Refined solution structure and backbone dynamics of 15N-labeled C12A-p8MTCP studied by NMR relaxation

MTCP1 (for Mature-T-Cell Proliferation) was the first gene unequivocally identified in the group of uncommon leukemias with a mature phenotype. The three-dimensional solution structure of the human p8^MTCP protein encoded by the MTCP1 oncogene has been previously determined by homonuclear proton two-dimensional NMR methods at 600 MHz: it consists of an original scaffold comprising three -helices, associated with a new cysteine motif. Two of the helices are covalently paired by two disulfide bridges, forming an -hairpin which resembles an antiparallel coiled-coil. The third helix is orientated roughly parallel to the plane defined by the -antiparallel motif and appears less well defined. In order to gain more insight into the details of this new scaffold, we uniformly labeled with nitrogen-15 a mutant of this protein (C12A-p8^MTCP1) in which the unbound cysteine at position 12 has been replaced by an alanine residue, thus allowing reproducibly high yields of recombinant protein. The refined structure benefits from 211 additional NOEs, extracted from ¹⁵N-edited 3D experiments, and from a nearly complete set of angular restraints allowing the estimation of the helical content of the structured part of the protein. Moreover, measurements of ¹⁵ N spin relaxation times and heteronuclear ¹⁵ N¹HNOEs provided additional insights into the dynamics of the protein backbone. The analysis of the linear correlation between J(0) and J() was used to interpret relaxation parameters. It appears that the apparent relative disorder seen in helix III is not simply due to a lack of experimental constraints, but associated with substantial contributions of sub-nanosecond motions in this segment.