Multiple Pathways in Capsid Assembly

For a three dimensional structure to spontaneously self-assemble from many identical components it must be kinetically accessible. Many icosahedral virus capsids can spontaneously assemble from hundreds of identical proteins, but how they navigate the assembly process is poorly understood. Capsid assembly is thought to involve stepwise addition of subunits to a growing capsid fragment. Course-grained models suggest that the reaction occurs on a downhill energy landscape, so intermediates are expected to be fleeting. In this work, charge detection mass spectrometry (CDMS) has been used to track assembly of the hepatitis B virus (HBV) capsid in real time. The icosahedral T=4 capsid of HBV is assembled from 120 capsid protein dimers. The results indicate that there are multiple pathways for assembly. A facile pathway occurs on a mainly downhill energy landscape with no large intermediates. This pathway dominates under the lower salt conditions examined here. Under higher salt conditions, where subunit interactions are strengthened, around half of the products of the initial assembly reaction have masses close to the T=4 capsid, and the other half are stalled intermediates which emerge abruptly at around 90 dimers. When incubated at room temperature, they gradually shift to higher mass and merge with the capsid peak. The stalled intermediates are not kinetically trapped by the lack of the subunits needed to proceed???. Thus they represent local minima on the energy landscape. They probably result from hole closure, where the growing capsid distorts to close the hole due to the missing capsid proteins. The threshold at around 90 dimers presumably reflects the size where hole closure becomes feasible (i.e., where the energy gained by hole closure overcomes the strain induced by distorting the capsid)