Protein transfer across microsomal membranes reassembled from separated membrane components

Abstract

THE synthesis of secretory proteins is initiated on ribosomes in the cell cytoplasm and the first part of the polypeptide chain to appear is a short sequence termed the signal sequence¹. This signal is thought to direct the ribosomal complex to the endoplasmic reticulum (ER) membrane where the synthesis of the rest of the secretory protein is tightly coupled to its transfer across the membrane into the cisternal space. In all but one case, it is clear that the polypeptide chain is processed during transfer to remove the signal sequence^2,3. Little is known about the membrane proteins involved in conveying the growing polypeptide chain across the membrane. A classical approach to learning more about these proteins would be to dissect the microsomal membrane into inactive components that can be reassembled subsequently into a functional entity. This would allow the purification of components involved in protein transfer. Here we report the first such reassembly.