The Structural Proteins of Thermophilic Bacteriophage P23-77: Expression and Characterization

P23-77 is a thermophilic bacteriophage that infects Thermus thermophilus bacteria. The genome of the virus is enclosed in an icosahedral capsid. This capsid is made of the small major capsid protein (VP16), the large major capsid protein (VP17), and the minor capsid protein (VP11). In addition to these three structural proteins, membrane-associated proteins (VP15, VP19, VP20, VP22, and VP23) have been identified in the virus and may serve as scaffold proteins to help with viral assembly. Previous studies have expressed VP11, VP16, and VP17 in E. coli. A mixture of these proteins can lead to the formation of complexes. However, the potential to express membrane-associated proteins has never been explored. Here, we demonstrated, for the first time, the expression and co-expression of some membrane-associated proteins with capsid (coat) proteins, both in the natural host and in E. coli. Co-expression of these proteins did not result in the assembly of virus-like particles. We explored further strategies to express and purify some of the proteins for future studies. We observed that the insertion of a purification tag (Strep-II tag, but not a histidine tag) significantly reduced the expression levels of some of the proteins. Six of the eight structural proteins were successfully purified to homogeneity using different approaches. We showed that VP20 and VP22 migrated on SDS PAGE gel at sizes larger than their predicted molecular weights. Predicted 3D structures of the proteins show that most of them are helical in nature with disordered regions. The work presented here will help pave the way for the expression and purification of these proteins. This will help determine their 3D structures and may shed light on the requirements for viral assembly.