Secretion of bacterial lipoproteins: through the cytoplasmic membrane, the periplasm and beyond

Abstract

Bacterial lipoproteins are peripherally anchored membrane proteins that play a variety of roles in bacterial physiology and virulence in monoderm (single membrane-enveloped, e.g., gram-positive) and diderm (double membrane-enveloped, e.g., gram-negative) bacteria. After export of prolipoproteins through the cytoplasmic membrane, which occurs predominantly but not exclusively via the general secretory or Sec pathway, the proteins are lipid-modified at the cytoplasmic membrane in a multistep process that involves sequential modification of a cysteine residue and cleavage of the signal peptide by the signal II peptidase Lsp. In both monoderms and diderms, signal peptide processing is preceded by acylation with a diacylglycerol through preprolipoprotein diacylglycerol transferase (Lgt). In diderms but also some monoderms, lipoproteins are further modified with a third acyl chain through lipoprotein N-acyl transferase (Lnt). Fully modified lipoproteins that are destined to be anchored in the inner leaflet of the outer membrane (OM) are selected, transported and inserted by the Lol (lipoprotein outer membrane localization) pathway machinery, which consists of the inner-membrane (IM) ABC transporter-like LolCDE complex, the periplasmic LolA chaperone and the OM LolB lipoprotein receptor. Retention of lipoproteins in the cytoplasmic membrane results from Lol avoidance signals that were originally described as the "+2 rule". Surface localization of lipoproteins in diderms is rare in most bacteria, with the exception of several spirochetal species. Type 2 (T2SS) and type 5 (T5SS) secretion systems are involved in secretion of specific surface lipoproteins of γ-proteobacteria. In the model spirochete Borrelia burgdorferi, surface lipoprotein secretion does not follow established sorting rules, but remains dependent on N-terminal peptide sequences. Secretion through the outer membrane requires maintenance of lipoproteins in a translocation-competent unfolded conformation, likely through interaction with a periplasmic holding chaperone, which delivers the proteins to an outer membrane lipoprotein flippase. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Keywords: Bacterial envelope; Chaperone; Lipoprotein; Membrane protein; Postranslational modification; Protein secretion.

Figure 1. Lipoprotein domain structure

Lipoproteins are translated in the bacterial cytoplasm as preprolipoprotein precursors. An Nterminal signal peptide (in green) targets the protein for export of the protein through the cytoplasmic membrane. Diacylation at a conserved Cys residue (in red) is mediated by Lgt via the Cys sulfhydryl group. Lsp recognizes the lipobox residues and cleaves the signal peptide. This makes the N-terminal amine group available for Lnt-mediated modification with a third acyl chain, which completes the membrane anchor. The N-terminus of the mature lipoprotein contains sorting signals (in blue) recognized by the Lol pathway in diderm bacteria. Generally, the Nterminus is also intrinsically disordered, providing a flexible “tether” (in yellow) for proper positioning and function in the bacterial envelope. The C-terminal portion of the polypeptide (in orange) assumes a fold specific for the protein’s function. The structure of B. burgdorferi OspA (PBD Accession # 1osp) is used for illustrative purposes. See text for details.

Figure 2. Modular model of lipoprotein secretion pathways in monoderm and diderm bacteria

Lipoprotein secretion is mediated by a sequence of pathway modules. (1) The first module exports lipoprotein precursors through the cytoplasmic or inner membrane via the Sec or TAT pathways. (2) The second module processes the N-terminus of the proteins to yield a mature lipidated protein; in monoderm bacteria, the last modification step is dispensable, as indicated by a vertical dashed line. In diderm bacteria, IM lipoproteins like E. coli Nlp are retained by failure to interact with downstream pathways. (3) In diderm bacteria, OM lipoproteins can subsequently interact with three modules: (a) the Lol periplasmic sorting module uses the energy from ATP hydrolysis to release lipoproteins like E. coli Lpp from the IM, provides a carrier chaperone for transport through the periplasm and an OM membrane receptor for insertion into the inner leaflet of the OM. Lpp also assumes an integral membrane protein conformation leading to surface exposure of its C terminus. (b) The T2SS module uses assembly of a pseudopilus on a periplasmic platform to drive secretion of a specific surface lipoprotein such as K. oxytoca PulA through an OM pore. (c) The T5SS module involves the BAM complex in the OM and the integral membrane protein chaperones Skp, SurA and DegP as well to facilitate OM insertion and pore formation of NalP’s translocator domain (T) and subsequent translocation of its N-terminal passenger domain (P) through the OM. N. gonorrhoeae NalP is released from the cell by autolytic cleavage. (d) A proposed module mediates complete surface localization of spirochetal lipoproteins such as B. burgdorferi OspA by interaction with a holding chaperone and an outer membrane lipoprotein flippase complex. Any specific involvement of the Lol pathway remains to be resolved. See text for details.