CLEAVABLE SIGNAL PEPTIDES ARE RARELY FOUND IN BACTERIAL CYTOPLASMIC MEMBRANE-PROTEINS (REVIEW)

Most proteins destined for secretion are synthesized with aminotermina l extensions, known as signal peptides, which play a vital role in the ir translocation across the membrane bordering the cytoplasm. Followin g translocation across the eukaryotic endoplasmic reticulum (ER) membr ane or the bacterial cytoplasmic membrane, signal peptides are proteol ytically removed from the preproteins. The process of membrane protein assembly can be likened to that of protein export in that it involves the translocation of portions of proteins across membranes. Moreover, the topological similarities between eukaryotic ER and plasma membran e proteins and bacterial cytoplasmic membrane proteins suggest that th e mechanisms of membrane protein assembly may, like those of protein e xport, share fundamental similarities in eukaryotic and bacterial cell s. However, whilst many of the ER and plasma membrane proteins of high er eukaryotes are synthesized with cleavable signal peptides, the same is true of only very few bacterial cytoplasmic membrane proteins. Thi s fact is not widely appreciated, probably because certain exceptional (signal peptide-containing) bacterial membrane proteins, such as the m ajor coat protein of bacteriophage M13, have been the subject of exten sive investigations. In this review we highlight this anomoly and disc uss it within the general context of membrane protein topology.