Rhesus TRIM5α (rhTRIM5α) potently restricts replication of human immunodeficiency virus type 1 (HIV-1). Restriction is mediated through direct binding of the C-terminal B30.2 domain of TRIM5α to the assembled HIV-1 capsid core. This host-pathogen interaction involves multiple capsid molecules within the hexagonal HIV-1 capsid lattice. However, the molecular details of this interaction and the precise site at which the B30.2 domain binds remain largely unknown. The human orthologue of TRIM5α (hsTRIM5α) fails to block infection by HIV-1 both in vivo and in vitro. This is thought to be due to differences in binding to the capsid lattice. To map the species-specific binding surface on the HIV-1 capsid lattice, we used microscale thermophoresis and dual-focus fluorescence correlation spectroscopy to measure binding affinity of rhesus and human TRIM5α B30.2 domains to a series of HIV-1 capsid variants that mimic distinct capsid arrangements at each of the symmetry axes of the HIV-1 capsid lattice. These surrogates include previously characterized capsid oligomers, as well as a novel chemically cross-linked capsid trimer that contains cysteine substitutions near the 3-fold axis of symmetry. The results demonstrate that TRIM5α binding involves multiple capsid molecules along the 2-fold and 3-fold interfaces between hexamers and indicate that the binding interface at the 3-fold axis contributes to the well-established differences in restriction potency between TRIM5α orthologues.IMPORTANCE TRIM5α is a cellular protein that fends off infection by retroviruses through binding to the viruses' protein shell surrounding its genetic material. This shell is composed of several hundred capsid proteins arranged in a honeycomb-like hexagonal pattern that is conserved across retroviruses. By binding to the complex lattice formed by multiple capsid proteins, rather than to a single capsid monomer, TRIM5α restriction activity persists despite the high mutation rate in retroviruses such as HIV-1. In rhesus monkeys, but not in humans, TRIM5α confers resistance to HIV-1. By measuring the binding of human and rhesus TRIM5α to a series of engineered HIV-1 capsid mimics of distinct capsid lattice interfaces, we reveal the HIV-1 capsid surface critical for species-specific binding by TRIM5α.

Caspases play important roles in cell death, differentiation, and proliferation. Due to their high homology, especially of the active site, specific targeting of a particular caspase using substrate analogues is very difficult. Although commercially available small molecules based on peptides are lacking high specificity due to overlapping cleavage motives between different caspases, they are often used as specific tools. We have selected designed ankyrin repeat proteins (DARPins) against human caspases 1-9 and identified high-affinity binders for the targeted caspases, except for caspase 4. Besides previously reported caspase-specific DARPins, we generated novel DARPins (D1.73, D5.15, D6.11, D8.1, D8.4, and D9.2) and confirmed specificity for caspases 1, 5, 6, and 8 using a subset of caspase family members. In addition, we solved the crystal structure of caspase 8 in complex with DARPin D8.4. This binder interacts with non-conserved residues on the large subunit, thereby explaining its specificity. Structural analysis of this and other previously published crystal structures of caspase/DARPin complexes depicts two general binding areas either involving active site forming loops or a surface area laterally at the large subunit of the enzyme. Both surface areas involve non-conserved surface residues of caspases.