Independently cloned halves of cytomegalovirus assemblin, A(n) and A(c), can restore proteolytic activity to assemblin mutants by intermolecular complementation

Herpesviruses encode an essential serine proteinase called assemblin that is responsible for cleaving the precursor assembly protein during the process of capsid formation. In cytomegalovirus (CMV), assemblin undergoes autoproteolysis at an internal (1) site located near the middle of the molecule. I-site cleavage converts the enzyme to an active two-chain form consisting of the subunits A(n) and A(c). We have recently shown that the recombinant A(n) and A(c) subunits can spontaneously associate within eukaryotic cells to yield active two-chain proteinase. This finding indicates that the subunits are able to independently assume their correct functional conformations and led us to lest whether they are capable of intermolecular complementation. This was done by coexpressing inactive mutant (point, deletion, and insertion) forms of assemblin together with the wild-type subunit (either A(n) or A(c)) corresponding to the domain of assemblin that was mutated. Results of these experiments showed that both A(n) and A(c) are able to rescue the enzymatic activity of assemblin mutants. I-site cleavage of the mutated assemblin occurred during complementation but was not absolutely required, as shown by effective complementation of inactive assemblins with nonclearable I sites. We have also shown that intermolecular complementation can rescue the activity of an inactive mutant full-length proteinase precursor and can occur between different species of CMV (e.g., human CMV subunit can rescue activity of mutant simian CMV assemblin). These results indicate that assemblin is able to form active multimeric structures that may be of functional importance.