Molecular modeling and in vitro activity of an HIV-1-encoded glutathione peroxidase

Based on theoretical evidence, it has been proposed that HIV-1 may encode s everal selenoprotein modules, one of which (overlapping the env gp41-coding region) has highly significant sequence similarity to the mammalian seleno protein glutathione peroxidase (GPx; EC 1.11.1.9). The similarity score of the putative HIV-1 viral GPx homolog relative to an aligned set of known GP x is 6.3 SD higher than expected for random sequences of similar compositio n. Based on that alignment, a molecular model of the HIV-1 GPx was construc ted by homology modeling from the bovine GPx crystal structure. Despite ext ensive truncation relative to the cellular GPx gene, the structural core an d the geometry of the catalytic triad of selenocysteine, glutamine, and try ptophan are well conserved in the viral GPx. All of the insertions and dele tions predicted by the alignment proved to be structurally feasible. The mo del is energetically favorable, with a computed molecular mechanics strain energy close to that of the bovine GPx structure, when normalized on a per- residue basis. However, considering the remote homology, this model is inte nded only to provide a working hypothesis allowing for a similar active sit e and structural core. To validate the theoretical predictions, we cloned t he hypothetical HIV-1 gene and found it to encode functional GPx activity w hen expressed as a selenoprotein in mammalian cells. In transfected canine kidney cells, the increase in GPx activity ranged from 21% to 43% relative to controls (average 30%, n = 9, P < 0.0001), whereas, in transfected MCF7 cells, which have low endogenous GPx activity, a near 100% increase was obs erved (average 99%, n = 3, P < 0.05).