Protein Secretion Systems in Gram-Negative Bacteria

P rotein export systems are present in all living organisms, including gram-negative bacteria and the eukaryotic organelles that derived from them (Table 1). In contrast to other organisms, however, gram-negative bacteria contain numerous, apparently independent systems for exporting proteins, including at least eight types that mediate export across or insertion into the inner cytoplasmic membrane and another eight that specifically mediate export across or insertion into the outer lipopolysaccharide-containing membrane. Three of the secretory pathway (SP) systems– namely, type I (ISP; ABC), type III (IIISP; Fla/ Path), and type IV Secretory Pathways (IVSP; Conj/Vir)–can export proteins across both the inner and outer bacterial membranes in a single energy-coupled step. A fourth generalized mechanism for exporting proteins across the two-membrane envelope in two distinct steps, which we refer to as type II secretory pathways (IISP), uses either the general secretory pathway (Sec) or the twinarginine targeting translocase (Tat) to export proteins across the inner membrane, plus any one of several types of systems to achieve export across the outer membrane. A variety of functionally restricted secretion systems can also be found in gramnegative bacteria (see box, p. 416, top of page). Further, periplasmic proteins destined for the external milieu cross the outer membrane via either the “main terminal branch” (MTB) or one of several systems for exporting specific proteins across the outer membrane (see box, p. 416, bottom of page). Comparisons among these different bacterial systems suggest that each evolved independently despite there being a number of specific biogenic, mechanistic, and evolutionary similarities among them.