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 DD3 domains. VWF multimer assembly occurs in the trans-Golgi at pH ϳ6.2 but not at pH 7.4, which suggests that protonation of one or more His residues (pK a ϳ6.0) mediates the pH dependence of multimerization. Alignment of 30 vertebrate VWF sequences identified 13 highly conserved His residues in the D1D2DD3 domains, and His-toAla 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.von Willebrand factor (VWF) 2 is a multimeric hemostatic protein that mediates platelet adhesion to sites of vascular injury. VWF is assembled from identical 350-kDa precursors consisting of multiple structural domains in the order D1-D2-