In situ proteolysis, crystallization and preliminary X-ray diffraction analysis of a V_HH that binds listeria internalin B

The variable region of camelid heavy-chain antibodies produces the smallest known antibody fragment with antigen-binding capability (a V_HH). The V_HH R303 binds internalin B (InlB), a virulence factor expressed by the pathogen Listeria monocytogenes. InlB is critical for initiation of Listeria infection, as it binds a receptor (c-Met) on epithelial cells, triggering the entry of bacteria into host cells. InlB is surface-exposed and is required for virulence, hence a V_HH targeting InlB has potential applications for pathogen detection or therapeutic intervention. Here, the expression, purification, crystallization and X-ray diffraction of R303 are reported. Crystals of R303 were obtained following in situ proteolysis with trypsin. Gel filtration and SDS–PAGE revealed that trypsin removed the C-terminal tag region of R303, facilitating crystal formation. Crystals of R303 diffracted to 1.3 Å resolution and belonged to the monoclinic space group P2₁, with unit-cell parameters a = 46.4, b = 31.2, c = 74.8 Å, β = 93.8°. The crystals exhibited a Matthews coefficient of 1.95 ų Da⁻¹ with two molecules in the asymmetric unit.