HHV-6 U53 encodes its own proteinase, which is essential for capsid maturation, DNA packaging and the ultimate formation of new virus particles (Tigue et al., 1996). The mature proteinase consists of 230 residues, but is synthesized in the form of a precursor, which has an additional 298 residues attached to the C terminus of the mature enzyme. Autolytic removal of these residues, which themselves constitute a form of the viral assembly protein) releases the N-terminal proteinase in its mature form; processing takes place at two locations positioned, respectively, at the C terminus of the proteinase. Autoprocessing of the precursor form of HHV-6 proteinase at two sites (termed M and R) is required to generate the mature enzyme, which could represent targets for novel antivirals against HHV-6 (Tigue and Kay, 1998a,b). Despite sharing 40% identity with other betaherpesvirus protein-ases such as human cytomegalovirus proteinase, the one-chain HHV-6 enzyme is distinguished from these two-chain proteinases by the absence of an internal autocatalytic cleavage site.
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