Complete assignment of 1H, 13C and 15N chemical shifts for bovine beta-lactoglobulin: secondary structure and topology of the native state is retained in a partially unfolded form.

Abstract

Although beta-lactoglobulin (beta-LG) has been studied extensively for more than 50 years, its physical properties in solution are not yet understood fully in terms of its three-dimensional (3D) structure. For example, despite a recent high-resolution crystal structure, it is still not clear why the two common variants of bovine beta-LG which differ by just two residues have different aggregation properties during milk processing. We have conducted solution-state NMR studies on a recombinant form of the A variant of beta-LG at low pH conditions where the protein is partially unfolded and exists as a monomer rather than a dimer. Using a 13C, 15N-labelled sample, expressed in Pichia pastoris, we have employed the standard combination of 3D heteronuclear NMR techniques to obtain near complete assignments of proton, carbon and nitrogen resonances. Using a novel pulse sequence we were able to obtain additional assignments, in particular those of methyl groups in residues preceding proline within the sequence. From chemical shifts and on the basis of inter-residue NOEs, we have inferred the secondary structure and topology of monomeric beta-LG A. It includes eight antiparallel beta-strands arranged in a barrel, flanked by an alpha-helix, which is typical of a member of the lipocalin family. A detailed comparison with the crystal structure of the dimeric form (for a mixture of A and B variants) at pH 6.5 reveals a close resemblance in both secondary structure and overall topology. Both forms have a ninth beta-strand which, at the higher pH, forms part of the dimer interface. These studies represent the first full NMR assignment of beta-LG and will form the basis for a complete characterisation of the solution structure and dynamics of this protein and its variants.