Multiple Pathways of Exoprotein Secretion in Gram-Negative Bacteria

Abstract

Molecular studies on protein secretion in Gram-negative bacteria began in earnest a little over a decade ago when the first mutations preventing the appearance of extracellular proteins were constructed and analysed. Many of these studies were (and still are) fed by an interest in the function of the exoproteins in pathogenicity and were started at a time when the characterisation of protein export across the Escherichia coli inner membrane (the Sec system) by genetical and biochemical methods was already in full swing. What did we expect to find? Are we surprised that many different bacteria use basically the same pathway to secrete one or many different proteins or that so many totally different systems can function simultaneously in a single bacterial cell? What, exactly, are we all trying to figure out and how much have we achieved? How can we exploit the knowledge we have accrued to design more efficient systems for heterologous protein production and secretion in Gram-negative bacteria? Despite the very great differences between the secretion systems, the main questions being asked are very similar: • How is an exoprotein recognised by its cognate machinery and where is/are the secretion signal(s)? Are exoproteins in folded or unfolded configurations before and during translocation through the envelope? Where does folding occur and what additional factors are required to assist the folding process? Are these processes specific to the exoprotein(s) under study or can they operate on heterologous proteins that are engineered to be secreted by the same pathway? Does secretion occur in one or two steps and is the Sec system directly involved? Is protein translocation energy-driven and if so, what is the source of energy and how is it coupled to translocation? How are components of the secretion machinery assembled? Is the assembled machinery stable or dynamic? Is energy involved in assembly? What prevents periplasmic proteins from leaking across the outer membrane? •