Preparation of a highly translocation-competent proOmpA/SecB complex

Interplay between MPIase, YidC, and PMF during Sec-independent insertion of membrane proteins

Charges in the N-terminal region and the synthesis level of membrane proteins with N-out topology determine YidC and PMF dependencies with the interplay between glycolipid MPIase, YidC, and PMF.

Integral membrane proteins with the N-out topology are inserted into membranes usually in YidC- and PMF-dependent manners. The molecular basis of the various dependencies on insertion factors is not fully understood. A model protein, Pf3-Lep, is inserted independently of both YidC and PMF, whereas the V15D mutant requires both YidC and PMF in vivo. We analyzed the mechanisms that determine the insertion factor dependency in vitro. Glycolipid MPIase was required for insertion of both proteins because MPIase depletion caused a significant defect in insertion. On the other hand, YidC depletion and PMF dissipation had no effects on Pf3-Lep insertion, whereas V15D insertion was reduced. We reconstituted (proteo)liposomes containing MPIase, YidC, and/or F₀F₁-ATPase. MPIase was essential for insertion of both proteins. YidC and PMF stimulated Pf3-Lep insertion as the synthesis level increased. V15D insertion was stimulated by both YidC and PMF irrespective of the synthesis level. These results indicate that charges in the N-terminal region and the synthesis level are the determinants of YidC and PMF dependencies with the interplay between MPIase, YidC, and PMF.

Trigger factor is a bona fide secretory pathway chaperone that interacts with SecB and the translocase

Bacterial secretory preproteins are translocated across the inner membrane post-translationally by the SecYEG-SecA translocase. Mature domain features and signal peptides maintain preproteins in kinetically trapped, largely soluble, folding intermediates. Some aggregation-prone preproteins require chaperones, like trigger factor (TF) and SecB, for solubility and/or targeting. TF antagonizes the contribution of SecB to secretion by an unknown molecular mechanism. We reconstituted this interaction in vitro and studied targeting and secretion of the model preprotein pro-OmpA. TF and SecB display distinct, unsuspected roles in secretion. Tightly associating TF:pro-OmpA targets the translocase at SecA, but TF prevents pro-OmpA secretion. In solution, SecB binds TF:pro-OmpA with high affinity. At the membrane, when bound to the SecA C-tail, SecB increases TF and TF:pro-OmpA affinities for the translocase and allows pro-OmpA to resume translocation. Our data reveal that TF, a main cytoplasmic folding pathway chaperone, is also a bona fide post-translational secretory chaperone that directly interacts with both SecB and the translocase to mediate regulated protein secretion. Thus, TF links the cytoplasmic folding and secretion chaperone networks.

CdsA is involved in biosynthesis of glycolipid MPIase essential for membrane protein integration in vivo

MPIase is a glycolipid that is involved in membrane protein integration. Despite evaluation of its functions in vitro, the lack of information on MPIase biosynthesis hampered verification of its involvement in vivo. In this study, we found that depletion of CdsA, a CDP-diacylglycerol synthase, caused not only a defect in phospholipid biosynthesis but also MPIase depletion with accumulation of the precursors of both membrane protein M13 coat protein and secretory protein OmpA. Yeast Tam41p, a mitochondrial CDP-diacylglycerol synthase, suppressed the defect in phospholipid biosynthesis, but restored neither MPIase biosynthesis, precursor processing, nor cell growth, indicating that MPIase is essential for membrane protein integration and therefore for cell growth. Consistently, we observed a severe defect in protein integration into MPIase-depleted membrane vesicles in vitro. Thus, the function of MPIase as a factor involved in protein integration was proven in vivo as well as in vitro. Moreover, Cds1p, a eukaryotic CdsA homologue, showed a potential for MPIase biosynthesis. From these results, we speculate the presence of a eukaryotic MPIase homologue.

Glycolipozyme MPIase is essential for topology inversion of SecG during preprotein translocation

Presecretory proteins are translocated across biological membranes through protein-conducting channels such as Sec61 (eukaryotes) and SecYEG (bacteria). SecA, a translocation ATPase, pushes preproteins out with dynamic structural changes through SecYEG. SecG, a subunit of the SecYEG channel possessing two transmembrane stretches (TMs), undergoes topology inversion coupled with SecA-dependent translocation. Recently, we characterized membrane protein integrase (MPIase), a glycolipozyme involved in not only protein integration into membranes but also preprotein translocation. We report here that SecG inversion occurs only when MPIase associates with SecYEG. We also found that MPIase modulates the dimer orientation of SecYEG. Cysteine-scanning mutagenesis mapped SecG TM 2 to a relatively hydrophilic environment. The dimer formation of SecG, crosslinked at TM 2, was not observed on SecG inversion, indicating that SecYEG undergoes a dynamic structural change during preprotein translocation.

Position-dependent effects of polylysine on Sec protein transport

The bacterial Sec protein translocation system catalyzes the transport of unfolded precursor proteins across the cytoplasmic membrane. Using a recently developed real time fluorescence-based transport assay, the effects of the number and distribution of positive charges on the transport time and transport efficiency of proOmpA were examined. As expected, an increase in the number of lysine residues generally increased transport time and decreased transport efficiency. However, the observed effects were highly dependent on the polylysine position in the mature domain. In addition, a string of consecutive positive charges generally had a more significant effect on transport time and efficiency than separating the charges into two or more charged segments. Thirty positive charges distributed throughout the mature domain resulted in effects similar to 10 consecutive charges near the N terminus of the mature domain. These data support a model in which the local effects of positive charge on the translocation kinetics dominate over total thermodynamic constraints. The rapid translocation kinetics of some highly charged proOmpA mutants suggest that the charge is partially shielded from the electric field gradient during transport, possibly by the co-migration of counter ions. The transport times of precursors with multiple positively charged sequences, or "pause sites," were fairly well predicted by a local effect model. However, the kinetic profile predicted by this local effect model was not observed. Instead, the transport kinetics observed for precursors with multiple polylysine segments support a model in which translocation through the SecYEG pore is not the rate-limiting step of transport.