Abstract
Many tripartite motif-containing (TRIM) proteins, comprising RING-finger, B-Box, and coiled-coil domains, carry additional B30.2 domains on the C-terminus of the TRIM motif and are considered to be pattern recognition receptors involved in the detection of higher order oligomers (e.g. viral capsid proteins). To investigate the spatial architecture of domains in TRIM proteins we determined the crystal structure of the TRIM20Δ413 fragment at 2.4 Å resolution. This structure comprises the central helical scaffold (CHS) and C-terminal B30.2 domains and reveals an anti-parallel arrangement of CHS domains placing the B-box domains 170 Å apart from each other. Small-angle X-ray scattering confirmed that the linker between CHS and B30.2 domains is flexible in solution. The crystal structure suggests an interaction between the B30.2 domain and an extended stretch in the CHS domain, which involves residues that are mutated in the inherited disease Familial Mediterranean Fever. Dimerization of B30.2 domains by means of the CHS domain is crucial for TRIM20 to bind pro-IL-1β in vitro. To exemplify how TRIM proteins could be involved in binding higher order oligomers we discuss three possible models for the TRIM5α/HIV-1 capsid interaction assuming different conformations of B30.2 domains.
Highlights
Tripartite motif-containing proteins represent a large family of proteins sharing a conserved domain architecture
To investigate the oligomerisation state and the spatial domain orientation of TRIM20 we designed a construct that comprises the TRIM20 coiled-coil and B30.2 domains. This design was guided by secondary structure prediction of tripartite motif-containing (TRIM) proteins, which suggested that the bipartite coiled-coil motif starts immediately after the B-box domain
A comparison of the isolated B30.2 domain structures between TRIM20, TRIM21/Fc25 and Ret finger protein-like 4A26 as well as mapping of Familial Mediterranean Fever (FMF)-associated mutations suggested that M694 of TRIM20 is involved in ligand binding[21]
Summary
Tripartite motif-containing proteins represent a large family of proteins sharing a conserved domain architecture. The tripartite motif (TRIM) consists of an N-terminal RING domain, one or two B-box domains and a C-terminal coiled-coil domain. The TRIM segment is considered to serve as a scaffold domain that is extended on either side by additional protein-protein interaction modules. In TRIM20 the RING domain is replaced by a PYD domain, which belongs to the death domain superfamily and is frequently involved in homotypic protein-protein interactions (reviewed in ref 3.). TRIM5α acts as a pattern recognition receptor that confers restriction against HIV-1, HIV-2 and other enveloped viruses in a species-specific manner[13,14,15,16], whereas TRIM21 recognizes opsonized viruses in the cytoplasm by binding to the Fc-fragment of the antibody[17,18]. TRIM proteins are flexible structural entities that tend to oligomerize upon ligand binding. The structures of TRIM69 coiled-coil[27], TRIM25 coiled-coil[28], and TRIM5α B-box/coiled-coil domains[29] provide insight into the assembly of the scaffold domain, but the spatial orientation between the coiled-coil and B30.2 domains remains elusive
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