MCE domain proteins: conserved inner membrane lipid-binding proteins required for outer membrane homeostasis

Abstract

Bacterial proteins with MCE domains were first described as being important for Mammalian Cell Entry. More recent evidence suggests they are components of lipid ABC transporters. In Escherichia coli, the single-domain protein MlaD is known to be part of an inner membrane transporter that is important for maintenance of outer membrane lipid asymmetry. Here we describe two multi MCE domain-containing proteins in Escherichia coli, PqiB and YebT, the latter of which is an orthologue of MAM-7 that was previously reported to be an outer membrane protein. We show that all three MCE domain-containing proteins localise to the inner membrane. Bioinformatic analyses revealed that MCE domains are widely distributed across bacterial phyla but multi MCE domain-containing proteins evolved in Proteobacteria from single-domain proteins. Mutants defective in mlaD, pqiAB and yebST were shown to have distinct but partially overlapping phenotypes, but the primary functions of PqiB and YebT differ from MlaD. Complementing our previous findings that all three proteins bind phospholipids, results presented here indicate that multi-domain proteins evolved in Proteobacteria for specific functions in maintaining cell envelope homeostasis.

Modulation of Haemophilus influenzae interaction with hydrophobic molecules by the VacJ/MlaA lipoprotein impacts strongly on its interplay with the airways

“…Moreover, vacJ disruption did not modify lipid A hexa-acylation, excluding a functional relationship between VacJ and the acyltransferase HtrB. Analysis of H. influenzae genome sequences revealed a homolog to the recently characterized E. coli pqiB gene, encoding a multi M ammalian C ell E ntry (MCE) domain-containing protein displaying a syringe-like architecture periplasmic bridge, which may be involved in maintaining cell envelope homeostasis 48 , 49 , and will be subject of future study. H. influenzae genome sequences also revealed homologs to the E. coli fadL and fadD genes, annotated as an exogenous long-chain fatty acid transporter and an acyl-CoA synthase/fatty acid-CoA ligase 11 , 19 , but they lack the fadE , fadB , fadH and fadA genes, encoding four enzymes involved in fatty acid degradation via β-oxidation 50 , and genes encoding enzymes of the so called β-oxidation complex II 51 .…”

Structure of MlaFB uncovers novel mechanisms of ABC transporter regulation

“…An ABC transporter is a critical component of the Mla pathway, which is a phospholipid transport system that is important for outer membrane (OM) integrity in many Gram-negative species, and has been implicated in the M aintenance of outer membrane L ipid A symmetry ( Malinverni and Silhavy, 2009 ; Thong et al, 2016 ; Kamischke et al, 2019 ; Chong et al, 2015 ; Sutterlin et al, 2016 ; Ekiert et al, 2017 ). The Mla system from E. coli consists of three main parts: 1) an inner membrane (IM) ABC transporter complex, MlaFEDB ( Thong et al, 2016 ; Ekiert et al, 2017 ; Isom et al, 2017 ); 2) an OM complex, MlaA-OmpC/F ( Chong et al, 2015 ; Abellón-Ruiz et al, 2017 ); and 3) a periplasmic shuttle protein, MlaC ( Ekiert et al, 2017 ), which ferries lipids between the IM and OM complexes ( Figure 1A ). Mla was originally proposed to drive the import of mislocalized phospholipids from the outer leaflet of the OM toward the IM, thereby maintaining the asymmetry of the OM ( Malinverni and Silhavy, 2009 ).…”

Intermembrane transport: Glycerophospholipid homeostasis of the Gram-negative cell envelope

“…Indeed, this appears to be the case. E. coli has 3 detectable MCE domain proteins—MlaD, PqiB, and YebT—each encoded in separate operons (83). Each of these proteins copurify with the major GPLs from E. coli , suggesting that all could have a role in lipid transport in Gram-negatives.…”

“…The quaternary structures suggest independent mechanisms for transporting substrates; however, the mechanism by which transport occurs through the Pqi and Yeb complexes is elusive. Neither is associated with an ABC transporter, making it more likely that different MCE domains within the given polypeptide have different affinities for substrate or conformational changes in vivo to push substrate via peristaltic action (83). Although shown to bind GPLs, there is no direct biochemical or genetic evidence as to the function of Pqi and Yeb in Gram-negative bacteria.…”