The complexity of translation is a classical dilemma in the evolution of biological systems. Efficient translation requires coordination of complex, highly evolved RNAs and proteins; however, complex, highly evolved RNAs and proteins could not evolve without efficient translation system. At the heart of this complexity is the ribosome, itself a remarkably complex molecular machine. Our work illustrates the ribosome as deconstructed units of modification. Here we have deconstructed a segment of the ribosome to interacting RNA-protein units. L23 interacts in vivo with both Domain III (DIII) and Domain IIIcore (DIIIcore) independently of the fully assembled ribosome. This suggests that DIIIcore represents the functional rRNA unit in DIII-L23 interaction. Furthermore, L23peptide sustains binding function in vitro with both DIII and DIIIcore independently of any stabilizing effects from the globular domain of L23. The ability of L23peptide to form a 1:1 complex with both DIII and DIIIcore suggests that L23peptide is the functional rProtein unit in DIII-L23 interaction. We believe that our results will stimulate interest and discussions in the significance of 3D architecture and units of evolution in the ribosome. The ubiquity of the ribosome in cellular life prognosticates that our results impact and appeal to biologists, chemists, bioinformaticists, as well as the general scientific community.

Small molecule screening is commonly used to discover lead compounds for drug development, but it can also be a powerful way to identify chemical probes for studying biological mechanisms. Our lab uses small molecules to study the mechanisms by which the protozoan parasite Toxoplasma gondii infects and replicates within its hosts. In this work, we employed a fluorescence-based assay to screen the Medicines for Malaria Venture (MMV) Open Access Malaria box for compounds that affect T. gondii growth. The box contains 400 previously identified small-molecule inhibitors of the related parasite, Plasmodium falciparum. We identified 79 hits, including a 2,4-diaminoquinazoline (MMV006169; IC50=1.15µM) that strongly inhibits T. gondii intracellular replication and invasion with no evidence of toxicity to mammalian cells. Extensive structure-activity relationship analyses with T. gondii identified a number of analogs with changed potency and altered effects on replication and invasion. These structure-activity analyses provided the information necessary to synthesize a bivalent chemical inducer of dimerization (CID) containing MMV006169 for use in yeast three-hybrid experiments. Yeast growth competition assays showed that this CID is capable of entering the yeast nucleus, as required for yeast three-hybrid screening. Yeast three-hybrid was used in a targeted format to test the hypothesis that MMV006169 works by inhibiting parasite CDC48, an ATPase involved in trafficking and the degradation of misfolded proteins. Large-scale cDNA library screening by yeast three-hybrid suggests that the compound may instead be working through inhibition of a host cell target. This work has provided insight into how MMV006169 affects the parasite's lytic cycle and generated a testable hypothesis for the biologically relevant target of the compound.