Strains of Escherichia coli in which lacZ (specifies beta-galactosidase) is fused to genes that specify exported proteins such as LamB (lambda receptor) exhibit unusual phenotypes. In particular, such strains are killed by high-level expression of the LacZ hybrid protein. Previous results suggest that this overproduction phenotype is the consequence of a lethal jamming of the cellular protein export machinery and this hypothesis is supported by the observed accumulation of the precursor forms of many noncytoplasmic proteins within the moribund cell. Under conditions in which protein export is compromised, biochemical and immunocytochemical analyses indicate that these hybrid proteins can be found in transmembrane orientation. To identify the cellular component rendered rate-limiting by the LacZ hybrid protein under jamming conditions we have utilized signal sequence mutations, which block entry of the hybrid protein into the export pathway, and a dominant suppressor of these lesions, prlA4. Data obtained with a series of merodiploids heterozygous and homozygous for prlA+ and prlA4 show that PrlA is the component sequestered by hybrid jamming. Taken together, these results suggest that PrlA is a component of the export machinery that functions in the translocation of proteins across the cytoplasmic membrane.