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S100A13 is a 98-amino acid, calcium binding protein. It is known to participate in the non-classical secretion of signal peptide-less proteins, such as the acidic fibroblast growth factor. In this study, we investigate the lipid binding properties of S10013 using a number of biophysical techniques, including multidimensional NMR spectroscopy. Isothermal titration calorimetry and steady state fluorescence experiments show that apoS100A13 exhibits preferential binding to small unilamelar vesicles of l-phosphatidyl serine (pS). In comparison, Ca2+-bound S100A13 is observed to bind weakly to unilamelar vesicles (SUVs) of pS. Equilibrium thermal unfolding and limited trypsin digestion analysis reveal that apoS100A13 is significantly destabilized upon binding to SUVs of pS. Results of the far UV circular dichroism and ANS (8-anilino-1-napthalene sufonate) binding experiments indicate a subtle conformational change resulting in the increase in the solvent-accessible hydrophobic surface in the protein. Availability of the solvent-exposed hydrophobic surface(s) in apoS10013 facilitates its interaction with the lipid vesicles. Our data suggest that Ca2+ binding dictates the membrane binding affinity of S100A13. Based on the results of this study, a model describing the sequence of molecular events that possibly can occur during the non-classical secretion of FGF-1 is presented.

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SUMMARY As the toad oocyte grows, the intracellular Na concentration rises and the K concentration falls. By stereological analysis using a point counting method in a projection apparatus, it is shown that as the oocyte grows the volume density of the nucleus, endoplasmic reticulum and mitochondria falls, while that of the yolk platelets and cytoplasmic vesicles rises. A multiple regression analysis applied to the sodium and potassium concentrations and to the volume densities of the subcellular components indicates that sodium is most concentrated in cytoplasmic vesicles while potassium is most concentrated in a component composed of the cell cytoplasmic ground substance, the mitochondria and the cell nucleus.

conformational changes involved in the centrin-Sfi1p 21 complex formation by FT-IR spectroscopy, two dimensional correlation spectroscopy and isothermal titration calorimetry. The binding was exothermic and the thermodynamic data for Hcen1-Sfi1p 21 was the following: N 1.33 5 0.0165, Ka 1.59 x10 7 5 2.48 x10 6 M, DH À1.72 x10 4 5 301.1 kcal/mol and DS À23.8 kcal/mol. We have also established the relative stability of these proteins by differential scanning calorimetry. Our experiments address key questions underlying the molecular basis of this complex interaction.