Structural and Mechanistic Insights into Protein Translocation

Abstract

Many proteins are translocated across the endoplasmic reticulum (ER) membrane in eukaryotes or the plasma membrane in prokaryotes. These proteins use hydrophobic signal sequences or transmembrane (TM) segments to trigger their translocation through the protein-conducting Sec61/SecY channel. Substrates are first directed to the channel by cytosolic targeting factors, which use hydrophobic pockets to bind diverse signal and TM sequences. Subsequently, these hydrophobic sequences insert into the channel, docking into a groove on the outside of the lateral gate of the channel, where they also interact with lipids. Structural data and biochemical experiments have elucidated how channel partners, the ribosome in cotranslational translocation, and the eukaryotic ER chaperone BiP or the prokaryotic cytosolic SecA ATPase in posttranslational translocation move polypeptides unidirectionally across the membrane. Structures of auxiliary components of the bacterial translocon, YidC and SecD/F, provide additional insight. Taken together, these recent advances result in mechanistic models of protein translocation.

Protein translocation by the SecA ATPase occurs by a power‐stroke mechanism

“…Examples include hexameric ATPases, such as the p97 ATPase (Cdc48 in yeast), which extracts proteins from membranes or tight complexes, the Clp's and the ATPases of the 26S proteasome, which push polypeptides into a proteolytic chamber, and the NSF protein, which disassembles SNARE complexes involved in membrane fusion (for review, see Zhao et al, 2007;Bodnar & Rapoport, 2017;Ye et al, 2017;Yedidi et al, 2017). Another important member of this ATPase family is SecA, which translocates polypeptides through the plasma membrane in bacteria (for review, see Corey et al, 2016;Rapoport et al, 2017;Cranford-Smith & Huber, 2018). SecA acts a monomer and uses the energy of ATP hydrolysis to move its substrates through the protein-conducting SecY channel (Economou & Wickner, 1994).…”

Stabilization of SecA ATPase by the primary cytoplasmic salt of Escherichia coli

“…Exhaustive biochemical and structural studies from numerous laboratories have revealed the essential features of SecA structure and function at the molecular level (see review by Crane and Randall 4 ), although the structural details of its interactions with the SecYEG translocase are just now beginning to emerge. 5 Since the first structure of SecA from Bacillus subtilis, 6 structures of SecA from several species have been determined (see reviews 7,8 ) and suggest, along with biochemical studies, 9,10 that SecA is dimeric in the cytoplasm. However, little is known about SecA conformational states in the cytoplasm of E. coli, which are important for understanding in vivo conformational transitions that occur during secretion of preproteins across the inner membrane.…”

Identification and characterization of a translation arrest motif in VemP by systematic mutational analysis

“…The former mutation dramatically decreased the VemP arrest efficiency, whereas the latter mutation did not; thus, specific interactions exist between the VemP nascent chain and the interior of the exit tunnel of the ribosome, and the interaction mode of VemP is different from that of SecM (6). However, the mechanism via which the conserved segment inactivates the peptidyl transfer center (PTC) 2 of the ribosome remains unclear.…”