2 ANGSTROM RESOLUTION STRUCTURE OF DPPA, A PERIPLASMIC DIPEPTIDE TRANSPORT CHEMOSENSORY RECEPTOR

The family of about 50 periplasmic binding proteins, which exhibit div erse specificity (e.g., carbohydrates, amino acids, dipeptides, oligop eptides, oxyanions, metals, and vitamins) and range in size from 20 to 58 kDa, is a gold mine for an atomic-level investigation of structure and molecular recognition. These proteins serve as initial receptors for active transport systems or permeases. About six of these proteins , including the dipeptide-binding protein (DppA), are also primary rec eptors for chemotaxis. The structure of the unbound form of DppA (M(r) = 57 400) has been determined and refined to an R-factor of 0.169 to 2 Angstrom resolution. DppA consists of two distinct domains (I and II ) connected by two ''hinge'' segments which form part of the base of t he wide groove between the two domains. The relative orientation of th e two domains gives the protein a pearlike shape, with domain I and do main II forming the larger and smaller apical ends, respectively. From the tip to the rounded bottom measures about 85 Angstrom, and the wid est diameter is about 60 Angstrom. Domain I, which consists of two int egrated subdomains, is folded from two separate polypeptide segments f rom the amino- and carboxyl-terminal ends. The more compact domain II. is formed from the intervening segment. Comparison of the dipeptide-b inding protein structure with that of the bound form of the similar ol igopeptide-binding protein [Tame, J.R.H., Murshudov, C.N., Dodson, E.J ., Neil, T.K., Dodson, G.G., Higgins, C.F., & Wilkinson, A.J. (1994) S cience 264, 1578-1581] reveals the major features that differentiate t he ligand specificity of the two proteins and describe the large hinge bending (about 55 degrees) between the two domains.