Opposing effects of the M368A point mutation and deletion of the SP1 region on membrane binding of human immunodeficiency virus type 1 Gag

The Gag protein of human immunodeficiency virus type 1 (HIV-1) contains a 14-amino-acid region termed SP1 that is located between the capsid (CA) and nucleocapsid (NC) domains. It has been previously observed that either a M368A substitution within SP1 or the ΔSP1 deletion impaired virus production. In this study, we further showed that the M368A point mutation, but not the ΔSP1 deletion, severely diminished the levels of membrane-associated Gag proteins. This membrane binding defect associated with M368A was corrected either by changing NC to the leucine zipper (LZ) motif derived from the yeast transcription factor GCN4 or by a L364A second-site mutation in the context of the first four residues of SP1. Yet, neither the L364A mutation nor the LZ substitution restored wild type levels of particle production to the M368A Gag. These results suggest that SP1 affects both Gag-membrane binding and the subsequent events of virus assembly such as capsid morphogenesis or virus budding.