Is Ffh required for export of secretory proteins?

In Escherichia coli, protein export from the cytoplasm may occur via the si gnal recognition particle (SRP)-dependent pathway or the See-dependent path way. Membrane proteins utilize the SRP-dependent route, whereas many secret ory proteins use the cytoplasmic See machinery. To examine the possibility that signal peptide hydrophobicity governs which targeting route is utilize d, we used a series of PhoA signal sequence mutants which vary only by incr emental hydrophobicity changes. We show that depletion of SRP, but not trig ger factor, affects all the mutants examined. These results suggest secreto ry proteins with a variety of signal sequences, as well as membrane protein s, require SRP for export. (C) 2001 Federation of European Biochemical Soci eties. Published by Elsevier Science B.V. All rights reserved.