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bioRxiv · 10.1101/2022.02.11.480155

Bacterial outer-membrane polysaccharide export (OPX) proteins occupy three structural classes with selective β-barrel porin requirements for polymer secretion

Saïdi, F.·Mahanta, U.·Panda, A.·Jolivet, N. Y.·Bitazar, R.·John, G.·Martinez, M.·Mellouk, A.·Calmettes, C.·Chang, Y.-W.·Sharma, G.·Islam, S. T.
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Abstract

Secretion of high-molecular-weight polysaccharides across the bacterial envelope is ubiquitous as it enhances prokaryotic survival in (a)biotic settings. Such polymers are often assembled by Wzx/Wzy- or ABC transporter-dependent schemes that implicate outer-membrane (OM) polysaccharide export (OPX) proteins in polymer translocation to the cell surface. In the social predatory bacterium Myxococcus xanthus, exopolysaccharide (EPS)-pathway WzaX, major spore coat (MASC)-pathway WzaS, and biosurfactant polysaccharide-pathway WzaB were herein found to be truncated OPX homologues of Escherichia coli Wza lacking OM-spanning -helices. Comparative genomics across all bacteria, complemented with cryo-electron tomography cell- envelope analyses, revealed WzaX/S/B architecture to be the most common amongst three defined OPX-protein structural classes independent of periplasmic thickness. Fold-recognition and deep- learning analyses revealed the conserved M. xanthus proteins MXAN_7418/3226/1916 (encoded adjacent to WzaX/S/B) to be integral OM {beta}-barrels, with structural homology to the poly-N-acetyl-D- glucosamine synthase-dependent pathway porin PgaA. Such porins were identified in bacteria near numerous genes for all three OPX-protein classes. Interior MXAN_7418/3226/1916 {beta}-barrel electrostatics were found to match known properties of their associated polymers. With MXAN_3226 essential for MASC export, and MXAN_7418 absence shown herein to compromise EPS translocation, these data support a novel secretion paradigm for Wzx/Wzy-dependent pathways in which those containing an OPX component that cannot span the OM instead utilize a {beta}-barrel porin to mediate polysaccharide transport across the OM.

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Saïdi, F., Mahanta, U., Panda, A., Jolivet, N. Y., Bitazar, R., John, G., Martinez, M., Mellouk, A., Calmettes, C., Chang, Y.-W., Sharma, G., Islam, S. T.. 2022-02-12. Bacterial outer-membrane polysaccharide export (OPX) proteins occupy three structural classes with selective β-barrel porin requirements for polymer secretion. https://doi.org/10.1101/2022.02.11.480155

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