Co-translational membrane integration of calcium pump protein without signal sequence cleavage
Mostov, K.E.; Defoor, P.; Fleischer, S.; Blobel, G.
Nature 292(5818): 87-88
1981
ISSN/ISBN: 0028-0836 PMID: 6456415 Document Number: 182468
The Ca pump protein (CCP) or Ca2+-ATPase is the predominant integral membrane protein of the sarcoplasmic reticulum (SR) of skeletal muscle. CPP is a single polypeptide chain of MW 119,000 which loops in and out of the membrane at least 3 times. A number of integral transmembrane proteins possessing a single hydrophilic domain on each side of the membrane were observed to use a signal sequence, cleaved or uncleaved, to initiate translocation of their ectoplasmic domain. Microsomal membranes when present during translation are capable of correctly integrating these do novo synthesized proteins into the membrane in the dog. This in vitro translocation system was used to investigate the integration of CPP into the microsomal membrane. CPP is synthesized without a cleaved signal sequence. It can be integrated into heterologous microsomal membranes only when these are present during translation, but not when they are present after completion of translation.