Capsid assembly and disassembly are critical steps in the retroviral replication cycle. A correctly formed capsid is essential to shuttle the enclosed viral genomic RNA into target cells, yet the capsid is metastable, and disassociates in a tightly regulated manner following entry. Extensive in vitro studies, coupled with low-resolution imaging of virions, have resulted in development of the “fullerene” model of the mature capsid. In this model ~1200 copies of the capsid protein (CA) assemble into a largely hexagonal array, with inclusion of the occasional pentamer allowing capsid closure. However some fundamental questions still remain regarding the physical forces that drive CA to assemble.

Many retroviral capsid proteins remain in the unassembled state when expressed, purified, and concentrated at physiological pH and salt concentration. However small perturbations in conditions will subsequently initiate capsid assembly. For example addition of NaCl promotes Human Immunodeficiency Virus (HIV) CA assembly, whereas acidification, or the addition of poly-anions, promotes Rous Sarcoma Virus (RSV) CA assembly. We have further investigated the effect of temperature and salinity on in vitro assembly of RSV CA. We find that RSV CA assembly efficiency increases dramatically with temperature. And that decreasing the salinity can also promote assembly of RSV CA.

Together these observations further demonstrate that CA is poised to assemble, and that environmental changes that modulate electrostatic attraction and repulsion, or the hydrophobic effect, will readily initiate assembly. The new in vitro assembly protocols we have developed for RSV CA should allow more detailed dissection of these effects.