An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>

ABSTRACT The outermost exosporium layer of spores of Bacillus anthracis, the causative agent of anthrax, is comprised of a basal layer and an external hairlike nap. The nap includes filaments composed of trimers of the collagenlike glycoprotein BclA. Essentially all BclA trimers are tightly attached...

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Autores principales:	Li Tan, Mei Li, Charles L. Turnbough
Formato:	article
Lenguaje:	EN
Publicado:	American Society for Microbiology 2011
Materias:	Microbiology QR1-502
Acceso en línea:	https://doaj.org/article/adff1e65a9db44f2a39831afbdbfea1d
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spelling	oai:doaj.org-article:adff1e65a9db44f2a39831afbdbfea1d2021-11-15T15:38:49ZAn Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>10.1128/mBio.00084-112150-7511https://doaj.org/article/adff1e65a9db44f2a39831afbdbfea1d2011-07-01T00:00:00Zhttps://journals.asm.org/doi/10.1128/mBio.00084-11https://doaj.org/toc/2150-7511ABSTRACT The outermost exosporium layer of spores of Bacillus anthracis, the causative agent of anthrax, is comprised of a basal layer and an external hairlike nap. The nap includes filaments composed of trimers of the collagenlike glycoprotein BclA. Essentially all BclA trimers are tightly attached to the spore in a process requiring the basal layer protein BxpB (also called ExsFA). Both BclA and BxpB are incorporated into stable, high-molecular-mass complexes, suggesting that BclA is attached directly to BxpB. The 38-residue amino-terminal domain of BclA, which is normally proteolytically cleaved between residues 19 and 20, is necessary and sufficient for basal layer attachment. In this study, we demonstrate that BclA attachment occurs through the formation of isopeptide bonds between the free amino group of BclA residue A20 and a side chain carboxyl group of an acidic residue of BxpB. Ten of the 13 acidic residues of BxpB can participate in isopeptide bond formation, and at least three BclA polypeptide chains can be attached to a single molecule of BxpB. We also demonstrate that similar cross-linking occurs in vitro between purified recombinant BclA and BxpB, indicating that the reaction is spontaneous. The mechanism of BclA attachment, specifically, the formation of a reactive amino group by proteolytic cleavage and the promiscuous selection of side chain carboxyl groups of internal acidic residues, appears to be different from other known mechanisms for protein cross-linking through isopeptide bonds. Analogous mechanisms appear to be involved in the cross-linking of other spore proteins and could be found in unrelated organisms. IMPORTANCE Isopeptide bonds are protein modifications found throughout nature in which amide linkages are formed between functional groups of two amino acids, with at least one of the functional groups provided by an amino acid side chain. Isopeptide bonds generate cross-links within and between proteins that are necessary for proper protein structure and function. In this study, we discovered that BclA, the dominant structural protein of the external nap of Bacillus anthracis spores, is attached to the underlying exosporium basal layer protein BxpB via isopeptide bonds formed through a mechanism fundamentally different from previously described mechanisms of isopeptide bond formation. The most unusual features of this mechanism are the generation of a reactive amino group by proteolytic cleavage and promiscuous selection of acidic side chains. This mechanism, which apparently relies only on short peptide sequences in protein substrates, could be a general mechanism in vivo and adapted for protein cross-linking in vitro.Li TanMei LiCharles L. TurnboughAmerican Society for MicrobiologyarticleMicrobiologyQR1-502ENmBio, Vol 2, Iss 3 (2011)
institution	DOAJ
collection	DOAJ
language	EN
topic	Microbiology QR1-502
spellingShingle	Microbiology QR1-502 Li Tan Mei Li Charles L. Turnbough An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>
description	ABSTRACT The outermost exosporium layer of spores of Bacillus anthracis, the causative agent of anthrax, is comprised of a basal layer and an external hairlike nap. The nap includes filaments composed of trimers of the collagenlike glycoprotein BclA. Essentially all BclA trimers are tightly attached to the spore in a process requiring the basal layer protein BxpB (also called ExsFA). Both BclA and BxpB are incorporated into stable, high-molecular-mass complexes, suggesting that BclA is attached directly to BxpB. The 38-residue amino-terminal domain of BclA, which is normally proteolytically cleaved between residues 19 and 20, is necessary and sufficient for basal layer attachment. In this study, we demonstrate that BclA attachment occurs through the formation of isopeptide bonds between the free amino group of BclA residue A20 and a side chain carboxyl group of an acidic residue of BxpB. Ten of the 13 acidic residues of BxpB can participate in isopeptide bond formation, and at least three BclA polypeptide chains can be attached to a single molecule of BxpB. We also demonstrate that similar cross-linking occurs in vitro between purified recombinant BclA and BxpB, indicating that the reaction is spontaneous. The mechanism of BclA attachment, specifically, the formation of a reactive amino group by proteolytic cleavage and the promiscuous selection of side chain carboxyl groups of internal acidic residues, appears to be different from other known mechanisms for protein cross-linking through isopeptide bonds. Analogous mechanisms appear to be involved in the cross-linking of other spore proteins and could be found in unrelated organisms. IMPORTANCE Isopeptide bonds are protein modifications found throughout nature in which amide linkages are formed between functional groups of two amino acids, with at least one of the functional groups provided by an amino acid side chain. Isopeptide bonds generate cross-links within and between proteins that are necessary for proper protein structure and function. In this study, we discovered that BclA, the dominant structural protein of the external nap of Bacillus anthracis spores, is attached to the underlying exosporium basal layer protein BxpB via isopeptide bonds formed through a mechanism fundamentally different from previously described mechanisms of isopeptide bond formation. The most unusual features of this mechanism are the generation of a reactive amino group by proteolytic cleavage and promiscuous selection of acidic side chains. This mechanism, which apparently relies only on short peptide sequences in protein substrates, could be a general mechanism in vivo and adapted for protein cross-linking in vitro.
format	article
author	Li Tan Mei Li Charles L. Turnbough
author_facet	Li Tan Mei Li Charles L. Turnbough
author_sort	Li Tan
title	An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>
title_short	An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>
title_full	An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>
title_fullStr	An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>
title_full_unstemmed	An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>
title_sort	unusual mechanism of isopeptide bond formation attaches the collagenlike glycoprotein bcla to the exosporium of <named-content content-type="genus-species">bacillus anthracis</named-content>
publisher	American Society for Microbiology
publishDate	2011
url	https://doaj.org/article/adff1e65a9db44f2a39831afbdbfea1d
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An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of <named-content content-type="genus-species">Bacillus anthracis</named-content>

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