Biology subjects

Gopaldass, N.

Publications and source records attributed to Gopaldass, N..

3 recordsLinked to original sources

bioRxiv2022

Retromer oligomerization drives SNX-BAR coat assembly and membrane constriction

The retromer coat mediates protein exit from endosomes and impacts many signaling pathways, lysosomal biogenesis, and diseases such as Parkinsons, Alzheimers and COVID-19. Retromer complexes (CSC in yeast) form coats by interconnecting sorting nexins (SNX). The dynamics of this process is poorly explored. Here, we analyze the oligomerization of CSC/SNX-BAR retromer coats on oriented synthetic lipid tubules. SNX-BARs and CSC assemble a static tubular coat that does not exchange subunits. Coat formation proceeds bidirectionally, adding new subunits at both ends of the coat. High concentrations of SNX-BARs alone suffice to constrict membrane tubes to an invariant radius of 19 nm. At lower concentrations, CSC-complexes must drive constriction, which requires their oligomerization. CSCs populate the SNX-BAR layer at densities that increase with the starting radius of the membrane tube. We hence propose that retromer-mediated crosslinking of SNX-BARs at variable densities tunes the coat according to the energy required to deform the membrane. This model is supported by the effects of mutations interfering with retromer oligomerization, which impair retromer function in yeast and human cells.

cell biology↗

bioRxiv2022

Structural plasticity of Atg18 oligomers: organization of assembled tubes and scaffolds at the isolation membrane

Autophagy-related protein 18 (Atg18) participates in the elongation of early autophagosomal structures in concert with Atg2 and Atg9 complexes. How Atg18 contributes to the structural coordination of Atg2 and Atg9 at the isolation membrane remains to be understood. Here, we determined the cryo-EM structures of Atg18 organized in helical tubes as well as soluble oligomers. The helical assembly is composed of Atg18 tetramers forming a lozenge cylindrical lattice with remarkable structural similarity to the COPII outer coat. When reconstituted with lipid membranes, using subtomogram averaging we determined tilted Atg18 dimer structures bridging two juxtaposed lipid membranes spaced apart by 80 [A]. Together with an AlphaFold Atg18-Atg2 model, we propose that Atg18 oligomers form a structural scaffold coordinating the Atg2 membrane bridge. The observed structural plasticity of Atg18s oligomeric organization and membrane binding provide a molecular framework for the positioning of downstream components of the autophagy machinery.

biophysics↗

bioRxiv2021

CROP: A Retromer-PROPPIN complex mediating membrane fission in the endo-lysosomal system

Endo-lysosomal compartments exchange proteins by fusing, fissioning, and through endosomal transport carriers. Thereby, they sort many plasma membrane receptors and transporters and control cellular signaling and metabolism. How the membrane fission events are catalyzed is poorly understood. Here, we identify the novel CROP complex as a factor acting at this step. CROP joins members of two protein families: the peripheral subunits of retromer, a coat forming endosomal transport carriers, and membrane inserting PROPPINs. Integration into CROP potentiates the membrane fission activity of the PROPPIN Atg18 on synthetic liposomes and confers strong preference for binding PI(3,5)P2, a phosphoinositide required for membrane fission activity. Disrupting CROP blocks fragmentation of lysosome-like yeast vacuoles in vivo. CROP-deficient mammalian endosomes accumulate micrometer-long tubules and fail to export cargo, suggesting that carriers attempt to form but cannot separate from these organelles. PROPPINs compete for retromer binding with the SNX proteins, which recruit retromer to the membrane during the formation of endosomal carriers. Transition from retromer-SNX complexes to retromer-PROPPIN complexes might hence switch retromer activities from cargo capture to membrane fission.

cell biology↗