Viral genomes are packaged into "procapsids" by powerful molecular motors. We report the crystal structure of the DNA packaging motor protein, gene product 17 (gp17), in bacteriophage T4. The structure consists of an N-terminal ATPase domain, which provides energy for compacting DNA, and a C-terminal nuclease domain, which terminates packaging. We show that another function of the C-terminal domain is to translocate the genome into the procapsid. The two domains are in close contact in the crystal structure, representing a "tensed state." A cryo-electron microscopy reconstruction of the T4 procapsid complexed with gp17 shows that the packaging motor is a pentamer and that the domains within each monomer are spatially separated, representing a "relaxed state." These structures suggest a mechanism, supported by mutational and other data, in which electrostatic forces drive the DNA packaging by alternating between tensed and relaxed states. Similar mechanisms may occur in other molecular motors.

Self-assembly of regular protein surfaces around nanoparticle templates provides a new class of hybrid biomaterials with potential applications in medical imaging and in bioanalytical sensing. We report here the first example of efficiently self-assembled virus-like particles (VLPs) having a brome mosaic virus protein coat and a functionalized gold core. The present study indicates that functionalized gold particles can initiate VLP assembly by mimicking the electrostatic behavior of the nucleic acid component of the native virus. These VLP constructs are symmetric, with the protein stoichiometry and packaging properties indicating similarity to the icosahedral packing of the capsid. Moreover, a pH-induced swelling transition of the VLPs is observed, in direct analogy to the native virus.Two-dimensional crystalline arrays of proteins self-assembled on flat supports have recently emerged as prime candidates for biotechnological applications. 1,2 Due to their intrinsic symmetry, these arrays can provide high density and equivalent environments for binding domains, characteristics that are difficult to achieve by any other technologies. 3 The ability to engineer specific regulatory switches into proteins by use of recombinant DNA technology also promises additional uses in these arrays.The protein coat, or capsid, of many viruses provides a discrete analogue to extended protein assemblies on planar surfaces. As a consequence, modified-virus capsids have been proposed as versatile biomolecular platforms for displaying engineered peptide sequences for triggering specific host responses. 3,4 Moreover, the regular motif of tubular or quasispherical capsids has been used for the templated synthesis of nanomaterials ranging from magnetic particles 5 to nanowires for electronic applications. 6 The majority of current studies on protein cage-based materials have focused on the use of the capsid surface as a nucleator and/or template for the growth or attachment of inorganic entities. The converse approach of using inorganic particles to nucleate viral capsids represents a new method for synthesizing hybrid inorganic/viral particles, thus widening the range of possible combinations of physical and chemical properties.Preliminary experiments on the encapsulation of functionalized gold nanoparticles into brome mosaic virus (BMV) capsids have shown that the VLP protein coat is likely organized in a closed shell, similar in its physicochemical properties to the native virus. 7,8 These previous examples of nanoparticle encapsidation were characterized by an extremely low yield relative to the formation of empty capsid (∼1%), 9 in stark contrast with the essentially quantitative in-vivo incorporation of the native RNA. 9 Practical application of these systems, however, requires a high yield of homogeneous material. Moreover, knowledge of the VLP shell structure and concomitant preservation of its functional attributes also requires homogeneous materials that can be used for high-resolution structural studies.The fact that...