Dissecting the Fine Details of Assembly of aT = 3 Phage Capsid

Abstract

The RNA bacteriophages represent ideal model systems in which to probe the detailed assembly pathway for the formation of aT = 3 quasi-equivalent capsid. For MS2, the assembly reaction can be probedin vitrousing acid disassembled coat protein subunits and a short (19 nt) RNA stem-loop that acts as the translational operator of the replicase gene and leads to sequence-specific sequestration and packaging of the cognate phage RNAin vivo. Reassembly reactions can be initiated by mixing these components at neutral pH. The molecular basis of the sequence-specific RNA–protein interaction is now well understood. Recent NMR studies on the protein demonstrate extensive mobility in the loops of the polypeptide that alter their conformations to form the quasi-equivalent conformers of the final capsid. It seems reasonable to assume that RNA binding results in reduction of this flexibility. However, mass spectrometry suggests that these RNA–protein complexes may only provide one type of quasi-equivalent capsid building block competent to form five-fold axes but not the full shell. Work with longer RNAs suggests that the RNA may actively template the assembly pathway providing a partial explanation of how conformers are selected in the growing shell.