Figure 6
Schematic of RNC targeting and nascent-chain transfer to SecYEG in a quaternary membrane-associated complex. In the activation phase, an RNC exposing the SAS of an inner-membrane protein forms a high-affinity complex with SRP that leads to activation of SRP for an efficient interaction with the SRP receptor FtsY via the exposed Ffh NG domain (green) and the homologous NG domain (yellow/orange) of FtsY which is exposed upon binding of the unstructured A domain to SecYEG14,16. Targeting complex formation prepares SRP for the transfer of the SAS to SecYEG in that SRP rearranges on the ribosome and vacates the binding site of SecYEG near the peptide exit. This results in the pre-transfer complex in which the SRP-RNC interaction is destabilized, but that of SRP─FtsY remains tight (lower panel). Elongation of the nascent chain to an insertion-competent length triggers the transfer of the SAS to SecYEG to form the post-transfer complex. After the transfer, the complex rearranges and the SRP-FtsY interaction becomes weaker. Following GTP hydrolysis on both Ffh/SRP and FtsY, the post-transfer complex is disassembled and the targeting machinery can associate with another RNC, or the same ribosome exposing an upstream TM segment, to enter a new round of targeting. Apparent Kd values for the disassembled state are for the complexes of SRP with ribosomes, FtsY with SRP bound to ribosomes1,18, or SRP with FtsY bound to SecYEG16.
