Protein Translocation into and across Archaeal Cytoplasmic Membranes

Abstract

Recent analyses of the archaeal Sec and Tat pathways have revealed novel and crucial information about archaeal protein translocation, as well as protein translocation in general. This chapter provides an overview on protein translocation into and across archaeal cytoplasmic membranes. The Sec pathway is the only known universally conserved protein translocation pathway. Protein translocation may be driven by one or several extracytoplasmic activities that provide directionality by preventing movement of the polypeptide chain back into the cytoplasm. In vitro studies suggest that the proton motive force (PMF), in concert with the action of SecA, facilitates bacterial secretion via the Sec pore. Furthermore, the PMF is apparently sufficient to drive translocation of proteins via the twin-arginine translocation (Tat) pore. Thus, it is possible that an ion gradient across the archaeal membrane is the sole source of energy for protein translocation. Many bacteria and archaea possess an additional general secretion pathway, described as the Tat pathway. The presence of the twin-arginine motif in the Tat signal sequence provided a means of identifying novel Tat substrates by computational pattern-matching techniques. Recent in vivo, in vitro, and in silico studies have led to a better understanding of archaeal protein translocation. Moreover, the elucidation of an archaeal Sec-pore X-ray crystal structure strikingly demonstrates how analysis of this pathway in archaea can significantly advance the field of protein translocation as a whole. In addition to standard molecular and biochemical approaches, it is now crucial to develop in vitro Sec and Tat protein translocation systems that will more clearly define the mechanisms of these pathways and reveal the energetics of these cellular processes in archaea.