Identification of two distinct cell binding sequences in the vitamin D binding protein

The vitamin D binding protein (DBP) is a multifunctional, albumin-like plasma protein that often requires cell surface binding to mediate some of its diverse functions. DBP binds to several different molecules on the external face of the plasma membrane indicating that it may possess distinct cell binding sequences. In this report, surface plasmon resonance was utilized to evaluate the relative binding of the human myeloid cell line U937 to immobilized recombinant expressed DBP in order to identify cell localization sequences. U937 cells showed robust binding to immobilized native DBP, but essentially no interaction when sensor chips were coated with β2-microglobulin or BSA. The cell–DBP interaction was completely eliminated if cells were pretreated with soluble DBP. Recombinant DBP domains and truncated domains were next evaluated to determine the location of cell binding regions. Domains I (amino acids 1–191) and III (379–458), but not domain II (192–378), could support cell binding. Further evaluation of domain I, using truncated proteins and overlapping peptides, demonstrated that a single amino acid sequence, residues 150–172 (NYGQAPLSLLVSYTKSYLSMVGS), mediated cell binding. The domain III cell binding region was investigated using truncated versions of domain III fused to full-length domain II that served as a scaffold. These experiments indicated that the cell binding sequence is located in the first portion of that domain (379–402: ELSSFIDKGQELCADYSENTFTEY). Overlapping peptides spanning this sequence could partially block cell binding only when used in combination. We conclude that DBP contains two cell localization sequences that may be required for some of the multiple functions of this protein.