Discrimination between SRP- and SecA/SecB-dependent substrates involves selective recognition of nascent chains by SRP and trigger factor

Besides SecA and SecB, Escherichia coli cells possess a signal recognition particle (SRP) to target exported proteins to the SecY translocon, Using ch emical and site-specific cross-linking ht vitro, we show that SRP recognize s the first signal anchor sequence of a polytopic membrane protein (MtlA) r esulting in cotranslational targeting of MtlA to SecY and phospholipids of the plasma membrane, In contrast, a possible interaction of SRP with the se cretory protein pOmpA is prevented by the association of trigger factor wit h nascent pOmpA, Trigger factor also prevents SecA from binding to the firs t 125 amino acids of pOmpA when they are still associated with the ribosome , Under no experimental conditions was SecA found to interact with MtlA, Li kewise, virtually no binding of trigger factor to ribosome-bound MtlA occur s even in the complete absence of SRP, Collectively, our results indicate t hat at the stage of nascent polypeptides, polytopic membrane proteins are s elected by SRP for co-translational membrane targeting, whereas secretory p roteins are directed into the SecA/SecB-mediated post-translational targeti ng pathway by means of their preferential recognition by trigger factor.