RESUMO
The Ca(2+) titration of the (15)N-labeled mutant V136G calmodulin has been monitored using (1)H-(15)N HSQC NMR spectra. Up to a [Ca(2+)]/[CaM] ratio of 2, the Ca(2+) ions bind predominantly to sites I and II on the N-domain in contrast with the behavior of the wild-type calmodulin where the C-terminal domain has the higher affinity for Ca(2+). Surprisingly, the Ca(2+)-binding affinity for the N-domain in the mutant calmodulin is greater than that for the N-domain in the wild-type protein. The mutated C-domain is observed as a mixture of unfolded, partially folded (site III occupied), and native-like folded (sites III and IV occupied) conformations, with relative populations dependent on the [Ca(2+)]/[CaM] ratio. The occupancy of site III independently of site IV in this mutant shows that the cooperativity of Ca(2+) binding in the C-domain is mediated by the integrity of the domain structure. Several NH signals from residues in the Ca(2+)-bound N-domain appear as two signals during the Ca(2+) titration indicating separate species in slow exchange, and it can be deduced that these result from the presence and absence of interdomain interactions in the mutant. It is proposed that an unfolded part of the mutated C-domain interacts with sites on the N-domain that normally bind to target proteins. This would also account for the increase in the Ca(2+) affinity for the N-domain in the mutant compared with the wild-type calmodulin. The results therefore show the wide-ranging effects of a point mutation in a single Ca(2+)-binding site, providing details of the involvement of individual residues in the calcium-induced folding reactions.
Assuntos
Cálcio/química , Calmodulina/química , Calmodulina/genética , Glicina/genética , Valina/genética , Substituição de Aminoácidos/genética , Animais , Sítios de Ligação/genética , Cálcio/metabolismo , Calmodulina/metabolismo , Drosophila melanogaster , Motivos EF Hand/genética , Glicina/química , Substâncias Macromoleculares , Músculo Liso/enzimologia , Quinase de Cadeia Leve de Miosina/química , Isótopos de Nitrogênio , Ressonância Magnética Nuclear Biomolecular/métodos , Conformação Proteica , Dobramento de Proteína , Estrutura Terciária de Proteína/genética , Prótons , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Soluções , Termodinâmica , Valina/químicaRESUMO
A new pH-dependent off-resonance ROESY-HSQC experiment has been used to characterize the degree of protection of the amide protons of cryptogein, a protein of the elicitin family, against solvent exchange. The study of the pH dependence of solvent-shielded amide protons in this protein reveals that the helices have different levels of stability. Two of the five helices exhibit strong protection of amide hydrogens against exchange with the solvent. By contrast, greater flexibility is observed in the other three helices, particularly in the C-terminal helix. These results provide information on the dynamic features of the protein and are consistent with the RMSD for the backbone atoms of residues involved in helical structures. In addition, the question of the flexibility in a hydrophobic cavity made of conserved residues, which represent a plausible binding site, is addressed by this method.
Assuntos
Proteínas de Algas , Proteínas Fúngicas/química , Ressonância Magnética Nuclear Biomolecular/métodos , Amidas/química , Concentração de Íons de Hidrogênio , Modelos Moleculares , Isótopos de Nitrogênio , Conformação Proteica , Estrutura Secundária de Proteína , Prótons , Solventes , Água/químicaRESUMO
Cryptogein belongs to a new family of 10-kDa proteins called elicitins. Elicitins are necrotic and signaling proteins secreted by Phytophthora spp. responsible for the incompatible reaction and systemic hypersensitive-like necroses of diverse plant species leading to resistance against fungal or bacterial plant pathogens. The solution structure of beta cryptogein from Phytophthora cryptogea fungus was determined by using multidimensional heteronuclear nuclear magnetic resonance spectroscopy. A set of 18 structures was calculated using 1360 NOE-derived distance restraints and 40 dihedral angle restraints obtained from 3JHNH alpha couplings. The RMS deviation from the mean structure is 0.87 +/- 0.14 A for backbone atoms and 1.34 +/- 0.14 A for all the non-hydrogen atoms of residues 2 to 98. The structure of beta cryptogein reveals a novel protein fold, with five helices and a double-stranded beta-sheet facing an omega-loop. One edge of the beta-sheet and the adjacent face of the omega-loop form a hydrophobic cavity. This cavity made of highly conserved residues represents a plausible binding site. Residue 13, which has been identified from directed mutagenesis and natural sequence comparison studies as a key amino acid involved in the differential control of necrosis, is surface exposed and could contribute to the binding to a ligand or a receptor. The solution structure is close to the X-ray structure, with slight differences lightly due to the crystal packing.