S100A13 - Biochemical characterization and subcellular localization in different cell lines

S100 proteins became of major interest because of their divergent cell- and tissue-specific expression, their close association with a number of human diseases, and their importance for clinical diagnostics. Here, we report f or the first time the purification and characterization of human recombinan t S100A13. Flow dialysis revealed that the homodimeric S100A13 binds four C a2+ in two sets of binding sites, both displaying positive cooperativity bu t of very different affinity. Fluorescence and difference spectrophotometry indicate that the Trp/Tyr signal changes are almost complete upon binding of Ca2+ to the two high affinity sites, which probably correspond to the C- terminal EF-hands in each subunit. The far-UV circular dichroic signal also changes upon binding of the first two Ca2+. So far, the tissue distributio n of S100A13 has not been well characterized. Here, we show that S100A13 is widely expressed in various types of tissues with a high expression level in thyroid gland. Using specific antisera against S100A13, high protein exp ression was detected in follicle cells of thyroid, Leydig cells of testis, and specific cells of brain. In human smooth muscle cells, which co-express S100A2 in the nucleus and S100A1 in stress fibers, S100A13 shows a unique subcellular localization in the perinuclear area. These data suggest divers e functions for this protein in signal transduction.