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VAMP-associated protein (VAP) is a type II integral transmembrane protein at the endoplasmic reticulum (ER). Mutations in human VAPB/ALS8 are associated with amyotrophic lateral sclerosis (ALS). The N-terminal major sperm protein (MSP) domain of VAPB (Drosophila Vap33) is cleaved, secreted, and acts as a signaling ligand for several cell-surface receptors. Although extracellular functions of VAPB are beginning to be understood, it is unknown how the VAPB/Vap33 MSP domain facing the cytosol is secreted to the extracellular space. Here we show that Vap33 uses the ER-Golgi conventional secretion pathway to reach the plasma membrane, where the MSP domain is exposed extracellularly by topological inversion. The externalized MSP domain is cleaved by Matrix metalloproteinase 1/2 (Mmp1/2). Overexpression of Mmp1 restores decreased levels of extracellular MSP domain derived from ALS8-associated Vap33 mutants. We propose an unprecedented secretion mechanism for an ER-resident membrane protein, which may contribute to ALS8 pathogenesis.

The periodic circumferential cytoskeleton supports various tubular tissues1-5. Radial expansion of the tube lumen causes anisotropic tensile stress, which can be exploited as a geometric cue6. However, the molecular machinery linking anisotropy to robust circumferential patterning is poorly understood. Here, we aimed to reveal the emergent process of circumferential actin cable formation in a Drosophila tracheal tube7. During luminal expansion, sporadic actin nanoclusters emerge and exhibit circumferentially biased motion and fusion. RNAi screening revealed the formin family protein, DAAM, as an essential component of tensile stress sensing, and non-muscle myosin II as a component required for nanocluster fusion. A coarse-grained molecular dynamics simulation demonstrated that crosslinkers play a crucial role in nanocluster formation and cluster-to-cable transition occurs in response to mechanical anisotropy. Altogether, we propose that an actin nanocluster is an organizational unit that senses stress in the cortical membrane and builds a higher-order cable structure.