S100A13 is an 11 kDa member of the S100 (soluble in 100% saturated ammonium sulfate) family of vertebrate EF-hand Ca2+-binding proteins. It is widely expressed as a homodimer with two 98 amino acid (aa) long subunits. Human S100A13 shares 83%, 90%, 91%, 87%, 78% and 47% aa identity with mouse, rat, cow, dog, opossum, and chicken S100A13, respectively. Like other S100 proteins, S100A13 is small and generally acidic, but contains a basic residue rich sequence at the C-terminus, and two EF hand motifs that bind with Ca2+ differing affinities.
Some S100 proteins, including S100A13, are able to bind the cell surface receptor for advanced glycation endproducts (RAGE). Despite lacking a signal sequence, S100A13 plays an important role in Cu2+-dependent export of FGF-1 (FGF acidic) and IL-1 alpha from the cell in response to stresses such as heat shock, anoxia and starvation. Binding of copper is necessary for formation of a multiprotein complex between S100A13, FGF-1 and p40 synaptotagmin1 (syt1). Cu2+ ions supplied by S100A13 are thought to oxidize and downregulate the activity of FGF-1 prior to export. calcium influx may also play a similar role in FGF-1 release from neuronal cells. With FGF-1 and syt1, S100A13 likely perturbs the membrane, which allows the S100A13 protein complex to exit the cell. S100A13 has been proposed as a marker for angiogenesis in tumors and endometrium, due to its role in stress induced export of FGF-1.