Electrophoretic analysis of the arrangement of spiralin and other major proteins in isolated Spiroplasma citri cell membranes.

The arrangement of the amphiphilic protein spiralin and of the other major polypeptides in the Spiroplasma citri cell membrane was investigated by one- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The analyses were performed on untreated membranes for the detection of disulfide bonds and on membranes treated with dimethylsuberimidate and dithiobis(succinimidyl propionate). All membranes were depleted of the bulk of extrinsic proteins. Spiralin monomers and oligomers (mainly dimers) were detected. Almost all the oligomers appeared to be stabilized by intermolecular disulfide bonds. Components D7 (39,000 daltons), D9 (51,000 daltons), D13 (69,000 daltons), D14b (76,000 daltons), D16 (89,000 daltons), and D17 (95,000 daltons), which are the other (presumably intrinsic) main polypeptides of the S. citri membrane, were also involved in homooligomers stabilized by disulfide bonds. However, in contrast to spiralin, larger amounts of D7, D9, and D14b were involved in high-molecular-weight multimers (molecular weight, greater than 400 X 10(3) after cross-linking with dithiobis(succinimidyl propionate). Extensive cross-linking with dimethylsuberimidate showed that spiralin was the polypeptide least readily integrated to large covalent complexes. These results suggest that spiralin probably does not form a two-dimensional network in the S. citri membrane depleted of the bulk of extrinsic proteins.