Examination of AsmA and its effect on the assembly of Escherichia coli outer membrane proteins

Abstract

asmA mutations were isolated as extragenic suppressors of an OmpF assembly mutant, OmpF315. This suppressor locus produced a protein that was present in extremely low levels and could only be visualized by Western blotting in cells where AsmA expression was induced from a plasmid. Detailed fractionation analyses showed that AsmA localized with the inner membrane. Curiously, however, the mutant OmpF assembly step influenced by AsmA occurred in the outer membrane, perhaps indicating an indirect involvement of AsmA in the assembly of outer membrane proteins. Biochemical examination of the outer membrane showed that asmA null mutations reduce lipopolysaccharide (LPS) levels, thereby lowering the ratios of glycerolphospholipids to LPS and envelope proteins to LPS in the outer membrane. Despite these quantitative alterations, no apparent structural changes in LPS or major phospholipids were noted. Reduced LPS levels in asmA mutants indicate a possible role of AsmA in LPS biogenesis. Data presented in this study suggest that asmA-mediated OmpF assembly suppression may have been achieved by altering the outer membrane fluidity, thus making it more amenable for the assembly of mutant proteins.

Modulations in lipid A and phospholipid biosynthesis pathways influence outer membrane protein assembly in Escherichia coli K‐12

“…The effect of sabA1 on asmB2-, asmB3-and asmAmediated OmpF315 assembly suppression Like the asmB1 mutation, asmB2, asmB3 and asmA mutations reduce LPS levels and suppress the OmpF315 assembly defect (Deng and Misra, 1996;Kloser et al, 1996). asmB2 and asmB3 carry an identical mutation in lpxC (t149→c), which is distinct from that present in the asmB1 allele (g628→a; Kloser et al, 1996).…”

“…These suppressors defined a previously uncharacterized locus asmA (Deng and Misra, 1996) and a known locus asmB, which is allelic to lpxC (envA, Kloser et al, 1996). Mutations in either locus lead to reduced LPS levels and an elevated ratio of phospholipid to LPS in the outer membrane.…”

Roles of the Outer Membrane Protein AsmA of Salmonella enterica in the Control of marRAB Expression and Invasion of Epithelial Cells

“…An interpretation was that the presence of AsmA might create an environment refractory to the assembly of mutant (misfolded) OMPs (11). In the absence of AsmA, a more permissive environment would thus permit mutant OMP assembly (11). For Salmonella enterica, the only AsmA reference in the literature is a recent study that found asmA among the genes required for survival of Salmonella enterica serovar Typhimurium in the swine gastric environment (4).…”

“…The idea of involvement of AsmA in the assembly of wild-type OMPs was, however, discarded (30). An interpretation was that the presence of AsmA might create an environment refractory to the assembly of mutant (misfolded) OMPs (11). In the absence of AsmA, a more permissive environment would thus permit mutant OMP assembly (11).…”

Inner-membrane protein MorC is involved in fimbriae production and biofilm formation in Aggregatibacter actinomycetemcomitans

“…The direct model relies on the high sequence homology that the C terminus of MorC shares with AsmA, an E. coli inner-membrane protein involved in the insertion of proteins into the outer membrane (Deng & Misra, 1996). TamB, the Enterobacteriaceae orthologue of MorC, is proposed to interact with Tam A (Omp67), which functions as a lever arm to drive proteins into bacterial outer membranes (Selkrig et al, 2015).…”