Influence of the signal sequence and chaperone SecB on the interaction between precursor protein prePhoE and phospholipids

Membrane Association of the Cycling Peroxisome Import Receptor Pex5p

Peroxisomal proteins carrying a peroxisome targeting signal type 1 (PTS1) are recognized in the cytosol by the cycling import receptor Pex5p. The receptor-cargo complex docks at the peroxisomal membrane where it associates with multimeric protein complexes, referred to as the docking and RING finger complexes. Here we have identified regions within the Saccharomyces cerevisiae Pex5p sequence that interconnect the receptor-cargo complex with the docking complex. Site-directed mutagenesis of the conserved tryptophan residue within a reverse WXXXF motif abolished two-hybrid binding with the N-terminal half of Pex14p. In combination with an additional mutation introduced into the Pex13p-binding site, we generated a Pex5p mutant defective in a stable association not only with the docking complex but also with the RING finger peroxins at the membrane. Surprisingly, PTS1 proteins are still imported into peroxisomes in these mutant cells. Because these mutations had no significant effect on the membrane binding properties of Pex5p, we examined yeast and human Pex5p for intrinsic lipid binding activity. In vitro analyses demonstrated that both proteins have the potential to insert spontaneously into phospholipid membranes. Altogether, these data strongly suggest that a translocation-competent state of the PTS1 receptor enters the membrane via protein-lipid interactions before it tightly associates with other peroxins.

Effects of lipids on the interaction of SecA with model membranes

The effects of nonlamellar-prone lipids, diacylglycerol and phosphatidylethanolamine (PE), on the kinetic association of SecA with model membranes were examined by measuring changes in the intrinsic emission fluorescence with a stopped-flow apparatus. Upon interaction with standard liposomes composed of 50 mol% dioleolyphosphatidylcholine (DOPC) and 50 mol% of dioleoylphosphatidylglycerol (DOPG), the intrinsic fluorescence intensity of SecA was decreased after a lapse of time with a rate constant of 0.0049 s(-1). When the DOPC of the standard vesicles was gradually replaced with either dioeloyl PE (DOPE) or Escherichia coli (E. coli) PE, the rate constant increased appreciably as a function of PE concentration, in the order DOPE > E. coli PE. In addition, when the PE of E. coli PE/DOPG (50/50) vesicles was replaced with more than 5 mol% dioleoylglycerol (DOG), the rate constant further increased by 40%. The incorporation of nonlamellar-prone lipids in the vesicles also enhanced the binding of SecA to model membranes in the order DOPE > or = E. coli PE/DOG > E. coli PE > DOPC. These results provide the first kinetic evidence for the importance of nonlamellar-prone phospholipids for the association rate of SecA with membranes.

The high affinity ATP binding site modulates the SecA-precursor interaction

SecA is the central component of the protein-translocation machinery of Escherichia coli. It is able to interact with the precursor protein, the chaperone SecB, the integral membrane protein complex SecYEG, acidic phospholipids and its own mRNA. We studied the interaction between prePhoE and SecA by using a site-specific photocrosslinking strategy. We found that SecA is able to interact with both the signal sequence and the mature domain of prePhoE. Furthermore, this interaction was dependent on the type of nucleotide bound. SecA in the ADP-bound conformation was unable to crosslink with the precursor, whereas the ATP-bound conformation was active in precursor crosslinking. The SecA-precursor interaction was maintained in the presence of E. coli phospholipids but was loosened by the presence of phosphatidylglycerol bilayers. Examining SecA ATP binding site mutants demonstrated that ATP hydrolysis at the N-terminal high affinity binding site is responsible for the changed interaction with the preprotein.

Non-bilayer lipids stimulate the activity of the reconstituted bacterial protein translocase

To determine the phospholipid requirement of the preprotein translocase in vitro, the Escherichia coli SecYEG complex was purified in a delipidated form using the detergent dodecyl maltoside. SecYEG was reconstituted into liposomes composed of defined synthetic phospholipids, and proteoliposomes were analyzed for their preprotein translocation and SecA translocation ATPase activity. The activity strictly required the presence of anionic phospholipids, whereas the non-bilayer lipid phosphatidylethanolamine was found stimulatory. The latter effect could also be induced by dioleoylglycerol, a lipid that adopts a non-bilayer conformation. Phosphatidylethanolamine derivatives that prefer the bilayer state were unable to stimulate translocation. In the absence of SecG, activity was reduced, but the phospholipid requirement was unaltered. Remarkably, non-bilayer lipids were found essential for the activity of the Bacillus subtilis SecYEG complex. Optimal activity required a mixture of anionic and non-bilayer lipids at concentrations that correspond to concentrations found in the natural membrane.

Phosphatidylethanolamine mediates insertion of the catalytic domain of leader peptidase in membranes

Leader peptidase is an integral membrane protein of E. coli and it catalyses the removal of most signal peptides from translocated precursor proteins. In this study it is shown that when the transmembrane anchors are removed in vivo, the remaining catalytic domain can bind to inner and outer membranes of E. coli. Furthermore, the purified catalytic domain binds to inner membrane vesicles and vesicles composed of purified inner membrane lipids with comparable efficiency. It is shown that the interaction is caused by penetration of a part of the catalytic domain between the lipids. Penetration is mediated by phosphatidylethanolamine, the most abundant lipid in E. coli, and does not seem to depend on electrostatic interactions. A hydrophobic segment around the catalytically important residue serine 90 is required for the interaction with membranes.

Calorimetric analyses of the interaction between SecB and its ligands

SecB is a chaperone in Escherichia coli dedicated to export of proteins from the cytoplasm to the periplasm and outer membrane. It functions to bind and deliver precursors of exported proteins to the translocation apparatus before they fold into their native structures, thus maintaining them in a competent state for translocation across the membrane. The natural ligands of SecB are precursor proteins containing leader sequences. There are numerous reports in the literature indicating that SecB does not specifically recognize the leader peptides. However, two published investigations have concluded that the leader peptide is the recognition element (Watanabe M, Blobel G. 1989. Cell 58:685-705; Watanabe M, Blobel G. 1995. Proc Natl Acad Sci USA 92:10133-10136). In this work we use titration calorimetry to show that SecB binds two physiological ligands, which contain leader sequences, with no higher affinity than the same molecules lacking their leader sequences. Indeed, for one ligand the presence of the leader sequence reduces the affinity. Therefore, it can be concluded that the leader sequence provides no positive contribution to the binding energy.