The structure determination of Sindbis virus core protein using isomorphous replacement and molecular replacement averaging between two crystal forms

The structure of Sindbis virus core protein has been determined by a combination of multiple isomorphous replacement and molecular replacement averaging techniques. The multiple isomorphous replacement phase determinations were made for two crystal forms (P2₁ and P4₃2₁2) of the core protein. The real-space molecular replacement averaging was subsequently carded out between two copies of the protein per asymmetric unit in the monoclinic form and one copy in the tetragonal form. This greatly improved the quality of the electron density maps. The Sindbis virus core protein polypeptide could be traced and related to the known amino acid sequence. The averaging procedure between different crystal forms, as described in this paper, should be generally applicable to other systems.