The organization of the major (L1) and minor (L2) proteins in the human papillomavirus capsid is still largely unknown. In this study we analysed the disulphide bonding between L1 proteins and the association of L2 proteins with capsomers using virus-like particles obtained in insect cells by co-expression of the L1 and L2 genes of human papillomavirus type 33. About 50% of the L1 protein molecules in these particles (1.29 g/cm) formed disulphide-bonded trimers. Reduction of the intermolecular disulphide bonds by dithiothreitol (DTT) treatment caused disassembly of virus-like particles into capsomers. This indicates that disulphide bonds between capsomers at the threefold symmetry positions of the capsid are essential for the assembly of the papillomavirus capsid. In contrast, the L2 protein was not engaged in intermolecular disulphide bonding. The L2 protein remained associated with capsomers on disassembly by treatment with DTT. When the disassembly was carried out in 0.65 -NaCl, complete L2 protein molecules bound preferentially to capsomer oligomers, whereas truncated L2 protein molecules bound only to monomers. In 0.15 -NaCl only complete L2 protein molecules remained bound to capsomers. This indicates that different regions of the L2 protein molecule are differentially involved in the association of the papillomavirus capsid.


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