vitamin D binding protein (DBP) is a 52Kda protein that binds monomeric actin in addition to vitamin D. The protein is 458 residues in length, and forms three domains, the first of which contains the sterol binding site. The three domains share limited sequence homology, with each other and to similar repeats in human serum albumin (HSA). The structure of DBP is also similar overall to HSA . It is presumed that the function of DBP binding actin is to clear up any actin that enters the blood stream as a result of cell injury especially crush injury to muscle. This function is also performed by gelsolin. The affinity for actin monomers is high (Kd = 10-9M) , and the actin binding site has been reported to reside within domain III between residues 350 and 403, although the recent structural data of the complex shows that the interface is more extensive than this and involves residues from all three domains. The determination of the structure of the protein confirms the domain structure derived from the biochemical work.