Bovine beta-lactoglobulin at 1.8 A resolution--still an enigmatic lipocalin.
BACKGROUND
beta-Lactoglobulin (beta-Lg) is the major whey protein in the milk of ruminants and many other mammals. Its function is not known, but it undergoes at least two pH-dependent conformational changes which may be important. Bovine beta-Lg crystallizes in several different lattices, and medium-resolution structures of orthorhombic lattice Y and trigonal lattice Z have been published. Triclinic lattice X and lattice Z crystals grow at pH values either side of the pH at which one of the pH-induced conformational changes occurs. A full understanding of the structure is needed to help explain both the conformational changes and the different denaturation behaviour of the genetic variants.


RESULTS
We have redetermined the structure of beta-Lg lattice Z at 3.0 A resolution by multiple isomorphous replacement and have partially refined it (R factor = 24.8%). Using the dimer from this lattice Z structure as a search model, the triclinic crystal form grown at pH 6.5 (lattice X) has been solved by molecular replacement. Refinement of lattice X at 1.8 A resolution gave an R factor of 18.1%. The structure we have determined differs from previously published structures in several ways.


CONCLUSIONS
Incorrect threading of the sequence in the published structures of beta-Lg affects four of the nine beta strands. The basic lipocalin fold of the polypeptide chain is unchanged, however. The relative orientation of the monomers in the beta-Lg dimer differs in the two lattices. On raising the pH, there is a rotation of approximately 5 degrees, which breaks a number of intersubunit hydrogen bonds. It is not yet clear, however, why the stability of the structure should depend so heavily upon the external loop around residue 64 or the beta strand with the free thiol, each of which shows genetic variation.
Brownlow
S
S
Leeds Centre for Molecular Recognition in Biological Systems, Department of Biochemistry and Molecular Biology, The University of Leeds, Leeds, LS2 9JT, UK.
Morais Cabral
J H
JH
Cooper
R
R
Flower
D R
DR
Yewdall
S J
SJ
Polikarpov
I
I
North
A C
AC
Sawyer
L
L
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D003198:Computer Simulation; D018360:Crystallography, X-Ray / Q000379:methods; D019281:Dimerization; D006860:Hydrogen Bonding; D007782:Lactoglobulins / Q000737:chemistry*; D008024:Ligands; D046911:Macromolecular Substances; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017386:Sequence Homology, Amino Acid
D007782:Lactoglobulins; D008024:Ligands; D046911:Macromolecular Substances