Cloning and inducible synthesis of poliovirus non-structural proteins in Saccharomyces cerevisiae

Cell killing by HIV-1 protease

The human immunodeficiency virus protease (HIV-1 PR) was expressed both in the yeast Saccharomyces cerevisiae and in mammalian cells. Inducible expression of HIV-1 PR arrested yeast growth, which was followed by cell lysis. The lytic phenotype included loss of plasma membrane integrity and cell wall breakage leading to the release of cell content to the medium. Given that neither poliovirus 2A protease nor 2BC protein, both being highly toxic for S. cerevisiae, were able to produce similar effects, it seems that this lytic phenotype is specific of HIV-1 PR. Drastic alterations in membrane permeability preceded the lysis in yeast expressing HIV-1 PR. Cell killing and lysis provoked by HIV-1 PR were also observed in mammalian cells. Thus, COS7 cells expressing the protease showed increased plasma membrane permeability and underwent lysis by necrosis with no signs of apoptosis. Strikingly, the morphological alterations induced by HIV-1 PR in yeast and mammalian cells were similar in many aspects. To our knowledge, this is the first report of a viral protein with such an activity. These findings contribute to the present knowledge on HIV-1-induced cytopathogenesis.

Similar structural basis for membrane localization and protein priming by an RNA-dependent RNA polymerase

Protein primers are used to initiate genomic synthesis of several RNA and DNA viruses, although the structural details of the primer-polymerase interactions are not yet known. Poliovirus polymerase binds with high affinity to the membrane-bound viral protein 3AB but uridylylates only the smaller peptide 3B in vitro. Mutational analysis of the polymerase identified four surface residues on the three-dimensional structure of poliovirus polymerase whose wild-type identity is required for 3AB binding. These mutants also decreased 3B uridylylation, arguing that the binding sites for the membrane tether and the protein primer overlap. Mutation of flanking residues between the 3AB binding site and the polymerase active site specifically decreased 3B uridylylation, likely affecting steps subsequent to binding. The physical overlap of sites for protein priming and membrane association should facilitate replication initiation in the membrane-associated complex.

The yeast Saccharomyces cerevisiae as a genetic system for obtaining variants of poliovirus protease 2A

The inducible expression of poliovirus protease 2A (2Apro) blocks the growth of Saccharomyces cerevisiae. A number of yeast colonies that grow after 2Apro induction have been isolated. The majority of these clones express 2Apro to control levels, suggesting that their ability to divide is not due to the loss of 2Apro gene inducibility. The sequences of the 2Apro genes isolated from 22 clones were determined. Most of the 2Apro sequences from these colonies contain point mutations in the poliovirus protease. The different variant protease sequences were transferred to an infectious poliovirus cDNA clone. Translation of genomic RNA obtained from these poliovirus mutants in cell-free systems revealed that some of them had defects in their ability to cleave P1-2A in cis. In addition, several of these variants cleaved the translation initiation factor eIF-4G inefficiently. Transfection of the RNA generated from the full-length poliovirus genomes mutated in 2Apro yielded five viable polioviruses with a small plaque phenotype. These five polioviruses efficiently cleaved p220 but showed defects in viral protein synthesis, transactivation of a leader-luciferase mRNA, and 3CD cleavage to 3C' and 3D'. All 2Apro mutant sequences, including those that did not yield viable viruses, were cloned in pTM1 vector under a T7 promoter. Only the 2Apro variants that have activity to cleave 3CD produced viable poliovirus. Our findings indicate that S. cerevisiae represents a useful system for obtaining poliovirus 2Apro variants that may provide further insight into the role of this protease during the poliovirus replication cycle.

Poliovirus 2Apro expression inhibits growth of yeast cells

Poliovirus encodes two proteases, 2Apro and 3Cpro that participate in the processing of the viral polyprotein and cleave a number of host proteins. Both proteases have been cloned and expressed in an inducible manner in Saccharomyces cerevisiae cells. The expression of 2Apro, but not 3Cpro, was highly toxic for yeast cells such that growth was arrested after 5 h of induction and cell survival sharply declined. Cellular morphology was profoundly modified by expression of poliovirus 2Apro, in such a way that electron dense granules and autophagosomic bodies arise in the cytoplasm. Experiments aimed at defining the yeast function affected by 2Apro suggested that translation was not the target of protease toxicity, but showed that RNA synthesis was profoundly blocked.