dos Santos, N. F. | Search bioRxiv

bioRxiv2024

Arg18 substitutions reveal the capacity of the HIV-1 capsid protein for non-fullerene assembly

In the fullerene cone HIV-1 capsid, the central channels of the hexameric and pentameric capsomers each contain a ring of arginine (Arg18) residues that perform essential roles in capsid assembly and function. In both the hexamer and pentamer, the Arg18 rings coordinate inositol hexakisphosphate, an assembly and stability factor for the capsid. Previously, it was shown that amino-acid substitutions of Arg18 can promote pentamer incorporation into capsid-like particles (CLPs) that spontaneously assemble in vitro under high salt conditions. Here, we show that these Arg18 mutant CLPs contain a non-canonical pentamer conformation and distinct lattice characteristics that do not follow the fullerene geometry of retroviral capsids. The Arg18 mutant pentamers resemble the hexamer in intra-oligomeric contacts, and forms a unique tetramer-of-pentamers that allows for incorporation of an octahedral vertex with a cross-shaped opening in the hexagonal capsid lattice. Our findings highlight an unexpected degree of structural plasticity in HIV-1 capsid assembly.

microbiology↗

bioRxiv2022

A molecular switch modulates assembly and host factor binding of the HIV-1 capsid

Upon entry into a new host cell, the HIV-1 capsid performs multiple essential functions, which include shielding the genome from innate immune sensors1, promoting reverse transcription2 and transporting the core from the entry site at the plasma membrane to the integration site inside the nucleus3,4. The HIV-1 capsid is a fullerene cone made of hexamers and pentamers of the viral CA protein5,6. The two types of capsomers are quasi-equivalent, with the same structural elements mediating distinct inter-subunit contacts. In other studied quasi-equivalent viruses, the capacity of genetically identical subunits to form hexamers and pentamers is conferred by molecular switches. Such a switch has not been previously found in retroviral CA proteins. Here, we report cryoEM structures of the HIV-1 CA pentamer within assembled in vitro capsids at nominal resolutions of 2.4-3.4 [A]. Comparison with the hexamer identified an internal loop that adopts distinct conformations, 310 helix in the pentamer and random coil in the hexamer. Designed manipulations of the coil/helix configuration allowed us to control pentamer and hexamer formation in a predictable manner, thus proving its function as a molecular switch. Importantly, the switch controls not only fullerene cone assembly, but also the capsids capacity to bind post-entry host factors that are critical for viral replication. Furthermore, the switch forms part of the binding site of the new ultra-potent HIV-1 inhibitor, lenacapavir. These studies reveal that a critical assembly element also controls the post-assembly functions of the capsid, and provide new insights on capsid inhibition and uncoating.

microbiology↗