9.B.121 The AsmA (AsmA) Family
AsmA mutations were isolated as extragenic suppressors of an OmpF assembly mutant, OmpF315 in E. coli (Deng and Misra, 1996). This suppressor locus encoded a protein that was present in low levels and could only be visualized by Western blotting in cells where AsmA was overproduced. AsmA localized with the inner membrane, but the mutant OmpF assembly step influenced by AsmA occurred in the outer membrane. AsmA null mutations reduced 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 indicated a possible role of AsmA in LPS biogenesis (Deng and Misra, 1996).
Bacterial AsmA-like proteins appear to bridge the gap in intermembrane phospholipid transport (Kumar and Ruiz 2023). In eukaryotic cells, nonvesicular lipid transport between organelles is mediated by lipid-transfer proteins. A class of these lipid transporters has been described to facilitate the bulk of inter-organelle lipid transport at contact sites by forming bridge-like structures with a hydrophobic groove through which lipids travel. Because their predicted structures are composed of repeating β-groove (RBG) domains, they have been named the RBG protein superfamily. Early studies on RBG proteins VPS13 and ATG2 recognized the resemblance of their predicted structures to that of the bacterial Lpt system, which transports newly synthesized lipopolysaccharides (LPS) between the inner and the outer membranes (IMs and OMs) of Gram-negative bacteria. In these didermic bacteria, the IMs and OMs are separated by an aqueous periplasmic compartment that is traversed by a bridge-like structure built with β-jelly roll domains from several Lpt proteins that provides a hydrophobic groove for LPS molecules to travel across the periplasm. Despite structural and functional similarities between RBG proteins and the Lpt system, the bacterial AsmA-like protein family has recently emerged as the likely ancestor of RBG proteins and long sought-after transporters that facilitate the transfer of phospholipids from the IM to the OM. Kumar and Ruiz 2023 reviewed the structures and functions of bacterial AsmA-like proteins, mainly focusing on studies that have led to the proposal that AsmA-like proteins mediate the bulk of phospholipid transfer between the IMs and OMs.
Gram-negative bacteria display either a flat or an irregular outer
membrane. The periodontal pathogen Aggregatibacter (Actinobacillus)
actinomycetemcomitans has an irregular outer membrane. Gallant et al. 2008 have
identified a gene that is associated with the biogenesis of this
morphology. The gene is part of a three-gene operon and codes for a
141-kDa protein designated morphogenesis protein C (MorC), which is
conserved in several gram-negative bacteria including Haemophilus
influenzae and Pasteurella multocida. Insertional inactivation of this
gene resulted in the conversion of an irregularly shaped membrane to a
flat membrane. Associated with this morphological change were the
autoaggregation of the bacteria during planktonic growth and a
concomitant increase in the surface hydrophobicity of the bacterium.
The absence of MorC also resulted in the loss of the secretion of
leukotoxin but not of ltxA transcription. MorC apparently is critical for membrane morphology and leukotoxin secretion in A.
actinomycetemcomitans (Gallant et al. 2008).
References:
AsmA (686aas; 2 TMSs at the N- and C-termini)
Bacteria
AsmA of E. coli (P37645)
Putative lipopolysaccharide biogenesis periplasmic protein
Bacteria
LPS biogenesis protein of Gluconaceobacter xylinus (G2I0S0)
Uncharacterized protein, YhjG (675aas; 2 TMSs)
Bacteria
YhjG of Salmonella enterica (G5LV43)
Putative Diguanylate cyclase (1112aas; 2 TMSs)
Bacteria
Diguanylate cyclase of Riftia pachyptila (G2D9D8)
DUF3971 domain-containing protein of 1278 aas and 2 TMSs, N- and C-terminal.
DUF3971 protein of Rahnella aquatilis
AsmA2 domain-containing protein YhdP of 1272 aas and 2 TMSs, N- and C-terminal.
YhdP of Pantoea rodasii
YhdP (YhdQ, YhdR) of 1266 aas and 2 TMSs, one N-terminal and one C-terminal. This protein is a member of the AsmA-like protein family (Kumar and Ruiz 2023). This protein is one of six paralogs.
YhdP of E. coli
AsmA family protein, YicH, of 569 aas and 1 N-terminal TMS.
AsmA family protein of E. coli
AsmA family protein of 631 aas and 1 N-terminal TMS.
AsmA protein of Vibrio breoganii
AsmA family protein of 652 aas and 1 N-terminal TMS.
AsmA family protein of Aliivibrio fischeri (Vibrio fischeri)
AsmA family protein of 614 aas and 1 N-terminal TMS.
AsmA family protein of Salinivibrio kushneri
YdbH protein of 879 aas and one N-terminal TMS. It may be a AsmA-like protein functioning in phospholipid transport between the two membranes of a Gram-negative bacterium.
YdbH of E. coli
YdbH family protein of 903 aas and 1 N-terminal TMS.
YdbH of Mannheimia granulomatis
Uncharacterized protein of 506 aas and 1 N-terminal TMS.
UP of Serratia fonticola
The AsmA protein of 617aas with 1 N-terminal α-TMS and 20 predicted β-strands. Associates with the inner and the outer membrane, probably spanning the envelope. It influences OMP and LPS insertion, but it is not know if these effects are direct or indirect (Deng and Misra 1996). In Salmonella species, AsmA (Q8Z5G2) activates expression of the marRAB operon and attenuates virulence with poor epithelial invasion (Prieto et al. 2009).
AsmA of E. coli
AsmA of 618 aas with 1 N-terminal α-TMS and 23 predicted transmembrane β-strands, presumably in a β-barrel. Probably spans the inner and outer membranes. In Salmonella species, AsmA (Q8Z5G2) activates expression of the marRAB operon and attenuates virulence with poor epithelial invasion (Prieto et al. 2009).
AsmA of Salmonella typhi
AsmA family protein of 611 aas and 1 N-terminal TMS. It is encoded at a locus identified as the suppressor of OmpF assembly mutants of E. coli K-12 (Misra and Miao 1995).
AsmA protein of Shewanella algae
AsmA family protein of 809 aas with 2 N-terminal TMSs.
AsmA protein of Hylemonella sp.
AsmA family protein of 906 aas and 1 N-terminal TMS.
AsmA protein of Providencia rettgeri
AsmA family protein of 758 aas and 1 or 2 TMSs.
AsmA protein of Candidatus Kentron sp. MB
The Morphogenesis protein, MorC (Gallant et al., 2008).
Bacteria
MorC of Aggregatibacter actinomycetemcomitans (Q4JI69)
AsmA family protein of 543 aas and 1 N-terminal TMS.
AsmA protein of Acidobacteriia bacterium
Chloroplastic TIC236 protein, also called EMBRYO DEFECTIVE 2410. It is of 2166 aas with three hydrophobic peaks at about residues 110, 1510 and 1925 that could be TMSs.
TIC236 of Arabidopsis thaliana (Mouse-ear cress)
Inner membrane autotransporter assembly factor TamB (YtfN). Part of the translocation and assembly module (TAM) autotransporter assembly complex, which functions together with TamA and Omp85, both in the outer membrane (TC#1.B.33) to translocate autotransporters across the outer membrane (Selkrig et al. 2015). TamB has a signal-anchor linkage to the inner membrane, beta-helical structure, conserved domain architecture and a C-terminal region that mimics outer membrane protein beta-strands (Heinz et al. 2015). It may function in folding of the passenger domain while BamA-TamAB catalyze membrane β-domain insertion.
TamB of E. coli
Uncharacterized protein of 1471 aas with a single N-terminal TMS.
UP of Acidovorax avenae
Uncharacterized protein of 1322 aas and 1 N-terminal TMS.
UP of Bdellovibrio bacteriovorus
TamB of 1578 aas. Functions with TamA (TC# 1.B.33.2.4).
TamB of Sagittula stellata
TamB homologue of 1369 aas
TamB of Koribacter versatilis
TamB homologue; DUF490 family member of 1726 aas
TamB of Salinibacter ruber
Uncharacterized DUF490 domain containing protein of 1461 aas.
DUF490 protein of Dialister invisus
TamB homologue of 1684 aas
TamB of Chthonomonas calidirosea