Prefused lysosomes cluster on autophagosomes regulated by VAMP8

Lysosome–autophagosome fusion is critical to autophagosome maturation. Although several proteins that regulate this fusion process have been identified, the prefusion architecture and its regulation remain unclear. Herein, we show that upon stimulation, multiple lysosomes form clusters around individual autophagosomes, setting the stage for membrane fusion. The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein on lysosomes—vesicle-associated membrane protein 8 (VAMP8)—plays an important role in forming this prefusion state of lysosomal clusters. To study the potential role of phosphorylation on spontaneous fusion, we investigated the effect of phosphorylation of C-terminal residues of VAMP8. Using a phosphorylation mimic, we observed a decrease of fusion in an ensemble lipid mixing assay and an increase of unfused lysosomes associated with autophagosomes. These results suggest that phosphorylation not only reduces spontaneous fusion for minimizing autophagic flux under normal conditions, but also preassembles multiple lysosomes to increase the fusion probability for resuming autophagy upon stimulation. VAMP8 phosphorylation may thus play an important role in chemotherapy drug resistance by influencing autophagosome maturation.

Conserved arginine residues in synaptotagmin 1 regulate fusion pore expansion through membrane contact

Synaptotagmin 1 is a vesicle-anchored membrane protein that functions as the Ca2+ sensor for synchronous neurotransmitter release. In this work, an arginine containing region in the second C2 domain of synaptotagmin 1 (C2B) is shown to control the expansion of the fusion pore and thereby the concentration of neurotransmitter released. This arginine apex, which is opposite the Ca2+ binding sites, interacts with membranes or membrane reconstituted SNAREs; however, only the membrane interactions occur under the conditions in which fusion takes place. Other regions of C2B influence the fusion probability and kinetics but do not control the expansion of the fusion pore. These data indicate that the C2B domain has at least two distinct molecular roles in the fusion event, and the data are consistent with a model where the arginine apex of C2B positions the domain at the curved membrane surface of the expanding fusion pore.

Investigation of the Calculation of Coupled Channels for Some Halo Systems

The fusion reaction for systems involving halo nuclei are investigated by two- and multicoupled channel calculations for the systems 8B+58Ni, 11Be+209Bi, and 15C+232Th. The effect of coupling between the breakup channel and the elastic channel have been considered using the Continuum Discretized Coupled Channels (CDCC) method in full quantum and semiclassical approaches. The calculation of the fusion cross-section qfus (mb), fusion barrier distribution Dfus (mb/MeV) and fusion probability Pfus reproduces the measured data for the systems under study quite well above and below the Coulomb barrier VB. In the case of two-channel coupling both in semiclassical and quantum mechanical approaches, the measured data above the Coulomb barrier VB are overestimated.

Study on the compound nucleus fusion probability PCN in 12C +93Nb reaction at above barrier energies using different degrees-of-freedom

In this work, we extend our earlier study [S. Chopra, Hemdeep, A. Kaur and R. K. Gupta, Phys. Rev. C 93 (2016) 024603] of [Formula: see text] reaction at below- and near- to above-barrier energies for both coplanar ([Formula: see text]) and noncoplanar ([Formula: see text]) degrees-of-freedom. Our main aim is to see the texture of variation of compound nucleus (CN) fusion probability [Formula: see text] at all the experimentally available center-of-mass energies ([Formula: see text]) in the range 41–69[Formula: see text]MeV, using the dynamical cluster-decay model (DCM). In this study, we worked for both degrees-of-freedom, having compact configurations ([Formula: see text]) and including higher multipole deformations (quadrupole to hexadecapole, [Formula: see text]) of nuclei. In the work of above-mentioned reference, for [Formula: see text]Ag[Formula: see text] (weakly fissioning nuclear system), we found a discrepancy in [Formula: see text], i.e., increasing function with increasing [Formula: see text], belongs to superheavy nuclei for the case of [Formula: see text] [see Fig. Fig.4 of [A. Kaur, S. Chopra and R. K. Gupta, Phys. Rev. C 90 (2014) 024619], but this erroneous result moved to correct one after the inclusion of [Formula: see text] (decreasing with increasing [Formula: see text], weakly fissioning class), which encouraged us to check the trend of [Formula: see text] at all available energies. Our evaluation framework shows the noncoplanar configuration, with higher multipole deformations [Formula: see text] included, as the most probable configuration for studying the heavy-ion reactions [S. Chopra, Hemdeep, P. Kaushal and R. K. Gupta, Phys. Rev. C 98 (2018) 041603(R)]. Second, our interest is to check the consistency of experimental data, where the experimentalists have given the data with the mixing of three different experiments for same reaction. In this study, we have found that our results are in good agreement and consistent with only the latest experimental data at five [Formula: see text]’s (=41.097, 47.828, 54.205, 60.051 and 65.454[Formula: see text]MeV), i.e., showing large ([Formula: see text]90%) noncompound nucleus contribution (or equivalently quasi-fission/Incomplete fusion) in the (total) fusion cross-section.

Fusion probability of massive nuclei in reactions leading to heavy composite nuclear systems

Reactions between massive nuclei show a considerable reduction in fusion-evaporation cross-sections at the Coulomb barrier according to the comparison of experimental values with those calculated by barrier passing (BP) and statistical model (SM) approximations. Reduced fusion cross-sections corresponding to fusion probability PCN<1 are accompanied by a high probability of deep-inelastic and quasi-fission processes arising on the way to fusion. At the same time, the excitation functions for evaporation residues (ERs) obtained in very mass-asymmetric projectile-target combinations are well described in the framework of the BP model (assuming PCN=1) and SM approximations. In the framework of SM, the survivability of produced heavy nuclei can be described with the use of adjusted macroscopic fission barriers. Fusion suppression appears in less asymmetric combinations, for which PCN values can be estimated using survivability obtained for very asymmetric ones leading to the same CN. An attempt was made to systemize the PCN data derived from different projectile-target combinations leading to ERs in the range from Pb to the most heavies, which are compared withPCN values obtained in fission experiments.

Variable priming of a docked synaptic vesicle

The priming of a docked synaptic vesicle determines the probability of its membrane (VM) fusing with the presynaptic membrane (PM) when a nerve impulse arrives. To gain insight into the nature of priming, we searched by electron tomography for structural relationships correlated with fusion probability at active zones of axon terminals at frog neuromuscular junctions. For terminals fixed at rest, the contact area between the VM of docked vesicles and PM varied >10-fold with a normal distribution. There was no merging of the membranes. For terminals fixed during repetitive evoked synaptic transmission, the normal distribution of contact areas was shifted to the left, due in part to a decreased number of large contact areas, and there was a subpopulation of large contact areas where the membranes were hemifused, an intermediate preceding complete fusion. Thus, fusion probability of a docked vesicle is related to the extent of its VM–PM contact area. For terminals fixed 1 h after activity, the distribution of contact areas recovered to that at rest, indicating the extent of a VM–PM contact area is dynamic and in equilibrium. The extent of VM–PM contact areas in resting terminals correlated with eccentricity in vesicle shape caused by force toward the PM and with shortness of active zone material macromolecules linking vesicles to PM components, some thought to include Ca2+ channels. We propose that priming is a variable continuum of events imposing variable fusion probability on each vesicle and is regulated by force-generating shortening of active zone material macromolecules in dynamic equilibrium.