NMR structural characterization of the N-terminal domain of the adenylyl cyclase-associated protein (CAP) from Dictyostelium discoideum.

Cyclase-associated proteins (CAPs) are highly conserved, ubiquitous actin binding proteins that are involved in microfilament reorganization. The N-termini of CAPs play a role in Ras signaling and bind adenylyl cyclase; the C-termini bind to G-actin. We report here the NMR characterization of the amino-terminal domain of CAP from Dictyostelium discoideum (CAP(1-226)). NMR data, including the steady state (1)H-(15)N heteronuclear NOE experiments, indicate that the first 50 N-terminal residues are unstructured and that this highly flexible serine-rich fragment is followed by a stable, folded core starting at Ser 51. The NMR structure of the folded core is an alpha-helix bundle composed of six antiparallel helices, in a stark contrast to the recently determined CAP C-terminal domain structure, which is solely built by beta-strands.

Structure of the N-terminal domain of the adenylyl cyclase-associated protein (CAP) from Dictyostelium discoideum.

Cyclase-associated proteins (CAPs) are widely distributed and highly conserved proteins that regulate actin remodeling in response to cellular signals. The N termini of CAPs play a role in Ras signaling and bind adenylyl cyclase; the C termini bind to G-actin and thereby alter the dynamic rearrangements of the microfilament system. We report here the X-ray structure of the core of the N-terminal domain of the CAP from Dictyostelium discoideum, which comprises residues 51-226, determined by a combination of single isomorphous replacement with anomalous scattering (SIRAS). The overall structure of this fragment is an alpha helix bundle composed of six antiparallel helices. Results from gel filtration and crosslinking experiments for CAP(1-226), CAP(255-464), and the full-length protein, together with the CAP N-terminal domain structure and the recently determined CAP C-terminal domain structure, provide evidence that the functional structure of CAP is multimeric.

Backbone dynamics of green fluorescent protein and the effect of histidine 148 substitution.

Green fluorescent protein (GFP) and its mutants have become valuable tools in molecular biology. GFP has been regarded as a very stable and rigid protein with the beta-barrel shielding the chromophore from the solvent. Here, we report the 15N nuclear magnetic resonance (NMR) studies on the green fluorescent protein (GFPuv) and its mutant His148Gly. 15N NMR relaxation studies of GFPuv show that most of the beta-barrel of GFP is rigid on the picosecond to nanosecond time scale. For several regions, including the first alpha-helix and beta-sheets 3, 7, 8, and 10, increased hydrogen-deuterium exchange rates suggest a substantial conformational flexibility on the microsecond to millisecond time scales. Mutation of residue 148 located in beta-sheet 7 is known to have a strong impact on the fluorescence properties of GFPs. UV absorption and fluorescence spectra in combination with 1H-15N NMR spectra indicate that the His148Gly mutation not only reduces the absorption of the anionic chromophore state but also affects the conformational stability, leading to the appearance of doubled backbone amide resonances for a number of residues. This suggests the presence of two conformations in slow exchange on the NMR time scale in this mutant.

Slow exchange in the chromophore of a green fluorescent protein variant.

Green fluorescent protein and its mutants have become valuable tools in molecular biology. They also provide systems rich in photophysical and photochemical phenomena of which an understanding is important for the development of new and optimized variants of GFP. Surprisingly, not a single NMR study has been reported on GFPs until now, possibly because of their high tendency to aggregate. Here, we report the (19)F nuclear magnetic resonance (NMR) studies on mutants of the green fluorescent protein (GFP) and cyan fluorescent protein (CFP) labeled with fluorinated tryptophans that enabled the detection of slow molecular motions in these proteins. The concerted use of dynamic NMR and (19)F relaxation measurements, supported by temperature, concentration- and folding-dependent experiments provides direct evidence for the existence of a slow exchange process between two different conformational states of CFP. (19)F NMR relaxation and line shape analysis indicate that the time scale of exchange between these states is in the range of 1.2-1.4 ms. Thermodynamic analysis revealed a difference in enthalpy (Delta)H(0) = (18.2 +/- 3.8) kJ/mol and entropy T(Delta)S(0) = (19.6 +/- 1.2) kJ/mol at T = 303 K for the two states involved in the exchange process, indicating an entropy-enthalpy compensation. The free energy of activation was estimated to be approximately 60 kJ/mol. Exchange between two conformations, either of the chromophore itself or more likely of the closely related histidine 148, is suggested to be the structural process underlying the conformational mobility of GFPs. The possibility to generate a series of single-atom exchanges ("atomic mutations") like H --> F in this study offers a useful approach for characterizing and quantifying dynamic processes in proteins by NMR.