The bacterial translocase: a dynamic protein channel complex

Abstract

The major route of protein translocation in bacteria is the so-called general secretion pathway (Sec-pathway). This route has been extensively studied in Escherichia coli and other bacteria. The movement of preproteins across the cytoplasmic membrane is mediated by a multimeric membrane protein complex called translocase. The core of the translocase consists of a proteinaceous channel formed by an oligomeric assembly of the heterotrimeric membrane protein complex SecYEG and the peripheral adenosine triphosphatase (ATPase) SecA as molecular motor. Many secretory proteins utilize the molecular chaperone SecB for targeting and stabilization of the unfolded state prior to translocation, while most nascent inner membrane proteins are targeted to the translocase by the signal recognition particle and its membrane receptor. Translocation is driven by ATP hydrolysis and the proton motive force. In the last decade, genetic and biochemical studies have provided detailed insights into the mechanism of preprotein translocation. Recent crystallographic studies on SecA, SecB and the SecYEG complex now provide knowledge about the structural features of the translocation process. Here, we will discuss the mechanistic and structural basis of the translocation of proteins across and the integration of membrane proteins into the cytoplasmic membrane.

An outer membrane channel protein of Mycobacterium tuberculosis with exotoxin activity

“…CpnT does not appear to have a classical Sec signal sequence, as do porins of Gram-negative bacteria (45) and MspA, the only known mycobacterial porin (46). In addition, CpnT is unusually large for a porin and seems to have at least two domains.…”

Identification of Two Interaction Sites in SecY that Are Important for the Functional Interaction with SecA

“…1,2 In bacteria, a central role in both processes is fulfilled by the integral membrane complex SecYEG, that forms the protein conducting channel within the cytoplasmic membrane. 3,4 Two different cytoplasmic partners can bind to SecYEG to induce opening of the channel and to provide the driving force for the translocation process.…”

Assembly pathway of a bacterial complex iron sulfur molybdoenzyme

“…In bacteria, secretory proteins are translocated either via the general secretion (Sec) pathway (120,121) or the Tat pathway (8). The fundamental functional difference between the two pathways is that the Sec system is involved in the secretion and insertion of unfolded proteins, while that Tat system is implicated in the secretion of folded and/or cofactor containing proteins (122,123). It has been proposed that the Tat system is utilized only when cytoplasmic folding of the protein substrate therefore omitted the use of the Sec system (124).…”

“…The bacterial Sec translocase is composed of a membrane embedded protein conducting channel (PCC) that consists of three integral membrane proteins, SecY, SecE, and SecG, and a peripheral associated ATPase, SecA, which functions as a molecular motor to drive membrane translocation (120,123,127). SecA associates peripherally to the PCC, where it accepts secretory proteins from chaperones to subsequently thread the unfolded protein through the narrow PCC transmembrane channel in an ATP dependent fashion.…”