To investigate whether, for the same mutation, M1 and M87 (M85 in mice) differ in their intracellular localization and biophysical properties, we generated WT constructs, that were C-terminally tagged with GFP or Flag, of these two isoforms (Fig. S1A), as well as constructs harboring the missense mutation C448Y (C445Y in mice), which has been previously described (Errico et al., 2002; White et al., 2007; Solowska et al., 2014; Leo et al., 2017) and found in HSP-SPG4 patients (Hazan et al., 1999; Fonknechten et al., 2000). Here, SPAST is linked to hereditary spastic paraplegia.