| Self-assembly is a process used by many macromolecular systems to build large highly-symmetric oligomers. Biological polyhedra, such as virus particles, are known to follow a directed assembly pathway following an obligate order of oligomeric states toward a final structure of minimal energy. Maize streak virus, the type member of the geminivirus family, is a virus known to form two distinct capsid morphologies. The mature infectious capsid is composed of 110 copies of a 244 amino acid coat protein arranged in two joined pseudo T=1 heads. This particle is known to package the full complement of viral ssDNA, and its assembly is thought to be genome-dependent.; A T=1 icosahedral particle is also known to form but it only packages fragmented sub-genomic DNA. The structures of both the single and geminate particles have been solved by cryoEM to 9.7 and 9.3 A resolution, respectively. Interpretation of the volume data for the geminate particle suggests a possible structural role for the viral ssDNA genome and this is further verified by disruption of mature particles by treatment with DNase I. Analysis by buried surface area calculations suggests that MSV assembles from pentameric intermediates.; Helical macromolecular complexes, such as actin filaments (F-actin), follow a distinct, dynamic, self-assembly pathway. In vitro, the first detectable species after initiation of F-actin polymerization is an anti-parallel G-actin dimer (held together by a disulfide bond at Cys-374) that does not exist in mature F-actin. This species has been crystallized and its structure solved in three different space groups. The dynamics illustrated by these different crystal forms are postulated to be examples of the types of orientation changes that occur to actin during polymerization. An assembly mechanism utilizing the structural intermediates trapped in these crystal lattices is described and discussed. |
