Immunochemical analysis of the plasma membrane from baker's yeast Saccharomyces cerevisiae.

Yeast plasma membranes have been isolated from homogenized yeast cells, identified as pure plasma membrane vesicles which were used as antigens. By crossed immunoelectrophoresis with anti-membrane immunoglobulins, 17 discrete antigens have been detected in Triton X-100 extracts from plasma membranes. Three different immunoabsorption experiments were performed with : a) isolated membranes exposing the cytoplasmic surfaces (PS) and the external surfaces (ES), b) yeast protoplasts exposing only antigenic determinants on the ES, c) lysed protoplasts which had been saturated on the ES with antibodies prior to lysis. These absorption experiments demonstrated that seven of the antigens are expressed on the ES while eight immunogens expose antigenic determinants on the PS. Four of the principal immunoprecipitates are not affected by absorption with surface antigens whereas two of the antigens indicate transmembrane characteristics. Of these 17 immunoprecipitates four were shown by zymograms to possess enzymatic activities: ATPase (EC 3.6.1.3) and NADH-dehydrogenase (EC 1.8.99.3) (three separate components). Three of these enzymes are expressed on the PS, and one NADH-dehydrogenase exposes determinants on the ES of the protoplasts. The presence of antigens on the PS of the plasma membrane could also be demonstrated on micrographs by the indirect ferritin-antibody labeling technique followed by freeze-etching and shadowing of the membranes.