An experimental system developed previously for the heterologous expression
of c-type cytochromes in Escherichia coil has been adapted to monitor prot
ein transfer across the bacteria's cytoplasmic membrane. Apocytochrome, lac
king the haem cofactor and probably in an unfolded state, was readily trans
ferred across the cytoplasmic membrane when fused to a Sec-specific signal
peptide. Furthermore, cytochrome fused to a signal peptide regarded as spec
ific for the twin arginine transport (Tat) system was translocated in an un
folded state by the Sec apparatus. After maturation and folding in the cyto
plasm, Tat-mediated transfer of holocytochrome to the periplasm occurred. W
e conclude that, in addition to the nature of the specific signal peptide,
the folding state of a particular protein also governs its acceptance by a
given transport system.