Baculovirus expression of the human papillomavirus type 16 capsid proteins: detection of L1-L2 protein complexes

The human papillomavirus (HPV) type 16 major capsid proteins L1 and L2 have been produced in a baculovirus expression system. Both proteins are expressed at a high level and can be readily solubilized. The L1 capsid protein migrates close to its expected Mr of 60K. On the other hand L2 exhibits a much higher Mr migrating at 73K, which is considerably greater than its predicted Mr of 50K. The identity of both proteins has been confirmed also by Western blot analysis. Both proteins are produced in drastically reduced amounts in the presence of tunicamycin. In addition both L1 and L2 show interesting patterns of phosphorylation. L1 is phosphorylated only weakly and this appears to be quite labile, whereas L2 is very heavily phosphorylated and this, in contrast, appears to be very stable. We have also made use of a dual expression vector for co-expressing the L1 and L2 proteins within the same baculovirus-infected cell. The results obtained from this system demonstrate the presence of protein complexes forming between the two capsid proteins. These studies indicate that at least the initial events in capsid assembly of HPVs can occur in the absence of viral DNA.