Objective: The HIV-1 nucleocapsid protein (NC) is involved in transfer RNA3 annealing to the primer binding site of viral genomic RNA by means of two basic regions that are similar to the N-terminal portion of the arginine-rich motif (ARM) of Tat. As Tat is known to be asymmetrically arginine dimethylated by protein arginine methyltransferase 6 (PRMT6) in its ARM, we investigated whether NC could also act as a substrate for this enzyme.
Methods: Arginine methylation of NC was demonstrated in vitro and in vivo, and sites of methylation were determined by mutational analysis. The impact of the arginine methylation of NC was measured in RNA annealing and reverse transcription initiation assays. An arginine methyltransferase inhibitor (AMI)3.4 was tested for its effects on viral infectivity and replication in vivo.
Results: NC is a substrate for PRMT6 both in vitro and in vivo. NC possesses arginine dimethylation sites in each of its two basic regions at positions R10 and R32, and methylated NC was less able than wild-type to promote RNA annealing and participate in the initiation of reverse transcription. Exposure of HIV-1-infected MT2 and primary cord blood mononuclear cells to AMI3.4 led to increased viral replication, whereas viral infectivity was not significantly affected in multinuclear-activation galactosidase indicator assays.
Conclusion: NC is an in-vivo target of PRMT6, and arginine methylation of NC reduces RNA annealing and the initiation of reverse transcription. These findings may lead to ways of driving HIV-infected cells out of latency with drugs that inhibit PRMT6.