InterPro domain: IPR018121

The seven in absentia (sina) gene was first identified in Drosophila. The Drosophila Sina protein is essential for the determination of the R7 pathway in photoreceptor cell development: the loss of functional Sina results in the transformation of the R7 precursor cell to a non-neuronal cell type. The Sina protein contains an N-terminal RING finger domain C3HC4-type. Through this domain, Sina binds E2 ubiquitin-conjugating enzymes (UbcD1) Sina also interacts with Tramtrack (TTK88) via PHYL. Tramtrack is a transcriptional repressor that blocks photoreceptor determination, while PHYL down-regulates the activity of TTK88. In turn, the activity of PHYL requires the activation of the Sevenless receptor tyrosine kinase, a process essential for R7 determination. It is thought that Sina targets TTK88 for degradation, therefore promoting the R7 pathway. Murine and human homologues of Sina have also been identified. The human homologue Siah-1 [ 1 ] also binds E2 enzymes (UbcH5) and through a series of physical interactions, targets beta-catenin for ubiquitin degradation. Siah-1 expression is enhanced by p53, itself promoted by DNA damage. Thus this pathway links DNA damage to beta-catenin degradation [ 2 , 3 ]. Sina proteins, therefore, physically interact with a variety of proteins. The N-terminal RING finger domain that binds ubiquitin conjugating enzymes is a C3HC4-type, and does not form part of the alignment for this family. The remainder C-terminal part is involved in interactions with other proteins, and is included in this alignment. In addition to the Drosophila protein and mammalian homologues, whose similarity was noted previously, this family also includes putative homologues from Caenorhabditis elegans, Arabidopsis thaliana.

1. Characterization of human homologs of the Drosophila seven in absentia (sina) gene. Genomics 46, 103-11
2. PHYL acts to down-regulate TTK88, a transcriptional repressor of neuronal cell fates, by a SINA-dependent mechanism. Cell 90, 459-67
3. Siah-1, SIP, and Ebi collaborate in a novel pathway for beta-catenin degradation linked to p53 responses. Mol. Cell 7, 915-26