Tetraspanins — characterized by four transmembrane helices spanning the lipid bilayer — are best known as membrane organizers, assembling into tetraspanin-enriched microdomains (TEMs) that recruit various protein partners and lipids. Notably, recent studies have revealed that some tetraspanins are capable of forming ordered polymeric assemblies — for instance, TSPAN22 and TSPAN23 form linear polymers in retinal photoreceptors, while TSPAN21 and TSPAN20 assemble into hexagonal arrays on urothelial plaques. Among them, TSPAN7 has been linked to intellectual disability, viral infection, diabetes, and cancer progression, and has been shown to promote the formation of filopodia and dendrites, yet the molecular mechanism by which it shapes and stabilizes tubular membranes has remained unclear.
In this study, the authors reveal that TSPAN7 senses membrane curvature and polymerizes into a helical assembly on tubular membranes. By combining live-cell imaging, in vitro reconstitution, and in situ cryo-electron tomography, they show that TSPAN7 protomers self-assemble into a multi-start helix through two conserved interaction interfaces, forming a "transmembrane skeleton" that resists mechanical deformation.
TSPAN7 was found to be highly enriched on retraction fibers and tunneling nanotubes (Fig. 1a), and its overexpression promotes the formation of both structures. To test whether TSPAN7 directly senses and stabilizes curvature, the authors performed in vitro reconstitution using giant unilamellar vesicles (GUVs). In GUVs without TSPAN7, the reconstituted tubular network quickly transformed into vesicles. However, in GUVs embedded with recombinant TSPAN7 protein, TSPAN7 rapidly accumulated on and stabilized the reconstituted tubular network, demonstrating that TSPAN7 alone is sufficient for curvature sensing and membrane stabilization (Fig. 1b).
Complementing these reconstitution experiments, fluorescence recovery after photobleaching (FRAP) analysis on cellular retraction fibers and tunneling nanotubes revealed that TSPAN7 was remarkably immobile — its fluorescence showed virtually no recovery after bleaching. This behavior was in sharp contrast to other tested tetraspanin family members, which exhibited normal lateral diffusion. The immobility was independent of F-actin, as latrunculin A treatment did not disrupt it, and was most likely attributable to TSPAN7 having assembled into an ordered polymeric structure.
To visualize this assembly, in situ cryo-EM and cryo-ET were employed. TSPAN7 assembled into a highly ordered right-handed helical structure on retraction fibers, with multiple protofilaments winding around the membrane tube (Fig. 2). Endogenous TSPAN7 in lymphatic endothelial cells formed identical spirals, confirming that the assembly was not an overexpression artifact. Structural analysis further identified two key interaction interfaces between TSPAN7 protomers. A double mutant that disrupted both interfaces lost the ability to form spirals, showed increased mobility, and failed to stabilize tubular membranes in vitro.
The biological significance of the spiral assembly was then examined using a microfluidic flow assay. Cells expressing the spiral-deficient mutant showed dramatic membrane protrusion elongation under shear flow, while cells expressing wild-type TSPAN7 resisted deformation. The spiral assembly effectively prevented tube constriction, functioning as a structural scaffold embedded within the lipid bilayer. Intriguingly, TSPAN7 and F-actin appear to occupy non-overlapping regions on retraction fibers, suggesting that the transmembrane skeleton and the classical cytoskeleton may work in tandem to maintain tubular membrane integrity.
This study provides two key conceptual advances in our understanding of membrane biology. First, it reveals that the transmembrane protein TSPAN7 can autonomously sense membrane curvature and polymerize into a helical assembly, establishing a "transmembrane skeleton" that operates from within the lipid bilayer — a previously unrecognized class of membrane skeletal architecture distinct from the classical cytoskeleton. Second, it demonstrates that tubular membrane stabilization does not rely solely on cytoplasmic scaffolding; instead, transmembrane proteins can serve as intrinsic structural reinforcement, providing mechanical resilience to the cell's most delicate membrane protrusions.
DOI: 10.15302/vita.2026.05.0039