s)
Genome, Human , Neoplasm Proteins/genetics , Amino Acid Sequence , Animals , Base Sequence , Caenorhabditis elegans/genetics , Cell Adhesion Molecules , Chromosome Mapping , Codon, Initiator , Drosophila melanogaster/genetics , Humans , Membrane Proteins/genetics , Membrane Proteins/isolation & purification , Molecular Sequence Data , Neoplasm Proteins/chemistry , Neoplasm Proteins/isolation & purification , Neoplasm Proteins/metabolism , Protein Biosynthesis , Protein Structure, Tertiary , Sequence Homology, Amino Acid , Stromal Interaction Molecule 1 , Stromal Interaction Molecule 2ABSTRACT
Alpha-fetoprotein (AFP) is a synthetic product of only the visceral yolk sac endoderm and fetal liver during mouse embryogenesis. To examine the involvement of transcriptional and post-transcriptional mechanisms in the regulation of tissue-specific expression of the AFP gene, the distribution of AFP mRNA in tissues at different stages of development was determined. Total RNA was isolated from mid-gestation tissues, and a microextraction procedure was developed for earlier stage embryos where the amount of tissue was limited. AFP mRNA was measured by a dot-hybridization assay using a 32P-labelled AFP cDNA clone as probe. Results show that only visceral yolk sac endoderm and fetal liver of midgestation embryos contained AFP mRNA. At earlier stages AFP mRNA was confined to regions within the visceral endoderm which have previously been shown to be AFP positive. These results indicate that expression of the AFP gene during post-implantation development is probably controlled at the level of AFP gene transcription. In addition, we show by in situ hybridization to tissue sections that all endoderm cells of the visceral yolk sac contain AFP mRNA, indicating that the visceral endoderm layer is a homogeneous population of cells with respect to transcription of the AFP gene.
