Cysteine proteases of the Clan CA (papain) family are the predominant protease group in primitive invertebrates. Cysteine protease inhibitors arrest infection by the protozoan parasite, Trypanosoma brucei. RNA interference studies implicated a cathepsin B-like protease, tbcatB, as a key inhibitor target. Utilizing parasites in which one of the two alleles of tbcatb has been deleted, the key role of this protease in degradation of endocytosed host proteins is delineated. TbcatB deficiency results in a decreased growth rate and dysmorphism of the flagellar pocket and the subjacent endocytic compartment. Western blot and microscopic analysis indicate that deficiency in tbcatB results in accumulation of both host and parasite proteins, including the lysosomal marker p67. A critical function for parasitism is the degradation of host transferrin, which is necessary for iron acquisition. Substrate specificity analysis of recombinant tbcatB revealed the optimal peptide cleavage sequences for the enzyme and these were confirmed experimentally using FRET-based substrates. Degradation of transferrin was validated by SDS-PAGE and the specific cleavage sites identified by N-terminal sequencing. Because even a modest deficiency in tbcatB is lethal for the parasite, tbcatB is a logical target for the development of new anti-trypanosomal chemotherapy.

In a forward genetic screen for mutations that destabilize the neuromuscular junction, we identified a novel long isoform of Drosophila ankyrin2 (ank2-L). We demonstrate that loss of presynaptic Ank2-L not only causes synapse disassembly and retraction but also disrupts neuronal excitability and NMJ morphology. We provide genetic evidence that ank2-L is necessary to generate the membrane constrictions that normally separate individual synaptic boutons and is necessary to achieve the normal spacing of subsynaptic protein domains, including the normal organization of synaptic cell adhesion molecules. Mechanistically, synapse organization is correlated with a lattice-like organization of Ank2-L, visualized using extended high-resolution structured-illumination microscopy. The stabilizing functions of Ank2-L can be mapped to the extended C-terminal domain that we demonstrate can directly bind and organize synaptic microtubules. We propose that a presynaptic Ank2-L lattice links synaptic membrane proteins and spectrin to the underlying microtubule cytoskeleton to organize and stabilize the presynaptic terminal.

BACKGROUND: Among the most prominent molecular constituents of a recycling synaptic vesicle is the clathrin triskelion, composed of clathrin light chain (Clc) and clathrin heavy chain (Chc). Remarkably, it remains unknown whether clathrin is strictly necessary for the stimulus-dependent re-formation of a synaptic vesicle and, conversely, whether clathrin-independent vesicle endocytosis exists at the neuronal synapse.

RESULTS: We employ FlAsH-FALI-mediated protein photoinactivation to rapidly (3 min) and specifically disrupt Clc function at the Drosophila neuromuscular junction. We first demonstrate that Clc photoinactivation does not impair synaptic-vesicle fusion. We then provide electrophysiological and ultrastructural evidence that synaptic vesicles, once fused with the plasma membrane, cannot be re-formed after Clc photoinactivation. Finally, we demonstrate that stimulus-dependent membrane internalization occurs after Clc photoinactivation. However, newly internalized membrane fails to resolve into synaptic vesicles. Rather, newly internalized membrane forms large and extensive internal-membrane compartments that are never observed at a wild-type synapse.

CONCLUSIONS: We make three major conclusions. (1) FlAsH-FALI-mediated protein photoinactivation rapidly and specifically disrupts Clc function with no effect on synaptic-vesicle fusion. (2) Synaptic-vesicle re-formation does not occur after Clc photoinactivation. By extension, clathrin-independent "kiss-and-run" endocytosis does not sustain synaptic transmission during a stimulus train at this synapse. (3) Stimulus-dependent, clathrin-independent membrane internalization exists at this synapse, but it is unable to generate fusion-competent, small-diameter synaptic vesicles.

The identification of synaptic partners is challenging in dense nerve bundles, where many processes occupy regions beneath the resolution of conventional light microscopy. To address this difficulty, we have developed GRASP, a system to label membrane contacts and synapses between two cells in living animals. Two complementary fragments of GFP are expressed on different cells, tethered to extracellular domains of transmembrane carrier proteins. When the complementary GFP fragments are fused to ubiquitous transmembrane proteins, GFP fluorescence appears uniformly along membrane contacts between the two cells. When one or both GFP fragments are fused to synaptic transmembrane proteins, GFP fluorescence is tightly localized to synapses. GRASP marks known synaptic contacts in C. elegans, correctly identifies changes in mutants with altered synaptic specificity, and can uncover new information about synaptic locations as confirmed by electron microscopy. GRASP may prove particularly useful for defining connectivity in complex nervous systems.