Sequence specific resonance assignment is the first step in determining the three-dimensional (3D) structure of proteins. Several double and triple-resonance NMR experiments were proposed in this direction for achieving unambiguous assignments. However, these methodologies are non-trivial for proteins with molecular weight beyond 15 kDa. Common problems in this process are spectral overlaps and rapid relaxation rates of the nuclei that result in broad cross peaks. Further, Pro residues, which lack 1 H N , complicate the scenario. This led to identify maximum possible starting points along the polypeptide chain of a given protein to aid in the sequence-specific resonance assignment procedure. In this paper, we propose a fast NMR methodology namely Ile, Thr and Val (ITV) specific (3, 2) D-CB(CACO)NNH, which shows the spectral signatures of the C-terminal sequential neighbors of Ile, Thr and Val residues in highly resolved manner. This aids in the generation of additional starting points along a given polypeptide chain, apart from the known canonical Ala, Gly, Ser and Thr residues, and thus accelerate resonance assignment process significantly.
IntroductionAdvent of labeling schemes to produce 13 C-or/and 15 N-enriched proteins paved the way to determine their 3D structures of medium size proteins (upto a M r = 50 kDa) by 3D triple resonance NMR. In this endeavor, 1 H, 13 C and 15 N sequence-specific resonance assignments remain the first step. Till date, several double-and triple-resonance experiments were proposed to carry out these assignments in isotope-labeled proteins. However, these methodologies are non-trivial for proteins with molecular weights even beyond 15 kDa. Common problems in this process are spectral overlaps and rapid relaxation rates of the nuclei that result in broad spectral signatures. Further, Pro residues, which lack 1 H N , complicate the situation. Hence maximum possible number of starting points is required along the polypeptide chain of a given protein to facilitate the sequence-specific resonance assignment procedure.It is a well-known fact that, spectral signatures of Ala, Gly, Ser and Thr residues are easily identifiable residues because of their characteristic 13 C α and 13 C β chemical shifts. The 13 C α chemical shifts of Gly resonate around 45.4±1.8 ppm, which is well separated from the 13 C α chemical shifts of the rest of the residues. On the other hand, the 13 (Atreya et al., 2000;Atreya et al., 2002). Thus, in principle, four tripleresonance experiments HNCACB, CBCA(CO)NH, HNCO and HN(CA)CO, should suffice for the resonance assignment of 1 H N , 15 N, 13 C α , 13 C β and 13 C' spins (Atreya et al., 2000;Atreya et al., 2002). However, in practice, the success rate in any automated resonance assignment procedure turns out to be low, primarily because of incomplete spectral signatures or/and spectral overlap. Hence, the quest has been to identify spectral signatures arising from residues other than Ala, Ser, Thr and Gly. Based on the analysis of BMRB data of 9000 odd protein...