Fc receptors play a major role in immune defenses against pathogens and in inflammatory processes. The crystal structure of a human immunoglobulin receptor, FcgammaRIIIb, has been determined to 1.8 A resolution. The overall fold consists of two immunoglobulin-like domains with an acute interdomain hinge angle of approximately 50 degrees. Trp-113, wedged between the N-terminal D1 and the C-terminal D2 domains, appears to further restrict the hinge angle. The putative Fc binding region of the receptor carries a net positive charge complementary to the negative-charged receptor binding regions on Fc. A 1:1 binding stoichiometry between the receptor and Fc was measured by both the equilibrium and nonequilibrium size-exclusion chromatography. Two separate parallel dimers are observed in the crystal lattice, offering intriguing models for receptor aggregation.

Transforming growth factor beta (TGF-beta) is involved in a wide range of biological functions including development, carcinogenesis, and immune regulation. Here we report the 1.1 A resolution crystal structure of human TGF-beta type II receptor ectodomain (TBRII). The overall structure of TBRII is similar to that of activin type II receptor ectodomain (ActRII) and bone morphogenic protein receptor type IA (BRIA). It displays a three-finger toxin fold with fingers formed by the beta strand pairs beta1-beta2, beta3-beta4, and beta5-beta6. The first finger in the TBRII is significantly longer than in ActRII and BRIA and folds tightly between the second finger and the C terminus. Surface charge distributions and hydrophobic patches predict potential TBRII binding sites.

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Three constructs (residues 15-136, 22-136 and 27-136) of the truncated extracellular domain of human transforming growth factor beta type II receptor (TBRII) were overexpressed in Escherichia coli. The constructs are referred to as TBRII(15-136), TBRII(22-136) and TBRII(27-136). The refolded receptors were purified using a combination of ion-exchange and size-exclusion chromatography. The purified receptors have an apparent molecular weight of 14 kDa as judged by size-exclusion chromatography. In the crystallization trials, TBRII(15-136) and TBRII(22-136) formed mostly crystal-like spheres but failed to produce data-quality crystals. TBRII(27-136) yielded large single crystals from hanging drops using the vapor-diffusion procedure with PEG 2000 or 4000 at pH 5.0. The crystals diffracted to 1.05 A [using the X9B beamline operated at lambda = 1.0092 A of the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory] and belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 35.5, b = 40.7, c = 76.2 A. There was one molecule in the asymmetric unit, which corresponds to a solvent content of 42.1%.

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