How proteins cross the bacterial cytoplasmic membrane.

Abstract

The study of protein translocation across the bacterial cytoplasmic membrane is a rapidly advancing area of current biological research. In Escherichia coli, proteins are exported to the periplasmic space and outer membrane. These proteins are synthesized in the cytosol as precursors, mostly with a cleavable signal sequence. The pioneering work of Beckwith, Silhavy, Ito, and coworkers [for reviews, see refs. 17, 63, and 132; for a chronological perspective of important findings in bacterial protein export, see ref. 162] has established that the targeting and translocation of precursor proteins across the cytoplasmic membrane requires the products of at least six sec-genes. These genes, secA, secB, secD, secE, secF, and secY, encode interacting core-components of a complex translocation apparatus that consists of soluble, peripheral and membrane integrated proteins (Table, and Fig. 1) [For review, see 162]. Homologous genes have been found in many other Bacteria and even in Archaea, indicating that this pathway is widely distributed. In addition, several other factors have been found that may contribute to the translocation reaction. Sec proteins have been purified to homogeneity and reconstituted into liposomes [25]. These proteoliposomes translocate precursor proteins across the lipid bilayer in an energydependent manner. The mechanism of protein translocation across the bacterial cytoplasmic membrane and the membrane of the endoplasmic reticulum (ER) or thylakoid of eukaryotic cells may share many features in common [52, 57, 75]. It has been known for some time