Biochemical and preliminary crystallographic characterization of the vitamin D sterol- and actin-binding by human vitamin D-binding protein

Arch Biochem Biophys. 2002 Jun 1;402(1):14-23. doi: 10.1016/S0003-9861(02)00033-4.


Vitamin D-binding protein (DBP), a multi-functional serum glycoprotein, has a triple-domain modular structure. Mutation of Trp145 (in Domain I) to Ser decreased 25-OH-D(3)-binding by 80%. Furthermore, recombinant Domain I (1-203) and Domain I + II (1-330) showed specific and strong binding for 25-OH-D(3), but Domain III (375-427) did not, suggesting that only Domains I and II might be required for vitamin D sterol-binding. Past studies have suggested that Domain III is independently capable of binding G-actin. We exploited this apparently independent ligand-binding property of DBP to purify DBP-actin complex from human serum and rabbit muscle actin by 25-OH-D(3) affinity chromatography. Competitive (3)H-25-OH-D(3) binding curves for native DBP and DBP-actin complex were almost identical, further suggesting that vitamin D sterol- and actin-binding activities by DBP might be largely independent of each other. Trypsin treatment of DBP produced a prominent 25 kDa band (Domain I, minus 5 amino acids in N-terminus), while actin was completely fragmented by such treatment. In contrast, tryptic digestion of purified DBP-actin complex showed two prominent bands, 52 (DBP, minus 5 amino acids in the N-terminus) and 34 kDa (actin, starting with amino acid position 69) indicating that DBP, particularly its Domains II and III were protected from trypsin cleavage upon actin-binding. Similarly, actin, except its N-terminus, was also protected from tryptic digestion when complexed with DBP. These results provided the basis for our studies to crystallize DBP-actin complex, which produced a 2.5 A crystal, primitive orthorhombic with unit cell dimensions a=80.2A, b=87.3A, and c=159.6A, P2(1)2(1)2(1) space group, V(m)=2.9. Soaking of crystals of actin-DBP in crystallization buffer containing various concentrations of 25-OH-D(3) resulted in cracking of the crystal, which was probably a reflection of a ligand-induced conformational change in the complex, disrupting crystal contacts. In conclusion, we have provided data to suggest that although binding of 25-OH-D(3) to DBP might result in discrete conformational changes in the holo-protein to influence actin-binding, these binding processes are largely independent of each other in solution.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Actins / metabolism*
  • Amino Acid Sequence
  • Amino Acid Substitution
  • Animals
  • Binding, Competitive
  • Calcifediol / metabolism*
  • Calcitriol / metabolism*
  • Chromatography, Affinity
  • Crystallography, X-Ray
  • Humans
  • Molecular Sequence Data
  • Molecular Weight
  • Mutagenesis, Site-Directed
  • Protein Binding
  • Protein Conformation
  • Protein Structure, Secondary
  • Rabbits
  • Trypsin / metabolism
  • Vitamin D-Binding Protein / genetics
  • Vitamin D-Binding Protein / metabolism*


  • Actins
  • Vitamin D-Binding Protein
  • Trypsin
  • Calcitriol
  • Calcifediol