The RING domain of Siaz, the zebrafish homologue of Drosophila seven in absentia, is essential for cellular growth arrest.

Siah is a mammalian homologue of Drosophila seven in absentia (sina) that is required for R7 photoreceptor development. Both the SINA and Siah family interact with ubiquitin-conjugating enzymes via an N-terminal RING domain and the C-terminal domain of SINA/ Siahs interacts with proteins targeted for degradation. Siah induces cell growth arrest by promoting beta-catenin degradation in a phosphorylation-independent manner as a result of indirect binding to beta-catenin. We previously cloned a zebrafish homologue (Siaz) of Siah. Siaz shares high sequence homology with vertebrate Siah-2. We have now examined the role of Siaz in growth regulation using the trypan blue exclusion assay and flow cytometry and found that Siaz induces cellular growth arrest by inhibiting the G2/M transition. The C-terminal domain of Siaz that interacts with target proteins is not required for growth inhibition. We conclude that the N-terminal RING and central domain of Siaz are sufficient to block the G2/M phase transition.

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