Failure of E. coli K-12 to transport PhoE-LacZ hybrid proteins out of the cytoplasm.

A phoE-lacZ hybrid gene encoding the N-terminal 300 amino acid residues of pre-PhoE protein, fused to an almost complete beta-galactosidase molecule was constructed in vitro. Cell fractionation experiments suggested that the hybrid gene product is transported to the outer membrane. However, by using immuno-cytochemical labelling on ultra-thin cryosections it was shown that the hybrid protein accumulated in the cytoplasm. Thus, it appears that: (i) data on the localization of hybrid proteins merely based on cell fractionation experiments are not reliable, and (ii) either the C-terminal 15% of PhoE protein contain information which is essential for transport, or PhoE-LacZ hybrid proteins can never be transported out of the cytoplasm. The implications of these results for current models on the translocation of outer membrane proteins are discussed.