S100A6 (calcyclin), a small calcium-binding protein from the S100 family, interacts with several target proteins in a calcium-regulated manner. One target is Calcyclin-Binding Protein/Siah-1-Interacting Protein (CacyBP/SIP), a component of a novel pathway of beta-catenin ubiquitination. A recently discovered yeast homolog of CacyBP/SIP, Sgt1, associates with Skp1 and regulates its function in the Skp1/Cullin1/F-box complex ubiquitin ligase and in kinetochore complexes. S100A6-binding domain of CacyBP/SIP is in its C-terminal region, where the homology between CacyBP/SIP and Sgt1 is the greatest. Therefore, we hypothesized that Sgt1, through its C-terminal region, interacts with S100A6. We tested this hypothesis by performing affinity chromatography and chemical cross-linking experiments. Our results showed that Sgt1 binds to S100A6 in a calcium-regulated manner and that the S100A6-binding domain in Sgt1 is comprised of 71 C-terminal residues. Moreover, S100A6 does not influence Skp1-Sgt1 binding, a result suggesting that separate Sgt1 domains are responsible for interactions with S100A6 and Skp1. Sgt1 binds not only to S100A6 but also to S100B and S100P, other members of the S100 family. The interaction between S100A6 and Sgt1 is likely to be physiologically relevant because both proteins were co-immunoprecipitated from HEp-2 cell line extract using monoclonal anti-S100A6 antibody. Phosphorylation of the S100A6-binding domain of Sgt1 by casein kinase II was inhibited by S100A6, a result suggesting that the role of S100A6 binding is to regulate the phosphorylation of Sgt1. These findings suggest that protein ubiquitination via Sgt1-dependent pathway can be regulated by S100 proteins.
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