Hereditary spastic paraplegia (HSP) is a genetically heterogeneous disease caused by mutations in many genes, including those encoding spastin, strumpellin, or REEP1. Allison et al. show that similar lysosomal phenotypes are associated with mutations in different classes of HSP proteins and suggest that defective ER–endosome contacts and endosome tubule fission may be a common cause of axon degeneration in the disease.

Mutations in the gene encoding the microtubule severing ATPase spastin are the most frequent cause of hereditary spastic paraplegia, a genetic condition characterised by length-dependent axonal degeneration. Here, we show that HeLa cells lacking spastin and embryonic fibroblasts from a spastin knock-in mouse model become highly polarised and develop cellular protrusions. In HeLa cells, this phenotype was rescued by wild-type spastin, but not by forms unable to sever microtubules or interact with endosomal ESCRT-III proteins. Cells lacking the spastin-interacting ESCRT-III-associated proteins IST1 or CHMP1B also developed protrusions. The protrusion phenotype required protrudin, a RAB-interacting protein that interacts with spastin and localises to ER-endosome contact sites, where it promotes KIF5-dependent endosomal motility to protrusions. Consistent with this, the protrusion phenotype in cells lacking spastin also required KIF5. Lack or mutation of spastin resulted in functional consequences for receptor traffic of a pathway implicated in HSP, as Bone Morphogenetic Protein receptor distribution became polarised. Our results, therefore, identify a novel role for ESCRT-III proteins and spastin in regulating polarised membrane traffic.

The beta-coronavirus SARS-CoV-2 is the causative agent of the current global COVID-19 pandemic. Coronaviruses are enveloped RNA viruses. Assembly and budding of coronavirus particles occur at the Endoplasmic Reticulum-Golgi Intermediate Compartment (ERGIC), with the structural proteins Nucleocapsid, Spike, Membrane and Envelope facilitating budding and release of virions into the secretory pathway lumen. This allows viral release which can occur through delivery of virus particles to deacidified lysosomes and subsequent lysosomal secretion. Coronaviral Envelope proteins are necessary for coronavirus assembly, play important roles in replication and can form oligomeric cation channels. Whilst synthesised in the ER, the mechanism by which Envelope achieves its steady state localisation to the ERGIC remains unclear. Here, we used fluorescent reporters to illuminate the Envelope protein from SARS-CoV-2. We discovered that internal tagging of this protein is necessary to preserve the functionality of a C-terminal ER-export motif and to allow localisation of Envelope to the ERGIC. Using this non-disruptive form of tagging, we used proximity biotinylation to define the vicinal proteome of wild type and ER-restricted versions of Envelope. We show that both Envelope and the presence of its ER-export motif contribute to the packaging of nucleocapsid into virus like particles. Finally, using our labelled versions of Envelope, we discovered that a minor pool of this protein is delivered to lysosomes. We show that lysosomal Envelope is oligomeric and can contribute to pH neutralisation in these organelles.

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