Ca2+-dependent association of S100A6 (Calcyclin) with the plasma membrane and the nuclear envelope

Expression of S100A6 (Calcyclin), a member of the S100 family and of Zn2+-b inding proteins is elevated in a number of malignant tumors. In vitro the p rotein associates with several actin-binding proteins and annexins in a Ca2 +-dependent manner. We have now studied the subcellular localization of S10 0A6 using a new, specific monoclonal antibody. Immunofluorescence microscop y of unfixed, ultrathin, frozen sections demonstrated a dual localization o f S100A6 at the nuclear envelope and the plasma membrane of porcine smooth muscle only in the presence of Ca2+. The same localization was found by imm unofluorescence and immunogold electron microscopy as well as by confocal l aser scanning microscopy with cultured, fixed, human CaKi-2 and porcine ST interphase cells. Upon cell division, however, S100A6 was found exclusively in the cytoplasm. Cell fractionation studies showed that S100A6 was presen t in the microsomal fraction in the presence of Ca2+ and was released from this fraction by the addition of EGTA/EDTA but not by Triton X-100. The dat a demonstrate that S100A6 is localized both at the plasma membrane and the nuclear envelope in vivo and suggest a Ca2+-dependent interaction with anne xins or other components of the nuclear envelope.