Phylogenetic and Functional Analysis of Histidine Residues Essential for pH-dependent Multimerization of von Willebrand Factor

von Willebrand factor (VWF) is a multimeric plasma protein that mediates platelet adhesion to sites of vascular injury. The hemostatic function of VWF depends upon the formation of disulfide-linked multimers, which requires the VWF propeptide (D1D2 domains) and adjacent D'D3 domains. VWF multimer assembly occurs in the trans-Golgi at pH similar to 6.2 but not at pH 7.4, which suggests that protonation of one or more His residues (pK(a) similar to 6.0) mediates the pH dependence of multimerization. Alignment of 30 vertebrate VWF sequences identified 13 highly conserved His residues in the D1D2D'D3 domains, and His-to-Ala mutagenesis identified His(395) and His(460) in the D2 domain as critical for VWF multimerization. Replacement of His(395) with Lys or Arg prevented multimer assembly, suggesting that reversible protonation of this His residue is essential. In contrast, replacement of His(460) with Lys or Arg preserved normal multimer assembly, whereas Leu, Met, and Gln did not, indicating that the function of His(460) depends primarily upon the presence of a positive charge. These results suggest that pH sensing by evolutionarily conserved His residues facilitates the assembly and packaging of VWF multimers upon arrival in the trans-Golgi.