Studies on the Location and Orientation of Proteins in the Sarcoplasmic Reticulum

The adenosine triphosphatase of sarcoplasmic reticulum has been split into three parts by digestion with trypsin. A rapid split into fragments of molecular weight 60000 and 55000 was followed by a slow splitting of the 60000‐mol. wt piece into fragments of molecular weight 33000 and 24000. further digestion caused breakdown to small peptides and loss of protein from the vesicles. Vesicles reconstituted from purified ATPase lipoprotein have the same appearance in the electron microscope and suffer the same specific digestions as do the sarcoplasmic reticular vesicles; we conclude, therefore, that the characteristic projections seen on vesicles of the sarcoplasmic reticulum are part of the ATPase molecule. The two smallest fragments, molecular weight 33000 and 24000 were extensively iodinated by lactoperoxidase showing that they were exposed on the outer surface. Further, the phosphorylated active site has been located on the 33000‐mol. wt fragment. We suggest, therefore, that the 60000‐mol. wt fragment is responsible for the observed projections and that the active site is located in this projection and is outside of the membrane. We have shown that calsequestrin is readily extracted from sarcoplasmic reticulum by EDTA and is significantly iodinated by lactoperoxidase. It is likely therefore, that this protein is located on the outer surface of the membrane. Comparison of polyacrylamide gels of intact and digested sarcoplasmic reticulum suggests that several minor proteins of the sarcoplasmic reticulum may be proteolytic breakdown products of the ATPase.