Macroautophagy (hereafter, autophagy) is essential for the degradation of mitochondria from yeast to humans. Mitochondrial autophagy in yeast is initiated when the selective autophagy scaffolding protein Atg11 is recruited to mitochondria through its interaction with the selective autophagy receptor Atg32. This also results in the recruitment of small 30-nm vesicles that fuse to generate the initial phagophore membrane. We demonstrate that Atg11 can bind to autophagic-like membranes in vitro in a curvature-dependent manner in part via a predicted amphipathic helix. Deletion of the amphipathic helix from Atg11 results in a delay in the formation of mitophagy initiation sites in yeast. Furthermore, using a novel biochemical approach, we demonstrate that the interaction between Atg11 and Atg32 results in the tethering of autophagic-like vesicles in clusters to giant unilamellar vesicles containing a lipid composition designed to mimic the outer mitochondrial membrane. We also demonstrate that the N-terminal region of Atg11 is an important mediator of vesicle tethering to cargo mimetics and that clustering of autophagic-like vesicles requires the C-terminal region of Atg11. Taken together, our results reveal that Atg11 clusters into high-order oligomers, can tether autophagic-like membranes and due to its ability to oligomerize can cluster vesicles on the surface of cargo mimetics. This work provides new insight into the mechanisms of protein and membrane clustering by Atg11. Given the increasing importance of protein oligomerization and clustering in autophagy, these results have important implications in the initiation of mitochondrial autophagy.Abbreviations Atg11: autophagy related 11; Atg11-Cterm: C-terminal region of Atg11; Atg11-Nterm: N-terminal region of Atg11; Atg32: autophagy related 32; COV: coefficient of variance; DOPC: 1,2-dioleoyl-sn-glycero-3-phosphocholine; DOPE: 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine; DOPS: 1,2-dioleoyl-sn-glycero-3-phospho-L-serine; FRAP: fluorescence recovery after photobleaching; GLT: GUV and liposome tethering; GUV: giant unilamellar vesicle; MKO: multiple knockout; OMM: outer mitochondrial membrane; PC: phosphatidylcholine; PE: phosphatidylethanolamine; PtdIns: phosphatidylinositol; PtdIns3P: phosphatidylinositol-3-phosphate; RhPE: rhodamine phosphatidylethanolamine; SAR: selective autophagy receptor; SEC: size-exclusion chromatography; SLB: supported lipid bilayers; SMrT: supported membrane templates; YPL: yeast polar lipids.

Many bacteria have hemerythrin (Hr) proteins that bind O-2, including Pseudomonas aeruginosa, in which microoxia-induced Hr (Mhr) provide fitness advantages under microoxic conditions. Mhr has a 23 amino-acid extension at its C-terminus relative to a well-characterized Hr from Methylococcus capsulatus, and similar extensions are also found in Hrs from other bacteria. The last 11 amino acids of this extended, C-terminal tail are highly conserved in gammaproteobacteria and predicted to form a helix with positively charged and hydrophobic faces. In cellular fractionation assays, wild-type (WT) Mhr was found in both membrane and cytosolic fractions, while a Mhr(W143*) variant lacking the last 11 residues was largely in the cytosol and did not complement Mhr function in competition assays. Mhr(L112Y), a variant that has a much longer-lived O-2-bound form, was fully functional and had a similar localization pattern to that of WT Mhr. Both Mhr(W143*) and Mhr(L112Y) had secondary structures, stabilities, and O-2-binding kinetics similar to those of WT Mhr. Fluorescence studies revealed that the C-terminal tail, and particularly the fragment corresponding to its last 11 residues, was sufficient and necessary for association with lipid vesicles. Molecular dynamics simulations and subsequent cellular analysis of Mhr variants have demonstrated that conserved, positively charged residues in the tail are important for Mhr interactions with negatively charged membranes and the contribution of this protein to competitive fitness. Together, these data suggest that peripheral interactions of Mhr with membranes are guided by the C-terminal tail and are independent of O-2-binding.

Eukaryotes maintain cellular health through the engulfment and subsequent degradation of intracellular cargo using macroautophagy. The function of Atg23, despite being critical to the efficiency of this process, is unclear due to a lack of biochemical investigations and an absence of any structural information. In this study, we use a combination of in vitro and in vivo methods to show that Atg23 exists primarily as a homodimer, a conformation facilitated by a putative amphipathic helix. We utilize small-angle X-ray scattering to monitor the overall shape of Atg23, revealing that it contains an extended rod-like structure spanning approximately 320 Å. We also demonstrate that Atg23 interacts with membranes directly, primarily through electrostatic interactions, and that these interactions lead to vesicle tethering. Finally, mutation of the hydrophobic face of the putative amphipathic helix completely precludes dimer formation, leading to severely impaired subcellular localization, vesicle tethering, Atg9 binding, and autophagic efficiency.

Discovery-based proteomics workflows that identify novel interactors rely on immunoprecipitations or pull-downs with genetically-tagged bait proteins immobilized on appropriate matrices. But strategies to analyse protein interactions on a diffusible membrane surface combined with the practical ease of pull-downs remain unavailable. Such strategies are important to analyse protein complexes that mature in composition and stability because of diffusion-based encounter between participant proteins. Here, we describe a generic pull-down strategy to analyse such complexes using chelating lipid-containing supported bilayers formed on silica beads. These templates can display desired His-tagged bait proteins on a diffusible membrane surface. Using clathrin-mediated endocytosis as a paradigm, we find that the clathrin-binding adaptor protein epsin1 displayed as bait on these templates pulls down significantly higher amounts of clathrin from brain lysates than when immobilized on conventional matrices. Together, our results establish the potential of such templates as superior matrices for analysing protein-protein interactions and resultant complexes formed on membrane surfaces.

CSF levels of 5-hydroxyindolacetic acid (5-HIAA), the serotonin metabolite, were assayed in 10 violent and 10 matched nonviolent patients with schizophrenia. Mean group levels of 5-HIAA in cerebrospinal fluid were found to be nearly identical. Possible explanations, including effects of medications, are discussed.

Small 30-nm vesicles containing the integral membrane protein Atg9 provide the initial membrane source for autophagy in yeast. Atg23 is an Atg9 binding protein that is required for Atg9 vesicle trafficking but whose exact function is unknown. In our recent paper, we explored the function of Atg23 using an approach combining cellular biology and biochemistry on purified protein. We determined that Atg23 is an elongated dimer spanning 320 angstrom in length. We also demonstrated that Atg23 is a membrane-binding and -tethering protein. Furthermore, we identified a series of amino acids residing in a putative coiled-coil region that when mutated prevent Atg23 dimer formation resulting in a stable Atg23 monomer. Last, we demonstrated that when monomeric Atg23 is expressed in yeast lacking Atg23, this leads to a loss of Atg23 puncta, a reduction in Atg9 puncta, a reduction in nonselective autophagy and a complete block in the cytoplasm-to-vacuole targeting (Cvt) pathway.

Clathrin-mediated endocytosis manages the vesicular transport of the bulk of membrane proteins from the plasma membrane and the trans-Golgi network. During this process, discrete sets of adaptor proteins recognize specific classes of membrane proteins, which recruit and assemble clathrin lattices on the membrane. An important determinant to the success of this vesicular transport reaction is the intrinsic ability of adaptors to polymerize clathrin on a membrane surface. Adaptor-induced clathrin assembly has traditionally been analyzed using static electron microscopy-based approaches. Here, we describe a methodology to follow adaptor-induced clathrin assembly in real-time using fluorescence microscopy on a facile model membrane assay system of supported membrane tubes (SMrT). Results from such assays can be conveniently run through routine image analysis procedures to extract kinetic parameters of the clathrin assembly reaction.