The HIV-1 Capsid: Assembly and Restriction by TRIM5α

Abstract

The mature HIV-1 capsid packages the viral genome and facilitates the reverse transcription step in early stages of the viral replication cycle. The capsid is a cone-shaped fullerene shell composed of ∼1,500 copies of the virally encoded CA protein. The subunits form a two-dimensional array of CA hexamers that is closed by incorporation of 12 CA pentamers. Despite the locally symmetric arrangement of the subunits, the capsid particle itself is non-symmetric. Therefore, we have used a “hybrid methods” strategy to study the capsid architecture. Initially, we used electron cryomicroscopy (cryoEM) to obtain low-resolution views of the quaternary interactions that mediate hexamer formation by analyzing capsid-like structures formed in vitro. Docking atomic resolution structures of CA subdomains into the cryoEM map yielded a Cα model, which we used as a template to design disulfides that stabilized the hexamer, and the quasi-equivalent pentamer. The structures of the building blocks were then solved by X-ray crystallography. By comparative analysis of all the available structures, we identified conformational switches that facilitated the formation of a non-symmetric capsid. The structural and mechanistic elements were then combined computationally into an atomic model for the complete capsid. TRIM5α is a cellular innate immune factor that interferes with retroviral capsid function. TRIM5α directly binds the capsid, but its pair-wise affinity for the CA subunit is extremely weak. We have found that TRIM5α assembles into a net-like lattice that matches the symmetry and spacing of the capsid lattice. This suggests that recognition of the HIV-1 capsid by TRIM5α occurs through lattice-lattice interactions, and that the weak pair-wise affinity of TRIM5α for the CA subunit is amplified by avidity. These studies support a model wherein TRIM5α constitutes a soluble surveillance mechanism in the cell that intercepts and disables the incoming viral capsid.