Abstract

Proteasomal degradation of proteins plays important roles in cell physiology. In eukaryotic cells, this process involves the specific covalent modification by a highly conserved small regulatory protein, ubiquitin, which labels target proteins for proteolysis and subsequent degradation. Ubiquitination reaction is carried out by the E1 (ubiquitin activating proteins) - E2 (ubiquitin conjugating proteins) - E3 (ubiquitin ligases) cascade of enzymes. The specificity for targeting substrates for ubiquitination is mainly conferred by the E3 proteins.

Siah proteins are members of an evolutionarily highly conserved family of E3 ubiquitin ligases which regulate the ubiquitination and proteasome-dependent degradation of several proteins. The mammalian Siah genes are homologues of the Drosophila SINA (seven in absentia homologue), which is required for the R7 cell determination in the developing eyes, and they are characterized by the presence of a RING finger domain that is responsible for E3 activity.

SIAH/sina proteins interact with several proteins and participate in the regulation of some of them through ubiquitination and proteasome-dependent degradation. Degraded proteins include Drosophila repressor Tramtrack (Ttk88) (Li et al., 1997; Tang et al., 1997), netrin-1 receptor deleted in colorectal cancer (DCC) (Hu et al., 1997), coactivator BOB.1/OBF.1 (Boehm et al., 2001; Tiedt et al., 2001), nuclear receptor corepressor (NcoR) (Zhang et al., 1998), the transcriptional repressor TIEG-1 (Johnsen et al., 2002) and Kid (Germani et al., 2000), a protein necessary for chromosome movement during mitosis and meiosis. SIAH-1 also interacts with a complex including step1, Ebi, Sip (SIAH interacting protein) and APC (adenomatosis polyposis colonic protein) which facilitates the degradation of beta-catenin in a p53-dependent manner (Liu et al., 2001; Matsuzawa and Reed, 2001).

The C-terminus region of the Siah proteins has been demonstrated to be a substrate- and cofactor- interaction module (Substrate-Binding Domain, SBD. The X-ray analysis of the Siah SBD revealed an eight-stranded beta sandwich fold (Polekhina et al., 2002). House and colleagues (2003) have identified a binding motif PxAxVxP (named Siah degron), present the Drosophila protein PHYL, which binds with high affinity to the SINA and Siah SBDs. The degron motif is present in several Siah degradation targets. The structure of the Siah1 in complex with a degron-containing peptide from the adaptor protein Phyllopod has been recently solved (House et al., 2006).
Other Siah interacting proteins, including KID, APC, synphylin-1, carry similar peptides that do not perfectly match the reported consensus degron motif.