Structural studies of bacteriophage α-3 assembly
Abstract
Bacteriophage α3 is a member of the Microviridae , a family of small, single-stranded, icosahedral phages that include &phis;X174. These viruses have a ssDNA genome associated with approximately 12 copies of an H pilot protein and 60 copies of a small J DNA binding protein. The surrounding capsid consists of 60 F coat proteins decorated with 12 pentameric spikes of G protein. Assembly proceeds via a 108S empty procapsid that requires the external D and internal B scaffolding proteins for its formation. The α3 “open” procapsid structural intermediate was determined to 15Å resolution by cryo-electron microscopy (cryo-EM). Unlike the &phis;X174 “closed” procapsid and the infectious virion, the α3 open procapsid has 30Å wide pores at the three-fold vertices and 20Å wide gaps between F pentamers as a result of the disordering of two helices in the F capsid protein. The large pores are probably used for DNA entry and internal scaffolding protein exit during DNA packaging. Protein B appears to have autoproteolytic activity that cleaves at a Arg-Phe motif and probably facilitates the removal of the protein through the 30Å wide pores. The structure of the α3 mature virion was solved to 3.5Å resolution by X-ray crystallography and was used to interpret the open procapsid cryo-EM structure. The main differences between the α3 and &phis;X174 virion structures are in the spike and the DNA binding proteins. The icosahedrally ordered structural component of the ssDNA appears to be substantially increased in α3 compared to &phis;X174, allowing the building of about 10% of the ribose-phosphate backbone.
Degree
Ph.D.
Advisors
Rossmann, Purdue University.
Subject Area
Biophysics
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