Background: The BTB domain (also known as the POZ domain) is a versatile protein-protein interaction motif that participates in a wide range of cellular functions, including transcriptional regulation, cytoskeleton dynamics, ion channel assembly and gating, and targeting proteins for ubiquitination. Several BTB domain structures have been experimentally determined, revealing a highly conserved core structure. Results: We surveyed the protein architecture, genomic distribution and sequence conservation of BTB domain proteins in 17 fully sequenced eukaryotes. The BTB domain is typically found as a single copy in proteins that contain only one or two other types of domain, and this defines the BTB-zinc finger (BTB-ZF), BTB-BACK-kelch (BBK), voltage-gated potassium channel T1 (T1-Kv), MATH-BTB, BTB-NPH3 and BTB-BACK-PHR (BBP) families of proteins, among others. In contrast, the Skp1 and ElonginC proteins consist almost exclusively of the core BTB fold. There are numerous lineage-specific expansions of BTB proteins, as seen by the relatively large number of BTB-ZF and BBK proteins in vertebrates, MATH-BTB proteins in Caenorhabditis elegans, and BTB-NPH3 proteins in Arabidopsis thaliana. Using the structural homology between Skp1 and the PLZF BTB homodimer, we present a model of a BTB-Cul3 SCF-like E3 ubiquitin ligase complex that shows that the BTB dimer or the T1 tetramer is compatible in this complex. Conclusion: Despite widely divergent sequences, the BTB fold is structurally well conserved. The fold has adapted to several different modes of self-association and interactions with non-BTB proteins.

Striga uses a hypersensitive receptor: The strigolactone hormones govern plant growth and development. A nasty parasitic weed, Striga , senses traces of these hormones to identify its targets. After functionally characterizing several strigolactone receptors from Striga , Toh et al. solved the crystal structure of one that is especially sensitive. The structure shows an unexpectedly large ligand-binding pocket, which may explain how Striga manages to sense picomolar concentrations of the range of strigolactones. Science , this issue p. 203

The sulfonamides (sulfas) are the oldest class of antibacterial drugs and inhibit the bacterial dihydropteroate synthase (DHPS, encoded by folP), through chemical mimicry of its co-substrate p-aminobenzoic acid (pABA). Resistance to sulfa drugs is mediated either by mutations in folP or acquisition of sul genes, which code for sulfa-insensitive, divergent DHPS enzymes. While the molecular basis of resistance through folP mutations is well understood, the mechanisms mediating sul-based resistance have not been investigated in detail. Here, we determine crystal structures of the most common Sul enzyme types (Sul1, Sul2 and Sul3) in multiple ligand-bound states, revealing a substantial reorganization of their pABA-interaction region relative to the corresponding region of DHPS. We use biochemical and biophysical assays, mutational analysis, and in trans complementation of E. coli ΔfolP to show that a Phe-Gly sequence enables the Sul enzymes to discriminate against sulfas while retaining pABA binding and is necessary for broad resistance to sulfonamides. Experimental evolution of E. coli results in a strain harboring a sulfa-resistant DHPS variant that carries a Phe-Gly insertion in its active site, recapitulating this molecular mechanism. We also show that Sul enzymes possess increased active site conformational dynamics relative to DHPS, which could contribute to substrate discrimination. Our results reveal the molecular foundation for Sul-mediated drug resistance and facilitate the potential development of new sulfas less prone to resistance.

Significance: Legionella pneumophila is the causative agent of Legionnaires’ disease. It translocates a large repertoire of effectors into the host cell through a specialized secretion system to subvert cellular defenses. A key characteristic of this pathogen is that the majority of its effectors are encoded by eukaryotic-like genes acquired through horizontal gene transfer. We determined the crystal structure of one of these effectors, sphingosine-1 phosphate lyase ( Lp Spl), and show that it has high similarity with its eukaryotic homologue. We demonstrate that Lp Spl possesses lyase activity and that it disrupts sphingolipid metabolism in the host cells. Lp Spl plays a critical and previously unknown role in decreasing autophagy and is a unique virulence factor facilitating intracellular replication of L. pneumophila .