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Members of the DEAD/H box-containing helicase superfamily include proteins essential to genome replication, repair, and expression. We report here the cloning and initial characterization of a novel human member of this protein family, designated hHel1 (human helicase 1), now designated SMARCAD1 by HUGO. This DEAD/H box-containing molecule has seven highly conserved sequence regions that allow us to place it in the SNF2 family of the helicase superfamily. Uniquely, though, hHel1 contains two DEAD/H box motifs, a property not reported to be shared by any other SNF2 family members. This defines a new subfamily consisting of hHel1 and its homologues. In addition to these DEAD/H box/ATP-binding motifs, hHel1 has a putative nuclear localization signal and several regions that may mediate protein–protein interactions. Expression analysis indicates that hHel1 transcripts are ubiquitous, with particularly high levels in endocrine tissue. We have mapped the gene for hHel1 to human chromosome 4q22–q23; this region is rich in breakpoints and deletion mutants of genes involved in several human diseases, notably soft tissue leiomyosarcoma, hepatocellular carcinoma, and hematologic malignancies. Our observation that human Hel1 gene overexpression is present in an E1A-expressing cell line with increased capacity for gene reactivation events by genomic rearrangement suggests that human Hel1 may play a role in genetic instability development.

GDP-dissociation inhibitors (GDIs) play a primary role in modulating the activity of GTPases. We recently reported the identification of a new GDI for the Rho-related GTPases named RhoGDIgamma. This gene is now designated ARHGDIG by HUGO. Here, in a detailed analysis of tissue expression of ARHGDIG, we observe high levels in the entire brain, with regional variations. The mRNA is also present at high levels in kidney and pancreas and at moderate levels in spinal cord, stomach, and pituitary gland. In other tissues examined, the mRNA levels are very low (lung, trachea, small intestine, colon, placenta) or undetectable. RT-PCR analysis of total RNA isolated from exocrine pancreas and islets shows that the gene is expressed in both tissues. We also report the genomic structure of ARHGDIG. The gene spans over 4 kb and is organized into six exons and five introns. The upstream region lacks a canonical TATA box and contains several putative binding sites for ubiquitous and tissue-specific factors active in central nervous system development. Using FISH, we have mapped the gene to chromosome band 16p13.3. This band is rich in deletion mutants of genes involved in several human diseases, notably polycystic kidney disease, alpha-thalassemia, tuberous sclerosis, mental retardation, and cancer. The promoter structure and the chromosomal location of RhoGDIgamma suggest its importance and underscore the need for further investigation into its biology.