Optical properties and small-angle neutron scattering of bovine heart mitochondrial oligomycin sensitivity conferring protein

The mitochondrial oligomycin sensitivity conferring protein (OSCP) was isolated from beef heart mitochondria according to Senior (1979) [Senior, A.E. (1979) Methods Enzymol. 55,391-397] with a supplementary step of Sephadex chromatography for more extensive purification. The ultraviolet and fluorescence spectra of OSCP were consistent with the presence of tyrosyl residues and the absence of tryptophanyl residues. From the circular dichroism spectrum of OSCP, 43% alpha-helical structure was calculated; the dichroism spectra of OSCP in H2O and D2O were identical. A molecular weight (Mr) of 22 000 for OSCP was determined by sodium dodecyl sulfate gel electrophoresis at different concentrations of the polyacrylamide gel. The radius of gyration (Rg) and the shape of OSCP in H2O and D2O were studied by small-angle neutron scattering. The experimentally determined Rg value of OSCP in H2O was 24 +/- 1 A, and its Mr was 25 000 +/- 3000. Comparison of the experimental Rg value with that expected for a compact globular protein of the same molecular weight (17 A) led to the conclusion that OSCP is a considerably elongated molecule protein with an axial ratio higher than 3. In D2O buffer, the Rg value was higher than that in H2O, a situation in contrast with that observed for most globular hydrophilic proteins; this might be due to a preferential location of the positively charged lysine residues.