Mono-UFMylation promotes misfolding-associated secretion of -synuclein

Stressed cells secret misfolded proteins lacking signaling sequence via an unconventional protein secretion (UcPS) pathway, but how misfolded proteins are targeted selectively in UcPS is unclear. Here, we report that misfolded UcPS clients are subject to modification by a ubiquitin-like protein named ubiquitin-fold modifier 1 (UFM1). Using alpha-synuclein (alpha-Syn) as a UcPS model, we show that mutating the UFMylation sites in alpha-Syn or genetic inhibition of the UFMylation system mitigates alpha-Syn secretion, whereas overexpression of UFBP1, a component of the endoplasmic reticulum-associated UFMylation ligase complex, augments alpha-Syn secretion in mammalian cells and in model organisms. UFM1 itself is cosecreted with alpha-Syn, and the serum UFM1 level correlates with that of alpha-Syn. Because UFM1 can be directly recognized by ubiquitin specific peptidase 19 (USP19), a previously established UcPS stimulator known to associate with several chaperoning activities, UFMylation might facilitate substrate engagement by USP19, allowing stringent and regulated selection of misfolded proteins for secretion and proteotoxic stress alleviation.