The location of membrane components in sarcoplasmic-reticulum membranes by using free and immobilized lactoperoxidase

The plasma membrane of Saccharomyces cerevisiae. Molecular structure and asymmetry

The molecular structure of the plasma membrane of the haploid strain Saccharomyces cerevisiae X-2180 1 A has been studied by means of sodium dodecyl sulfate polyacrylamide gel electrophoresis. Protein and glycoprotein components have been identified and their apparent Mr determined. A glycoprotein showing an apparent Mr of 27500 has been shown to be the main structural component. Treatment of the cells with cycloheximide prior to plasma membrane isolation resulted in a redistribution of the relative amounts of each protein band and a drastic reduction in the number of Schiff positive bands. It is postulated that treatment with this drug rids the plasma membrane of glycoprotein secretory components which are in the process of being secreted to the periplasmic space, thus allowing the study of the basic structural components of the organelle. The electrophoretic pattern of the internal membranes revealed close similarities with that of the plasma membrane and though two-dimensional electrophoresis might disclose greater differences, these similarities suggest a common origin for most of the components of both membranous systems. Finally, radioiodination techniques, have been used in studying the asymmetric disposition of some of the components of the plasma membrane. At least five polypeptides were identified as located to the outer layer of the plasma membrane and two more glycopeptides were shown to span across the bilayer.

Lactoperoxidase-coupled iodination of cardiac sarcoplasmic reticulum proteins

The peptide compositions of rabbit skeletal- and canine cardiac-muscle sarcoplasmic reticulum preparations have been compared by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. The cardiac preparations contain many proteins in addition to the 105 000 dalton peptide which has been previously identified as the Ca2+ stimulated ATPase. Four peptide components iodinated in the presence of either free or Sepharose 4B-bound lactoperoxidase have molecular weights of 130 000 (component I), 105 000 (component II), 52 000 (component III) and 47 000 (component IV). Comparison of the labelling patterns in the presence of the detergent Triton X-100 suggests that components I, III and IV have part of their peptide internally located. Although part of component II is externally accessible to free lactoperoxidase, its iodination is decreased by Triton X-100. Iodination of phospholamban, the 22 000 dalton substrate for cyclic AMP-dependent protein kinase, was not observed under the conditions investigated.