EXPRESSION OF THE HUMAN PAPILLOMAVIRUS TYPE-11 L1-CAPSID PROTEIN IN ESCHERICHIA-COLI - CHARACTERIZATION OF PROTEIN DOMAINS INVOLVED IN DNA-BINDING AND CAPSID ASSEMBLY

The L1 major capsid protein of human papillomavirus type 11 (HPV-11) w as expressed in Escherichia coli, and the soluble recombinant protein was purified to near homogeneity. The recombinant L1 protein bound DNA as determined by the Southwestern assay method, and recombinant mutan t L1 proteins localized the DNA-binding domain to the carboxy-terminal 11 amino acids of L1. Trypsin digestion of the full-length L1 protein yielded a discrete 42-kDa product (trpL1), determined by sodium dodec yl sulfate-polyacrylamide gel electrophoresis, resulting from cleavage at R415, 86 amino acids from the L1 carboxy terminus. Sucrose gradien t sedimentation analysis demonstrated that trpL1 sedimented at 11S, wh ile L1 proteins with amino-terminal deletions of 29 and 61 residues se dimented at 4S. Electron microscopy showed that the full-length L1 pro tein appeared as pentameric capsomeres which self-assembled into capsi d-like particles. The trpL1 protein also had a pentameric morphology b ut was unable to assemble further. In an enzyme-linked immunosorbent a ssay, the trpL1 and L1 capsids reacted indistinguishably from virus-li ke particles purified after expression of HPV-11 L1 in insect cells. T he carboxy terminus of L1 therefore constitutes the interpentamer link er arm responsible for HPV-11 capsid formation, much like the carboxy- terminal domain of the polyomavirus VP1 protein. The trypsin susceptib ility of HPV-11 L1 capsids suggests a possible mechanism for virion di sassembly.