Non-infectious in-cell HIV-1 protease assay utilizing translocalization of a fluorescent reporter protein and apoptosis induction

This study describes a non-infectious in-cell imaging assay for HIV-1 protease inhibitor screening. It is based on re-distribution of a fluorescence reporter protein upon protease cleavage and the fact that HIV-infected cells undergo apoptosis. The in-cell assay utilizes fluorescent reporter proteins consisting of an intracellular translocation signal sequence, a caspase-3-specific cleavage sequence, and a fluorescent tagging protein. The reporter proteins are designed to change their intracellular localization upon cleavage, either from the cytosol to a subcellular organelle (type I) or from a subcellular organelle to the cytosol (type II). Inhibition of protease activity can be monitored at the single cell level. Interestingly, the expression of HIV-1 protease induced endogenous caspase-3 activation; thus, the fluorescence reporter protein containing the caspase-3 cleavage sequence translocalized upon cleavage. This is the first time that HIV-1 protease expression, not whole virus infection of the cell, was observed to trigger the apoptotic pathway, including caspse-3 activation. A validation of this assay was performed with a known HIV-1 protease inhibitor, Ac-Leu-Val-phenylalanine. The clear cellular change in fluorescence pattern makes this system an ideal tool for various types of life science and drug discovery research, including high throughput and high content screening applications.