SIAH2 (seven in absentia homolog 2 (Drosophila))

	Genomic organization of human Siah2. The line indicates intron and boxes indicate coding regions (exon 1-2) of the gene. Exon and intron lengths, the ATG transcription start site and the TGA stop codon are indicated.
Description	The human Siah2 gene is composed of 2 exons spanning a genomic region of about 22.4 Kb.
Transcription	The transcript length of human Siah2 is 2632 bp. The open reading frame of the coding region is 975 bp.
Pseudogene	No pseudogene of Siah2 has been reported.

	Domains of human Siah2 protein.
Description	Human Siah2 protein consists of 324 amino acids, with a molecular weight of 36 KDa. Siah protein consists of an N-terminal ring domain, followed by two zinc finger motifs, and a C-terminal substrate binding domain (SBD). The ring domain is the catalytic domain that recruits E2 ubiquitin-conjugating enzymes, while the SBD mediates the binding of adaptor proteins or some Siah substrate proteins. The structure of murine Siah1a SBD has been solved. The structure reveals that Siah is a dimeric protein, and the SBD adopts an eight-stranded beta-sandwich fold (Polekhina et al., 2001). The substrate binding groove is formed by the beta-sandwich fold and the beta-strand that connects to the second zinc finger domain (House et al., 2005).
Expression	Siah2 mRNA is widely expressed in the embryonic and adult mouse tissues. It is expressed at a higher level in the olfactory epithelium, retina, forebrain and proliferating cartilage of developing bone (Della et al., 1993). Siah2 mRNA is also expressed in most human tissues (Hu et al., 1997).
Localisation	Siah protein can be localized in both cytoplasm and nucleus.
Function	Siah2 is the mammalian homolog of Drosophila SINA (seven in absentia), which interacts with transcriptional repressor Tramtrack via adaptor protein PHYL (Phyllopod) and induces the proteasomal degradation of Tramtrack, thereby determining R7 cell fate (Li et al., 1997; Tang et al., 1997). As a ring-finger E3 ubiquitin ligase, Siah targets the degradation of diverse substrates via ubiquitin-proteasome pathway, and affects multiple signaling pathways such as HIF (Nakayama et al., 2004), Ras (Nadeau et al., 2007; Schmidt et al., 2007), NF-kB (Polekhina et al., 2002; Habelhah et al., 2002), and beta-catenin (Liu et al., 2001; Matsuzawa and Reed, 2001). Siah2 transcription is upregulated by hypoxia (Nakayama et al., 2004); p38-mediated phosphorylation of mouse Siah2 on Thr24 and Ser29 alters its subcellular localization (Khurana et al., 2006); HIPK2-mediated phosphorylation of human Siah2 on Thr 26, Ser 28 and Ser 68 decreases the stability of Siah2 and impairs its interaction with HIPK2 (Calzado et al., 2009). Over 20 Siah substrates have been reported (Nakayama et al., 2009) and some of them can be degradated by Siah2, Siah1 or both of them. In contrast to Siah1a knockout mice which exibit growth retardation and spermatogenesis defect, Siah2 knockout mice display no apparent phenotype, whereas Siah2 and Siah1a double knockout mice are embryonic or neonatal lethal, suggesting that the two Siah homologs have both overlapping and distinct functions in vivo (Frew et al., 2003). Despite the diverse substrates of Siah identified in vitro, loss of Siah2 (or both Siah2 and Siah1a) in vivo largely has no effect on the levels of many Siah substrates and the physiological processes associated with these substrates (Frew et al., 2002; Frew et al., 2003). Siah2 is implicated in the regulation of hypoxia response through its effect on HIF prolyl hydroxylases or HIPK2 (Nakayama et al., 2004; Calzado et al., 2009). Siah2 knockout mice subject to hypoxia showed impaired respiratory response and defect to adjust levels of red blood cells (Nakayama et al., 2004). Siah2 has been shown to be required for development and progression of several types of cancers via its regulation of HIF or Ras pathways (House et al., 2009). Siah2-dependent degradation of Pard3A is found to control germinal zone exit of neuronal progenitors or immature neurons in mice (Famulski et al., 2010).
Homology	Homologs: Human has two Siah genes (Siah1 and Siah2) (Hu et al., 1997), while mouse has three Siah genes (Siah2, Siah1a, Siah1b) (Della et al., 1993). Human Siah2 shares 77% identity with human Siah1 (Hu et al., 1997). Orthologs: Highly conserved Siah2 orthologs have been identified in all multicellular organisms examined (Nakayama et al., 2009).

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