Structure of membrane tethers and their role in fusion

Abstract

Vesicular transport between different membrane compartments is a key process in cell biology required for the exchange of material and information. The complex machinery that executes the formation and delivery of transport vesicles has been intensively studied and yielded a comprehensive view of the molecular principles that underlie the budding and fusion process. Tethering also represents an essential step in each trafficking pathway. It is mediated by Rab GTPases in concert with so-called tethering factors, which constitute a structurally diverse family of proteins that share a similar role in promoting vesicular transport. By simultaneously binding to proteins and/or lipids on incoming vesicles and the target compartment, tethers are thought to bridge donor and acceptor membrane. They thus provide specificity while also promoting fusion. However, how tethering works at a mechanistic level is still elusive.We here discuss the recent advances in the structural and biochemical characterization of tethering complexes that provide novel insight on how these factors might contribute the efficiency of fusion. K E Y W O R D S CATCHR, golgin, membrane fusion, Rab GTPases, Sec1/Munc18, SNARE, tethering, vesicular transport 1 | INTRODUCTION Compartmentalization of eukaryotic cells into organelles is the prerequisite for the formation of chemically distinct environments that allow the implementation of specialized functions. Communication and the exchange of substances between different membrane compartments, however, is also essential for proper function of cells. In the endomembrane system, comprising the secretory pathway and the endolysosomal system, membrane vesicles have long been established as transport carriers that fulfill this function. 1 Trafficking can be dissected into distinct steps: at the donor membrane, small GTPases like Arf, Sar1 or Arf-like (Arl) and coat proteins, which assemble into vesicle coat complexes, orchestrate cargo sorting and concentration followed by vesicle budding and scission. Vesicles are then transported along cytoskeletal filaments with the help of motor proteins. At the target membrane, incoming vesicles are recruited by Rab GTPases and tethering factors. Rabs are markers of organelle identity and conserved part of the fusion machinery. 2,3 Common to all small GTPases, Rabs cycle between an inactive GDP-bound and an active GTP-bound form, and only the latter form can interact with effectors that mediate downstream functions of Rabs. Tethering factors from different classes are evolutionarily unrelated proteins and structurally divers, but share a function as Rab effectors and are thought to bridge two opposing membranes. After tethering, SNAREs (soluble N-ethylmaleimide sensitive factor attachment protein receptor) on each membrane are necessary. 4 SNARE proteins drive fusion, but function of SNAREs requires S/M (Sec1/Munc18) family proteins that act as chaperones of SNARE assembly and the NSF (N-ethylmaleimide sensitive factor) ATPase that with the adaptor protein α-SNAP disasse...

Membrane Curvature, Trans-Membrane Area Asymmetry, Budding, Fission and Organelle Geometry

“…An important aspect of membrane geometry in cells is the speed of the curvature delivery/extraction during fusion of membrane organelles, and the speed of alterations in their volume and surface area. For instance, the pulling tether is stable when it is formed slowly, and can be disrupted at higher speed [ 26 , 27 , 28 , 29 ]. Experiments with quick freezing of synapses, after their stimulation, demonstrated that lipid physics is responsible for these extremely fast formations of buds, on the presynaptic membrane after the fusion of synaptic vesicles with it [ 30 , 31 ].…”

Structural basis for the binding of SNAREs to the multisubunit tethering complex Dsl1

“…In vivo, SNAREs collaborate with other factors including Sec1/Munc18 (SM) proteins and multisubunit tethering complexes (MTCs). SM proteins function as SNARE chaperones, regulating the assembly of SNARE complexes (11,(15)(16)(17), whereas MTCs act upstream of SNARE complex assembly, mediating the initial attachment of a vesicle to its target membrane (17)(18)(19). Vesicle tethering canonically involves interactions between MTCs and membrane-associated Rab proteins, but can also involve MTC•SNARE, MTC•coat, MTC•golgin, and/or MTC•phospholipid interactions.…”

“…The largest family of MTCs is the CATCHR (Complexes Associated with Tethering Containing Helical Rods) family, whose members function largely in trafficking to and from the Golgi (19,26,27). The CATCHR-family complexes -GARP/EARP, exocyst, conserved oligomeric Golgi (COG), and Dsl1 -contain 3-8 subunits each.…”

Structural basis for the binding of SNAREs to the multisubunit tethering complex Dsl1

“…In vivo, SNAREs collaborate with other factors including Sec1/Munc18 (SM) proteins and multisubunit-tethering complexes (MTCs). SM proteins function as SNARE chaperones, regulating the assembly of SNARE complexes (11,(15)(16)(17), whereas MTCs act upstream of SNARE complex assembly, mediating the initial attachment of a vesicle to its target membrane (17)(18)(19). Vesicle tethering canonically involves interactions between MTCs and membrane-associated Rab proteins but can also involve MTCSNARE, MTCcoat, MTCgolgin, and/or MTCphospholipid interactions.…”