RECENT PROGRESS AND FUTURE-DIRECTIONS IN STUDIES OF THE MAIN TERMINALBRANCH OF THE GENERAL SECRETORY PATHWAY IN GRAM-NEGATIVE BACTERIA - AREVIEW

The main terminal branch (MTB) of the general secretory pathway is use d by a wide variety of Gram(-) bacteria to transport exoproteins from the periplasm to the outside milieu. Recent work has led to the identi fication of the function of two of its 14 (or more) components: an enz yme with type-IV prepilin peptidase activity and a chaperone-like prot ein required for the insertion of another of the MTB components into t he outer membrane. Despite these important discoveries, little tangibl e progress has been made towards identifying MTB components that deter mine secretion specificity (presumably by binding to cognate exoprotei ns) or which form the putative channel through which exoproteins are t ransported across the outer membrane. However, the idea that the singl e integral outer membrane component of the MTB could line the wall of this channel, and the intriguing possibility that other components of the MTB form a rudimentary type-IV pilus-like structure that might spa n the periplasm both deserve more careful examination. Although Escher ichia coli K-12 does not normally secrete exoproteins, its chromosome contains an apparently complete set of genes coding for MTB components . At least two of these genes code for functional proteins, but the op eron in which twelve of the genes are located does not appear to be ex pressed. We are currently searching for conditions which allow these g enes to be expressed with the eventual aim of identifying the protein( s) that E. coli K-12 can secrete. (C) 1997 Elsevier Science B.V.