Membrane traffic wardens and protein secretion in gram-negative bacteria.

Recent progress in the genetic analysis of protein secretion in diverse Gram-negative bacteria has revealed three major, highly conserved but functionally independent pathways that involve accessory apparatus proteins. Protein secretion via the Type I pathway is signal sequence-independent with no free periplasmic intermediate. Secretion by the Type II pathway is signal sequence-dependent and via the periplasm. Recent results also suggest that a third (Type III) secretory pathway exists in which protein secretion is signal sequence-independent.