STRUCTURE OF CALCIUM-FREE MANNOSE-BINDING PROTEIN Kenneth K.-S. Ng, Shaun S. Snyder, William I. Weis, Department of Structural Biology, Stanford University, Stanford, CA 94305-5400

Calcium ions play key roles in the structures and biological activities of the carbohydrate-binding proteins known as C-type lectins. In the liver asialoglycoprotein receptor, for example, the reversible conformational change that accompanies calcium ion binding and release is integral to the protein's role as an endocytic carrier. Crystal structures of the calcium-bound form of several C-type lectins have previously been determined. We now report the structure of the calcium-free form of the carbohydrate-recognition domain of rat liver mannose-binding protein (MBP-C) and the one-calcium form of rat serum mannose-binding protein (MBP-A). The structures were solved by molecular replacement using the calcium-bound structures of MBP-C and MBP-A as search models. The loops which are involved in calcium-binding in the native state adopt different conformations when calcium ions are not present. For MBP-C, the four copies of the protein in the asymmetric unit reveal an additional range of loop conformations. Structural changes correlate with the kinetics of calcium-dependent changes in intrinsic tryptophan fluorescence.