S100A6 is a member of the S100 domain family within the EF-hand Ca2+-binding proteins superfamily. Note that the S-100 hierarchy, to which this S-100A6 group belongs, contains only S-100 EF-hand domains, other EF-hands have been modeled separately []. S100 proteins exhibit unique patterns of tissue- and cell type-specific expression and have been implicated in the Ca2+-dependent regulation of diverse physiological processes, including cell cycle regulation, differentiation, growth, and metabolic control [, , , ]. S100A6 is normally expressed in the G1 phase of the cell cycle in neuronal cells. The function of S100A6 remains unclear, but evidence suggests that it is involved in cell cycle regulation and exocytosis. S100A6 may also be involved in tumorigenesis; the protein is overexpressed in several tumours. Ca2+binding to S100A6 leads to a conformational change in the protein, which exposes a hydrophobic surface for interaction with target proteins. Several such proteins have been identified: glyceraldehyde-3-phosphate dehydrogenase, annexins 2, 6 and 11 and Calcyclin-Binding Protein (CacyBP) [, ].
This CS p23-like domain is found in the CacyBP/SIP protein (S100A6 binding protein and Siah-1 interacting protein) []. CacyBP/SIP interacts with S100A6 (calcyclin), with some other members of the S100 family, with tubulin, and with Siah-1 and Skp-1 []. The latter two are components of the ubiquitin ligase that regulates beta-catenin degradation. The beta-catenin gene is an oncogene participating in tumorigenesis in many different cancers. Overexpression of CacyBP/SIP, in part through its effect on the expression of beta-catenin, inhibits the proliferation, tumorigenicity, and invasion of gastric cancer cells []. CacyBP/SIP is abundant in neurons and neuroblastoma NB2a cells. An extensive re-organization of microtubules accompanies the differentiation of NB2a cells. CacyBP/SIP may contribute to NB2a cell differentiation through binding to and increasing the oligomerization of tubulin. CacyBP/SIP is also implicated in differentiation of erythroid cells, rat neonatal cardiomyocytes, in mouse endometrial events, and in thymocyte development [].
