The HIV-1 Pr55gag polyprotein binds to plastidial membranes and leads to severe impairment of chloroplast biogenesis and seedling lethality in transplastomic tobacco plants

Chloroplast genetic engineering has long been recognised as a powerful technology to produce recombinant proteins. To date, however, little attention has been given to the causes of pleiotropic effects reported, in some cases, as consequence of the expres- sion of foreign proteins in transgenic plastids. In this study, we investigated the phenotypic alterations observed in transplastomic tobacco plants accumulating the Pr55gag polyprotein of human immunodeficiency virus (HIV-1). The expression of Pr55gag at high levels in the tobacco plastome leads to a lethal phenotype of seedlings grown in soil, severe impairment of plastid development and photosynthetic activity, with chloro- plasts largely resembling undeveloped proplastids. These alterations are associated to the binding of Pr55gag to thylakoids. During particle assembly in HIV-1 infected human cells, the binding of Pr55gag to a specific lipid [phosphatidylinositol-(4-5) bisphosphate] in the plasma membrane is mediated by myristoylation at the amino-terminus and the so-called highly basic region (HBR). Surprisingly, the non-myristoylated Pr55gag expressed in tobacco plastids was likely able, through the HBR motif, to bind to non phosphorous glycerogalactolipids or other classes of lipids present in plastidial membranes. Although secondary conse- quences of disturbed chloroplast biogenesis on expres- sion of nuclear-encoded plastid proteins cannot be ruled out, results of proteomic analyses suggest that their altered accumulation could be due to retrograde control in which chloroplasts relay their status to the nucleus for fine-tuning of gene expression.