Identification of non-basic matrix domain residues that impact HTLV-1 Gag membrane targeting and particle release

• Analyzed HTLV-1 MA non-basic residues in particle assembly. • Identified L19, D42, S70, and L71 as essential for plasma membrane targeting. • Several MA mutants redirected to internal structures reminiscent to late endosomes. • Mutant particle morphologies were either smaller or comparable to wild type. The matrix (MA) domain of the Gag polyprotein is critical for directing retroviral assembly at the plasma membrane (PM), yet the determinants mediating human T-cell leukemia virus type 1 (HTLV-1) Gag targeting remain incompletely defined. While Gag myristoylation and basic residue-mediated electrostatic interactions are known to be crucial for Gag-PM interactions, recent evidence with HTLV-1 MA has implicated limited dependency on specific lipid headgroup recognition for the ability of Gag to interact with the PM. Here, we have analyzed the role of non-basic residues in HTLV-1 MA in membrane interactions and particle assembly. We identified several residues ( i.e., L19, D42, S70, and L71) that were essential for Gag targeting to the PM, where mutation of these amino acid residues led to Gag targeting to internal locations that colocalized with late endosomal markers. Mutation of L19 and D42 was found to alter the MA structure. Taken together, these data indicate that mutation of MA non-basic amino acid residues allowed for particle production and also led to Gag localization to internal membranes that colocalized with late endosome markers. These observations indicate that non-basic residues play an important role in efficient particle assembly and release of HTLV-1 from cells.