%0 Journal Article
%T Recognition of secretory proteins in Escherichia coli requires signals in addition to the signal sequence and slow folding
%A Ipsita Mallik
%A Margaret A Smith
%A Ann M Flower
%J BMC Microbiology
%D 2002
%I BioMed Central
%R 10.1186/1471-2180-2-32
%X In the current work, we tested this hypothesis using a mutant form of ¦Ë repressor that folds slowly. No export of the mutant protein was observed, even in a prl strain. We then examined binding of the mutant ¦Ë repressor to SecB. We did not observe interaction by either of two assays, indicating that slow folding is not sufficient for SecB binding and targeting to translocase.These results strongly suggest that to be targeted to the export pathway, secretory proteins contain signals in addition to the canonical signal sequence and the rate of folding.The Sec-dependent protein export pathway of Escherichia coli is responsible for translocation of secretory proteins across the inner membrane to final destinations in the periplasm or outer membrane. Secretory proteins, also called preproteins, are synthesized with a cleavable amino terminal signal sequence that functions both to slow folding of the preprotein and to aid in recognition of the secretory protein by export factors. Export of many, but not all, secretory proteins is dependent on interaction with SecB, a cytoplasmic chaperone that maintains the preprotein in a loosely folded conformation competent for translocation. Both SecB and the preprotein provide binding sites for SecA, a peripheral membrane ATPase. SecA targets the preprotein to the membranous translocase complex composed of SecY, SecE, SecG, SecD, SecF, and YajC. Formation of the complete translocase complex promotes an ATP binding and hydrolysis cycle by SecA that results in segmental translocation of the secretory protein across the membrane [1-4].The signal sequence is crucial for efficient translocation; mutations in the signal sequence significantly reduce export of the preprotein and complete deletions of the signal sequence eliminate essentially all export [5-8]. Selection for extragenic suppressors of such export defective preproteins led to the identification of the prl alleles of secY (prlA), secE (prlG), secA (prlD), and more recently secG
%U http://www.biomedcentral.com/1471-2180/2/32