Probing protein interactions in the membrane-containing virus PRD1.

PRD1 is a Gram-negative bacteria infecting complex tailless icosahedral virus with an inner membrane. This type virus of the family Tectiviridae contains at least 18 structural protein species, of which several are membrane associated. Vertices of the PRD1 virion consist of complexes recognizing the host cell, except for one special vertex through which the genome is packaged. Despite extensive knowledge of the overall structure of the PRD1 virion and several individual proteins at the atomic level, the locations and interactions of various integral membrane proteins and membrane-associated proteins still remain a mystery. Here, we demonstrated that blue native PAGE can be used to probe protein-protein interactions in complex membrane-containing viruses. Using this technique and PRD1 as a model, we identified the known PRD1 multiprotein vertex structure composed of penton protein P31, spike protein P5, receptor-binding protein P2 and stabilizing protein P16 linking the vertex to the internal membrane. Our results also indicated that two transmembrane proteins, P7 and P14, involved in viral nucleic acid delivery, make a complex. In addition, we performed a zymogram analysis using mutant particles devoid of the special vertex that indicated that the lytic enzyme P15 of PRD1 was not part of the packaging vertex, thus contradicting previously published results.