The role of scaffolding proteins in the assembly of the small, single-stranded DNA virus phi X174

An empty precursor particle called the procapsid is formed during assembly of the single-stranded DNA bacteriophage phi X174. Assembly of the phi X174 procapsid requires the presence of the two scaffolding proteins, D and B, which are structural components of the procapsid, but are not found in the mature virion. The X-ray crystallographic structure of a "closed" procapsid particle has been determined to 3.5 A resolution. This structure has an ex ternal scaffold made from 240 copies of protein D, 60 copies of the interna lly located B protein, and contains 60 copies of each of the viral structur al proteins F and G, which comprise the shell and the 5-fold spikes, respec tively. The F capsid protein has a similar conformation to that seen in the mature virion, and differs from the previously determined 25 Angstrom reso lution electron microscopic reconstruction of the "open" procapsid, in whic h the F protein has a different conformation. The D scaffolding protein has a predominantly a-helical fold and displays remarkable conformational vari ability. We report here an improved and refined structure of the closed pro capsid and describe in some detail the differences between the four indepen dent D scaffolding proteins per icosahedral asymmetric unit, as well as the ir interaction with the F capsid protein. We re-analyze and correct the com parison of the closed procapsid with the previously determined cryo-electro n microscopic image reconstruction of the open procapsid and discuss the ma jor structural rearrangements that must occur during assembly. A model is p roposed in which the D proteins direct the assembly process by sequential b inding and conformational switching. (C) 1999 Academic Press.