Herpesviruses encode an essential, maturational serine protease whose catalytic domain, assemblin (28 kDa), is released by self-cleavage from a 74-kDa precursor (pPR, pUL80a). Although there is considerable information about the structure and enzymatic characteristics of assemblin, a potential pharmacologic target, comparatively little is known about these features of the precursor. To begin studying pPR, we introduced five point mutations that stabilize it against self-cleavage at its internal (I), cryptic (C), release (R), and maturational (M) sites and at a newly discovered "tail" (T) site. The resulting mutants, called ICRM-pPR and ICRMT-pPR, were expressed in bacteria, denatured in urea, purified by immobilized metal affinity chromatography, and renatured by a two-step dialysis procedure and by a new method of sedimentation into glycerol gradients. The enzymatic activities of the pPR mutants were indistinguishable from that of IC-assemblin prepared in parallel for comparison, as determined by using a fluorogenic peptide cleavage assay, and approximated rates previously reported for purified assemblin. The percentage of active enzyme in the preparations was also comparable, as determined by using a covalent-binding suicide substrate. An unexpected finding was that, in the absence of the kosmotrope Na 2 SO 4 , optimal activity of pPR requires interaction through its scaffolding domain. We conclude that although the enzymatic activities of assemblin and its precursor are comparable, there may be differences in how their catalytic sites become fully activated.Human cytomegalovirus (HCMV) is a ubiquitous, opportunistic pathogen of the herpesvirus family that causes lifethreatening illness in immunocompromised individuals (47,52,64). Identifying and characterizing antiviral targets is critical for the development of new therapeutic agents to treat infections and diseases caused by this virus (7,17,23,40,44,75). One such target or potential set of targets to come from these efforts is the herpesvirus maturational protease (pPR, e.g., HCMV pUL80a) and its genetically related substrate proteins (Fig. 1A), including the precursor assembly protein (pAP, pUL80.5) (36,37,77,78), which are essential for the production of infectious virus (19,24,58).Together, the UL80a proteins coordinate the process of capsid assembly and maturation to a form that is competent for DNA packaging (9, 25, 67). At the earliest stages of the process, pAP is required to translocate the major capsid protein (MCP; pUL86) into the nucleus and then to serve as an internal scaffold to direct its organization into the spherical procapsid shell (19,39,69,70,83). Once the procapsid is formed, the protease is required to sever pAP-MCP interactions, enabling elimination of the scaffolding proteins to make room for the viral DNA.The 28-kDa protease is incorporated into the scaffolding structure as a 74-kDa precursor that includes the entire pAP sequence as its carboxyl end (Fig. 1A) (60). This fusion sequence ensures protease targeting to the interior of...