Expression and Characterization of Genetically Linked Homo- and Hetero-Dimers of Hiv Proteinase

Infectious retroviral particles are composed of an inner core structure enclosed in a host-derived plasma membrane that contains the viral glycoproteins. In the case of HIV, this inner core consists of a ribonucleoprotein complex (two identical molecules of genomic RNA associated with the viral nucleocapsid [NC] protein and probably also with the viral enzymes reverse transcriptase [RT], integrase [IN] and proteinase [PR]) encased in a capsid [CA] shell (Gelderblom et al., 1989). All structural components of the viral core (derived from the viral gag gene) as well as the replication enzymes (derived from the pol gene) are synthesized and assembled as polyprotein precursors. Proteolytic processing by the virus-encoded, virion-associated proteinase takes place only during and after budding of the viral particle from the plasma membrane and processing is not required for the release of immature, non-infectious particles but is necessary for maturation of infectious virions (reviewed in Krausslich & Wimmer, 1988). This elaborate mechanism of synthesizing different stable polyproteins at defined rates enables the virus to target many components of the viral particle to the site of assembly using only a single targeting signal. It requires, however, that a proteolytic enzyme is packaged into the virion that is capable of separating the different functional domains, thus allowing viral replication to occur. The activity of such an enzyme has to be tightly regulated since premature processing would dissociate the components of the virion from the site of assembly and incomplete or unfaithful processing should interfere with viral uncoating and replication. Controlled limited proteolysis can be achieved by synthesizing an inactive form of the viral proteinase as part of the polyprotein which is activated upon assembly of the viral particle.