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 Å. 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.