Transport of cytochrome c derivatives by the bacterial Tat protein translocation system

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.