Genes for the Major Structural Components of Thermotogales Species’ Togas Revealed by Proteomic and Evolutionary Analyses of OmpA and OmpB Homologs

Abstract

The unifying structural characteristic of members of the bacterial order Thermotogales is their toga, an unusual cell envelope that includes a loose-fitting sheath around each cell. Only two toga-associated structural proteins have been purified and characterized in Thermotoga maritima : the anchor protein OmpA1 (or Ompα) and the porin OmpB (or Ompβ). The gene encoding OmpA1 ( ompA1 ) was cloned and sequenced and later assigned to TM0477 in the genome sequence, but because no peptide sequence was available for OmpB, its gene ( ompB ) was not annotated. We identified six porin candidates in the genome sequence of T. maritima . Of these candidates, only one, encoded by TM0476, has all the characteristics reported for OmpB and characteristics expected of a porin including predominant β-sheet structure, a carboxy terminus porin anchoring motif, and a porin-specific amino acid composition. We highly enriched a toga fraction of cells for OmpB by sucrose gradient centrifugation and hydroxyapatite chromatography and analyzed it by LC/MS/MS. We found that the only porin candidate that it contained was the TM0476 product. This cell fraction also had β-sheet character as determined by circular dichroism, consistent with its enrichment for OmpB. We conclude that TM0476 encodes OmpB. A phylogenetic analysis of OmpB found orthologs encoded in syntenic locations in the genomes of all but two Thermotogales species. Those without orthologs have putative isofunctional genes in their place. Phylogenetic analyses of OmpA1 revealed that each species of the Thermotogales has one or two OmpA homologs. T. maritima has two OmpA homologs, encoded by ompA1 (TM0477) and ompA2 (TM1729), both of which were found in the toga protein-enriched cell extracts. These annotations of the genes encoding toga structural proteins will guide future examinations of the structure and function of this unusual lineage-defining cell sheath.

‘Ménage à trois’: a selfish genetic element uses a virus to propagate within Thermotogales

“…In particular, the mechanisms of MPV1 adsorption and extrusion through the ‘toga’, an unusual cell envelope forming a loose sheet envelope around cells, remain unknown. The toga is mainly composed of two proteins, the anchor protein OmpA1 and the porin OmpB (Petrus et al ., ). Porins are known to serve as viral receptors: In E. coli , for example, T4 viral particles use the OmpC porin as a receptor (Hashemolhosseini et al ., ; Marti et al ., ).…”

L-form bacteria, cell walls and the origins of life

“…However, as shown in figure 1 a , these organisms lack an lpxC gene and therefore can probably not synthesize LipidA. Nevertheless, the toga was recently shown to contain classical Omp-like proteins [40], consistent with these organisms having a diderm ancestry. It seems likely that further detailed studies of the cell envelopes of deeply rooted bacterial groups will provide important insights into the early evolutionary history of the bacteria.…”

One or two membranes? Diderm Firmicutes challenge the Gram‐positive/Gram‐negative divide

“…OmpA1 remains associated to the OM through porin OmpB in the portion of the toga that it is detached from the inner envelope. OmpB is absent in E. coli (Petrus et al, ; Ranjit & Noll, ). In agreement with the lack of LPS, no related genes are present in currently available genomes of Thermotogae.…”

“…This atypical outer cell envelope differs from the classical outer cell membrane of Gram-negative bacteria as it lacks LPS (Raetz & Whitfield, 2002). OmpA1, an alpha-helical protein, serves to anchor the toga to the peptidoglycan (Petrus et al, 2012;Ranjit & Noll, 2016). It plays a similar role as the E. coli OmpA porin, but they are not evolutionary or structurally related.…”