Co-Translational Protein Targeting and Insertion by SecA

Abstract

Co-translational protein targeting is a conserved process for the biogenesis of membrane proteins. This pathway was generally thought to depend on signal-recognition particle (SRP) for recognition of nascent protein and delivery to the membrane. Recently, SecA was found to also bind ribosomes near the nascent polypeptide exit tunnel, but the function of SecA’s ribosome interaction remains unclear. A combination of in vitro reconstitution and in vivo targeting assays show that SecA is necessary and sufficient to direct the targeting and translocation of RodZ to the bacterial plasma membrane in an obligatorily co-translational mechanism. The N-terminal extension preceding the transmembrane domain and periplasmic domain sequences immediately downstream of the transmembrane domain of RodZ provide distinguishing features that allow RodZ to engage SecA instead of the SRP machinery. Biochemical and cryoEM analyses further show that the N- terminal amphipathic helix on SecA and the ribosomal protein uL23 together form a composite binding site for the transmembrane domain (TMD) on the nascent chain. This interaction positions additional sites on the ribosome and SecA for recognition of the charged residues on both sides of the TMD, explaining the substrate specificity of SecA recognition. Quantitative kinetic analyses demonstrate that membrane-embedded SecYEG can associate with and remodel the SecA-bound ribosome-nascent chain complex, which together with elongation of the nascent polypeptide facilitates handover of the translating ribosome to the translocase.