External Scaffolding Proteins

The ( X174 external scaffolding protein (protein D) performs many of the functions typically associated with internal species in one-scaffolding-protein systems the organization of assembly precursors into a procapsid and the stabilization of that structure. However, its function is physically and temporally dependent on the internal scaffolding protein, which induces the conformational changes in capsid pentamers to prevent their premature association. In the procapsid crystal structure, 20 D proteins are associated with each pentameric capsomer. Remarkably, there is little or no contact between capsid pentamers. The structure is primarily held together by 2-fold-related contacts between D proteins. 

In addition to Gly-61, individual subunit structures are influenced by unique sets of interactions made with the underlying coat and neighboring D proteins within and across asymmetric units. For example, o-helix 5 forms (3 structure in subunit D2, where it participates in interdimer contacts with D3, and forms helical structure in D3, where it participates in D4 intradimer contacts. In subunit D4, it mediates scaffolding contacts across the 2-fold axis symmetry and forms loop structure. o-Helix 7 forms only in the D4 subunit, where it mediates the most extensive coat protein interactions found in the entire lattice. 

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Perhaps one of the most intriguing questions of triplex function is the role of Vp23. Before its association with Vpl9C, the protein exists as a partially folded molten globule (Kirkitadze et at, 1998). And its structural variability in the 8.5-A image reconstruction of the B capsid is truly noteworthy (Zhou et at, 2000). Presumably the alternate structures of the protein in the B capsid and presumed transient structures in the procapsid are influenced by its local environment, as has been observed with 0X174 external scaffolding protein (Dokland et at, 1997, 1999). 

X174 assembly pathway, schematic of, 239 ()[Pg.543]

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