Comparing capsid assembly of primate lentiviruses and hepatitis B virus using cell-free systems

Many viruses that assemble their capsids in the eukaryotic cytoplasm require a threshold concentration of capsid protein to achieve capsid assembly. Strategies for achieving this include maintaining high levels of capsid protein synthesis and targeting to specific sites to raise the effective concentration of capsid polypeptides. To understand how different viruses achieve the threshold capsid protein concentration required for assembly, we used cell-free systems to compare capsid assembly of hepatitis B virus (HBV) and three primate lentiviruses. Capsid formation of these diverse viruses in a common eukaryotic extract was dependent on capsid protein concentration. HBV capsid assembly was also dependent on the presence of intact membrane surfaces. Surprisingly, not all of the primate lentiviral capsid proteins examined required myristoylation and intact membranes for assembly, even though all contain a myristoylation signal. These findings reveal significant diversity in how different capsid proteins assemble in the same cellular extract.