Fibrillin domain folding and calcium binding: significance to Marfan syndrome.

BACKGROUND: Marfan syndrome is a heritable disorder of connective tissue which has been associated with mutations in a gene encoding fibrillin, a 350 kD glycoprotein found in microfibrils. This protein consists of approximately 60 domains, 47 of which have similarity to epidermal growth factor (EGF). The first mutations to be detected were found in two sporadic cases that had identical Arg to Pro changes within one EGF-like domain. Based on sequence features common to 43 of the EGF-like domains, it was proposed that these domains might bind calcium. Through the synthesis and characterization of wild-type and mutated single domain peptides, we examined the structural and calcium-binding properties of an isolated EGF-like domain from fibrillin and the effects of the Arg to Pro sequence change. RESULTS: A peptide corresponding to the thirteenth putative calcium-binding, EGF-like domain of fibrillin (the site of the first detected mutations) was synthesized. This peptide could be easily oxidized and refolded. The structure of this domain was probed using NMR methods, indicating features characteristic of the known structures of EGF-like domains. The domain bound to calcium with moderate affinity (Kd = 0.6 +/- 0.1 mM) with no major changes in structure induced upon calcium binding. A synthetic peptide containing the Arg to Pro mutation was found to be drastically impaired in its ability to fold in vitro. CONCLUSIONS: As predicted, a fibrillin domain forms a calcium-binding, EGF-like module. As the putative calcium-binding sites are found at the amino-terminal end of the modules, we propose that calcium ions may bind in the interfaces between domains, affecting the overall structure of the protein. The Arg to Pro mutation blocks domain folding in vitro, suggesting that lack of proper domain folding in vivo may contribute to the molecular defects responsible for Marfan syndrome.

Conformation of beta-methylmelibiose bound to the ricin B-chain as determined from transferred nuclear Overhauser effects.

Transferred nuclear Overhauser effect (TRNOE) experiments have revealed a change in the torsion angles about the alpha-1-6 glycosidic bond of methyl beta-melibioside upon binding of the melibioside to the ricin B-chain (Rb). A full relaxation rate matrix simulation of experimental buildup curves aided in quantitative interpretation of 1D selective inversion recovery TRNOE experiments. The data are consistent with a model in which both major (omega approximately 170 degrees) and minor (omega approximately -60 degrees) conformers for methyl beta-melibioside are significantly populated in solution while the Rb/methyl beta-melibioside complex has little of the minor conformer populated. The results indicate that the ricin B-chain excludes binding of certain ligand conformations on the basis of unfavorable interactions between the protein surface and remote portions of the disaccharide system.

Conformation of methyl beta-lactoside bound to the ricin B-chain: interpretation of transferred nuclear Overhauser effects facilitated by spin simulation and selective deuteration.

Spin simulation and selective deuteration have been used to aid in the interpretation of 1D transferred nuclear Overhauser effect (TRNOE) NMR experiments on ricin B-chain/ligand systems. Application of these methods has revealed a change in the conformation of deuterated methyl beta-lactoside upon binding to the ricin B-chain which results in a slight change in glycosidic torsional angels which appear to dominate in the solution conformation. The combination of simulation and experiment also shows an important sensitivity of TRNOE magnitudes to dissociation rate constants and available spin-diffusion pathways for the ricin B-chain/ligand systems under study. The sensitivity to dissociation rates allows determination of rate constants for methyl beta-lactoside and methyl beta-galactoside of 50 and 300 s-1, respectively.