S-100A1: S-100A1 domain found in proteins similar to S100A1. S100A1 is a calcium-binding protein belonging to a large S100 vertebrate-specific protein family within the EF-hand superfamily of calcium-binding proteins. Note that the S-100 hierarchy, to which this S-100A1 group belongs, contains only S-100 EF-hand domains, other EF-hands have been modeled separately. As is the case with many other members of S100 protein family, S100A1 is implicated in intracellular and extracellular regulatory activities, including interaction with myosin-associated twitchin kinase, actin-capping protein CapZ, sinapsin I, and tubulin. Structural data suggests that S100A1 proteins exist within cells as antiparallel homodimers, while heterodimers with S100A4 and S100B also has been reported. Upon binding calcium S100A1 changes conformation to expose a hydrophobic cleft which is the interaction site of S100A1 with its more that 20 known target proteins.