title: Structural Basis for Recombinatorial Permissiveness in the Generation of Anaplasma marginale Msp2 Antigenic Variants.
abstract: Sequential expression of outer membrane protein antigenic variants is an evolutionarily convergent mechanism used by bacterial pathogens to escape host immune clearance and establish persistent infection. Variants must be sufficiently structurally distinct to escape existing immune effectors yet retain the core structural elements required for localization and function within the outer membrane. We examined this balance using Anaplasma marginale, which generates antigenic variants in the outer membrane protein Msp2 using gene conversion. The overwhelming majority of Msp2 variants expressed during long-term persistent infection are mosaics, derived by recombination of oligonucleotide segments from multiple alleles to form unique hypervariable regions (HVR). As a result, the mosaics are not under long-term selective pressure to encode a functional protein; consequently, we hypothesized that the Msp2 HVR is structurally permissive for mosaic expression. Using an integrated approach of predictive modeling with determination of the native Msp2 protein structure and function, we demonstrate that structured elements, most notably, β-sheets, are significantly concentrated in the highly conserved N- and C-terminal domains. In contrast, the HVR is overwhelmingly a random coil, with the structured α-helices and β-sheets being confined to the genomically defined structural tethers that separate the antigenically variable microdomains. This structure is supported by the surface exposure of the HVR microdomains and the slow diffusion-type porin function in native Msp2. Importantly, the predominance of the random coil provides plasticity for the formation of functional HVR mosaics and realization of the full potential of segmental gene conversion to dramatically expand the variant repertoire.
authors.affiliation: The Paul G. Allen School for Global Animal Health, Washington State University, Pullman, Washington, USA Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA telmo@vetmed.wsu.edu.
authors.forename: Telmo
authors.identifier: 0000-0001-7940-4283
authors.initials: T
authors.lastname: Graça
authors.affiliation: Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA.
authors.forename: Marta G
authors.initials: MG
authors.lastname: Silva
authors.affiliation: The Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington, USA.
authors.forename: Alla S
authors.initials: AS
authors.lastname: Kostyukova
authors.affiliation: The Paul G. Allen School for Global Animal Health, Washington State University, Pullman, Washington, USA Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, Washington, USA.
authors.forename: Guy H
authors.initials: GH
authors.lastname: Palmer
mesh_terms: D000595:Amino Acid Sequence; D042322:Anaplasma marginale / Q000235:genetics / Q000276:immunology* / Q000472:pathogenicity; D000712:Anaplasmosis / Q000276:immunology* / Q000382:microbiology; D000907:Antibodies, Bacterial / Q000737:chemistry* / Q000235:genetics / Q000276:immunology*; D000940:Antigenic Variation; D000942:Antigens, Bacterial / Q000737:chemistry* / Q000235:genetics / Q000276:immunology*; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000276:immunology*; D005785:Gene Conversion; D006801:Humans; D057131:Immune Evasion / Q000502:physiology*; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D000907:Antibodies, Bacterial; D000942:Antigens, Bacterial; D001425:Bacterial Outer Membrane Proteins; C103348:msp2 protein, Anaplasma marginale