Bacterial Lol-dependent outer-membrane lipoprotein trafficking

A diderm-bacterial module for delivery of mature lipoproteins from the inner membrane to the outer membrane. The inner-membrane ABC-superfamily complex LolCDE uses ATP hydrolysis by LolD to extract an outer-membrane-destined lipoprotein from the outer leaflet of the inner membrane, without moving it across the bilayer, and hands it to the periplasmic carrier LolA. LolA shields the lipid anchor while crossing the periplasm and passes the lipoprotein mouth-to-mouth to the outer-membrane lipoprotein LolB, which incorporates it into the outer membrane. Lipoproteins with an inner-membrane retention signal are not released. Lipoprotein maturation and the functions of individual lipoproteins lie outside the module. Escherichia coli K-12, in which the pathway was defined, and Pseudomonas putida KT2440 supply exact exemplars for all five components.

Boundary: the module begins with extraction of an already mature outer-membrane-destined lipoprotein and ends with outer-membrane delivery. Lgt/LspA/Lnt maturation is upstream and substrate-specific lipoprotein function is downstream. Generalization: the LolCDE-LolA-LolB relay is conserved beyond Pseudomonas, so the module is species-neutral and uses E. coli K-12 (the organism with the defining biochemistry and genetics) and P. putida KT2440 as exemplars; every leaf grounds to UniProt members of both. The modeled three-part relay is the canonical implementation and not a claim that every diderm lineage has LolB or a heterodimeric LolC/LolE extractor; LolB is mainly found in beta- and gammaproteobacteria, and some lineages encode a single LolC-type membrane subunit. GO terms: LolCDE carries GO:0140306 lipoprotein releasing activity (a protein transporter activity, not a transmembrane transporter activity) and LolA carries GO:0140598 lipoprotein carrier activity; GO has no molecular-function term for the LolB acceptance and insertion step, which the LolB gene reviews record as a proposed new term. Family grounding: PANTHER:PTHR30489 covers both LolC and LolE; its subfamilies do not separate the two roles outside enterobacteria (KT2440 LolC and LolE are both in SF0), so the operon order lolC-lolD-lolE is what assigns the roles. LolB has no PANTHER family assignment in UniProt and is grounded to the NCBIfam LolB family TIGR00548, which both exemplars match. Subunit functions are deliberately asymmetric: the integral subunits LolC and LolE enable GO:0140306 lipoprotein releasing activity (they recognize cargo and recruit LolA), whereas the cytoplasmic ATPase LolD enables GO:0016887 ATP hydrolysis activity and only contributes_to GO:0140306 in its gene reviews.