CacyBP/SIP, a calcyclin and Siah-1-interacting protein, binds EF-hand proteins of the S100 family

Recently, a human ortholog of mouse calcyclin (S100A6) -binding protein (CacyBP) called SIP (Siah-1-interacting protein) was shown to be a component of a novel ubiquitinylation pathway regulating beta-catenin degradation (Matsuzawa, S., and Reed, J. C. (2001) Mol. Cell 7, 915-926). In murine brain, CacyBP/SIP is expressed at a high level, but S100A6 is expressed at a very low level. Consequently we carried out experiments to determine if CacyBP/SIP binds to other S100 proteins in this tissue. Using CacyBP/SIP affinity chromatography, we found that S100B from the brain extract binds to CacyBP/SIP in a Ca2+-dependent manner. Using a nitrocellulose overlay assay with I-125-CacyBP/SIP and CacyBP/SIP affinity chromatography, we found that this protein binds purified S100A1, S100A6, S100A12, S100B, and S100P but not S100A4, calbindin D-9k, parvalbumin, and calmodulin. The interaction of S100 proteins with CacyBP/SIP occurs via its C-terminal fragment (residues 155-229). Co-immunoprecipitation of CacyBP/SIP with S100B from brain and with S100A6 from Ehrlich ascites tumor cells suggests that these interactions are physiologically relevant and that the ubiquitinylation complex involving CacyBP/SIP might be regulated by S100 proteins.