%0 Journal Article
%T The impact of exchanging the light and heavy chains on the structures of bovine ultralong antibodies.
%A Clarke JD
%A Douangamath A
%A Mikolajek H
%A Bonnet-Di Placido M
%A Ren J
%A Fry EE
%A Stuart DI
%A Hammond JA
%A Owens RJ
%J Acta Crystallogr F Struct Biol Commun
%V 80
%N 0
%D 2024 Jul 1
%M 38958188
%F 1.072
%R 10.1107/S2053230X2400606X
%X The third complementary-determining regions of the heavy-chain (CDR3H) variable regions (VH) of some cattle antibodies are highly extended, consisting of 48 or more residues. These `ultralong' CDR3Hs form β-ribbon stalks that protrude from the surface of the antibody with a disulfide cross-linked knob region at their apex that dominates antigen interactions over the other CDR loops. The structure of the Fab fragment of a naturally paired bovine ultralong antibody (D08), identified by single B-cell sequencing, has been determined to 1.6 Å resolution. By swapping the D08 native light chain with that of an unrelated antigen-unknown ultralong antibody, it is shown that interactions between the CDR3s of the variable domains potentially affect the fine positioning of the ultralong CDR3H; however, comparison with other crystallographic structures shows that crystalline packing is also a major contributor. It is concluded that, on balance, the exact positioning of ultralong CDR3H loops is most likely to be due to the constraints of crystal packing.