Cellular location affects protein stability in Escherichia coli.

We study the biosynthesis of preproinsulin and proinsulin in Escherichia coli by pulse-chase experiments. When E. coli is transformed with plasmids bearing a gene consisting of a fusion of preproinsulin DNA to part or to all of the DNA encoding a bacterial signal sequence, a variety of hybrid preproinsulin molecules can be made. Molecules with largely complete hybrid signal sequences are secreted into the periplasm. Molecules with defective signal sequences, lacking the hydrophobic core, are not secreted and remain in the cytoplasm. Such molecules are degraded with a 2-min half-life, whereas the molecules transported to the periplasm are at least 10 times more stable. A full-length preproinsulin precursor appears transiently before the signal sequence is cleaved off by the bacterial signal peptidase, and we propose a model to account for this.