The presynaptic regions of axons accumulate synaptic vesicles, active zone proteins and periactive zone proteins. However, the rules for orderly recruitment of presynaptic components are not well understood. We systematically examined molecular mechanisms of presynaptic development in egg-laying synapses of Caenorhabditis elegans, demonstrating that two scaffolding molecules, SYD-1 and SYD-2, have key roles in presynaptic assembly. SYD-2 (liprin-α) was previously shown to regulate the size and the shape of active zones. We now show that in syd-1 and syd-2 mutants, synaptic vesicles and numerous other presynaptic proteins fail to accumulate at presynaptic sites. SYD-1 and SYD-2 function cell-autonomously at presynaptic terminals, downstream of synaptic specificity molecule SYG-1. SYD-1 is likely to act upstream of SYD-2 to positively regulate its synaptic assembly activity. These data imply a hierarchical organization of presynaptic assembly, in which transmembrane specificity molecules initiate synaptogenesis by recruiting a few key scaffolding proteins, which in turn assemble other presynaptic components.Cellular and molecular processes during synapse formation and maturation dictate specificity and types and strength of synaptic connections between neurons, ultimately determining the functional properties of neural circuits. It is believed that synapse formation is triggered by contact between synaptic partners, which induces the transformation of a patch of unspecialized plasma membrane of the presynaptic neuron into a presynaptic apparatus. Presynaptic sites are structurally characterized by a pool of synaptic vesicles and active zones, where synaptic vesicles undergo exocytosis 1 . Functionally, neurotransmitter Author Contributions: M.R.P. and K.S. designed the experiments, analyzed the data and wrote the paper. M.R.P, E.K.L. and V.Y.P performed the experiments. J.G.C. and C.I.B. designed the ASI synapse screen. J.G.C. isolated syd-2(ky292) and M.Z. contributed the GFP::SYD-2 construct. Competing Interests Statement:The authors declare that they have no competing financial interests. release is a multistep process, which involves coordinated actions of many presynaptic proteins. How various molecular components are organized into such complex machinery during development is an unresolved question. NIH Public AccessA number of membrane molecules have been implicated in synapse development. Transmembrane molecules are attractive candidates for initiating presynaptic differentiation when an axon comes in contact with a potential postsynaptic target 2 . For example, postsynaptically expressed neuroligin is capable of clustering β-neurexin in the presynaptic neuron, which then causes accumulation of synaptic vesicles 3,4 . Similarly, synCAM, another homophilic trans-membrane protein, can initiate presynaptic assembly in vitro 5 . Other transmembrane proteins have a role in patterning synapses although they are not required for synapse formation per se. The immunoglobulin superfamily (IgSF) proteins Sdk-...

Summary Mutant mitochondrial genomes (mtDNA) can be viewed as selfish genetic elements that persist in a state of heteroplasmy despite having potentially deleterious metabolic consequences. We sought to study regulation of selfish mtDNA dynamics. We establish that the large 3.1kb deletion-bearing mtDNA variant uaDf5 is a selfish genome in Caenorhabditis elegans. Next, we show that uaDf5 mutant mtDNA replicates in addition to, not at the expense of, wildtype mtDNA. These data suggest existence of homeostatic copy number control that is exploited by uaDf5 to ‘hitchhike’ to high frequency. We also observe activation of the mitochondrial unfolded protein response (UPRmt) in uaDf5 animals. Loss of UPRmt causes a decrease in uaDf5 frequency whereas its constitutive activation increases uaDf5 levels. UPRmt activation protects uaDf5 from mitophagy. Taken together, we propose that mtDNA copy number control and UPRmt represent two homeostatic response mechanisms that play important roles in regulating selfish mitochondrial genome dynamics.

SUMMARY Hsp70s are important cancer chaperones that act upstream of Hsp90 and exhibit independent anti-apoptotic activities. To develop chemical tools for the study of human Hsp70, we developed a homology model that unveils a previously unknown allosteric site located in the nucleotide binding domain of Hsp70. Combining structure-based design and phenotypic testing, we discovered a previously unknown inhibitor of this site, YK5. In cancer cells, this compound is a potent and selective binder of the cytosolic but not the organellar human Hsp70s and has biological activity partly by interfering with the formation of active oncogenic Hsp70/Hsp90/client protein complexes. YK5 is a small molecule inhibitor rationally designed to interact with an allosteric pocket of Hsp70 and represents a previously unknown chemical tool to investigate cellular mechanisms associated with Hsp70.

Escape from antagonism by hepatitis C and related viruses has repeatedly evolved in antiviral factor MAVS via convergent evolution, revealing an ancient history of previous viral encounters in primates.

During synaptogenesis, macromolecular protein complexes assemble at the pre- and postsynaptic membrane. Extensive literature identifies numerous transmembrane molecules sufficient to induce synapse formation and several intracellular scaffolding molecules responsible for assembling active zones and recruiting synaptic vesicles. However, little is known about the molecular mechanisms coupling membrane receptors to active zone molecules during development. Using C.elegans, we identify an F-actin network present at nascent presynaptic terminals required for presynaptic assembly. We unravel a sequence of events where specificity-determining adhesion molecules define the location of developing synapses and locally assemble F-actin. Next, an adaptor protein NAB-1/Neurabin binds to F-actin and recruits active zone proteins, SYD-1 and SYD-2/Liprin-α by forming a tripartite complex. NAB-1 localizes transiently to synapses during development and is required for presynaptic assembly. Together, we identify a role for the actin cytoskeleton during presynaptic development and characterize a molecular pathway where NAB-1 links synaptic partner recognition to active zone assembly.