SecB: A Molecular Chaperone of Escherichia Coli Protein Secretion Pathway

Abstract

Publisher Summary This chapter focuses on the role that SecB and other molecular chaperones play in sponsoring efficient protein secretion in Escherichia coli. The molecular chaperone SecB binds a subset of E.coli envelope protein precursors and promotes their efficient export from the cytoplasm by (1) maintaining them in a loosely folded or export competent conformation, (2) blocking nonproductive interactions with the cytoplasmic membrane, (3) preventing aggregation, and (4) targeting them to SecA, the peripheral membrane component of the preprotein translocase. SecB is involved in the targeting of certain precursors to the membrane components of the translocation machinery. SecB is an acidic, soluble, tetramer of identical 17-kDa subunits. It comprises only 0.08% of the cytosolic proteins. The secB gene is “conditionally essential” in that secB null strains cannot grow on rich media or at subbasal levels of heat-shock protein synthesis, but can grow on minimal media. Overexpression of heat-shock proteins can partially suppress the export and growth defects in secB null strains. Hybrid proteins composed of the signal peptide and a substantial portion of the mature MBP fused to LacZ enter the export pathway. SecB as a component of secretion machinery involves isolation, phenotype of SecB mutants, SecB sequence, and purification of SecB.