Limited Proteolysis and a Reactive Cysteine Residue Define Accessible Regions in the Native Conformation of the Adenovirus Hexon Protein

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Abstract
The sulfhydryl group of one cysteine residue in the adenovirus hexon protein is accessible to alkylation in the absence of denaturating agents. This residue was [14C]carboxymethylated and characterized in peptides after proteolytic treatments. It was identified in two different types of digest and corresponded to the cysteine residue most accessible to oxidation. It is located close to residue 670 in the tentative amino acid sequence of the entire polypeptide. Trypsin has a highly limited action on the native hexon protein. Only three peptide bonds are fully susceptible to cleavage. These were found to occur in the N-terminal third of the protein, and were characterized as cleavages close to positions 150, 180 and 320 in the tentative structure of the polypeptide. Trypsin therefore liberates four fragments in major yield, with approximate Mr of 3000, 15000, 17000 and 78000. The smallest fragment corresponds to a highly acidic region. A limited, but less restricted susceptibility of hexon to chymotryptic cleavage was also found, and four major sites were characterized. The cysteine labelling and the proteolytic treatments define accessible regions in the native conformation of hexon. Together with previously assigned residue distributions, they suggest the subdivision of the entire polypeptide chain into three approximately equal parts, with accessible sites in the border regions, as well as in the middle of at least the N-terminal third. One model compatible with all data is presented.