A new pentameric structure of rotavirus NSP4 revealed by molecular replacement

The region spanning residues 95-146 of the rotavirus nonstructural protein NSP4 from the asymptomatic human strain ST3 has been purified and crystallized and diffraction data have been collected to a resolution of 2.6 angstrom. Several attempts to solve the structure by the molecular-replacement method using the available tetrameric structures of this domain were unsuccessful despite a sequence identity of 73% to the already known structures. A more systematic approach with a dimer as the search model led to an unexpected pentameric structure using the program Phaser. The various steps involved in arriving at this molecular-replacement solution, which unravelled a case of subtle variation between different oligomeric states unknown at the time of solving the structure, are presented in this paper.