S1 proteinis a family of low molecular weight protein found in vertebrates characterized by twoEF-hand calcium-binding motifs. There are at least 21 different S1 proteins, and the name is derived from the fact that the protein is1%soluble in ammonium sulfateat neutralpH. Most S1 proteins are disulfide-linked homodimer, and is normally present in cells derived from theneural crest, chondrocytes, macrophages, dendritic cells, etc. S1 proteins have been implicated in a variety of intracellular and extracellular functions. They are involved in regulation of protein phosphorylation, transcription factors, the dynamics of cytoskeleton constituents, enzyme activities, cell growth and differentiation, and the inflammatory response.
Protein S1-A1, also known as Calpactin I light chain, Cellular ligand of annexin II, S1 calcium-binding protein A1, p1 protein, p11, ANX2LG and S1A1, is a member of the S1 family of small, dimeric EF hand-type Ca(2+)-binding proteins that generally modulate cellular target proteins in response to intracellular Ca(2+) signals. In contrast to all other S1 proteins, S1A1 is Ca(2+) insensitive because of amino acid replacements in its Ca(2+)-binding loops that lock the protein in a permanently active state. S1A1 forms a heterotetramer with annexin IIH and promotes carcinoma invasion and metastasis by plasminogen activation. S1A1 and annexin II contribute to the aggressive characteristics of anaplastic carcinoma, while playing a constitutive role in papillary carcinoma. S1A1 induces the dimerization of ANXA2 / p36, it may function as a regulator of protein phosphorylation in that the ANXA2 monomer is the preferred target of tyrosine-specific kinase. S1A1 functions as a linker tethering certain transmembrane proteins to annexin A2 thereby assisting their traffic to the plasma membrane and/or their firm anchorage at certain membrane sites.