Animation: Mechanisms of Action of Protease Inhibitors (PIs) The production of HIV viral proteins involves the interaction of both viral mRNA and host cell ribosomes. Viral mRNA is translated into long non-functional precursor proteins, which are cleaved into functional subunits by viral protease enzymes. This animation describes how PI drugs work by directly binding viral proteases and blocking their ability to cleave non-functional precursors into functional protein subunits. Halted viral protein production prevents the assembly and formation of new viruses.