A diacidic motif determines unconventional secretion of wild-type and ALS-linked mutant SOD1

Abstract

Starvation-induced unconventional secretion of Acb1 requires ESCRT-I, -II, and -III and Grh1. Cruz-Garcia et al. report that SOD1 and its mutant form linked to amyotrophic lateral sclerosis are also secreted upon nutrient starvation in a Grh1- and ESCRT-I–, -II–, and -III–dependent process. The authors identify a conserved diacidic motif in Acb1 and SOD1 that is necessary for their export in yeast and human cells.

Reactive oxygen species (ROS) triggers unconventional secretion of antioxidants and Acb1

“…Culturing yeast in potassium acetate promotes the unconventional secretion of SOD1, but is secreted SOD1 functionally active (Cruz-Garcia et al, 2017)? Wild-type and mutant SOD1 linked to the neurodegenerative disorder amyotrophic lateral sclerosis (ALS) are reportedly secreted in a misfolded state when expressed in a motor neuron cell line (Grad et al, 2014), therefore the enzymatic activity of extracellular SOD1 cannot be presumed.…”

“…We employed a zymography-based assay to directly test whether secreted SOD1 is enzymatically active in yeast cells cultured in potassium acetate. A mild cell wall extraction assay developed to detect starvation specific secretion of Acb1 and SOD1 was performed in non-denaturing conditions (Curwin et al, 2016;Cruz-Garcia et al, 2017). Intracellular and secreted proteins were then separated by native-gel electrophoresis and a standardized in-gel SOD1 activity assay was performed as described previously (Beauchamp and Fridovich, 1971).…”

“…1 C; Table S1). Secretion of SOD1 (and Acb1) requires Grh1 (Curwin et al, 2016;Cruz-Garcia et al, 2017), so we first tested the involvement of Grh1 in secretion of these enzymes. A secretion assay of wild-type and grh1D starved cells revealed that the release of Ahp1, Trx1 and Trx2 upon starvation is also Grh1 dependent ( Fig.…”

“…FGF2 follows a Type II route, whereas, Acb1 takes a Type III path via that involves a cellular compartment called CUPS, the cytoplasmic proteins Grh1 and a subset of ESCRTs (Duran et al, 2010;Cruz-Garcia et al, 2014;Steringer et al, 2015;Curwin et al, 2016;Steringer et al, 2017;Cruz-Garcia et al, 2018). More recently, the export of cytoplasmic enzyme superoxide dismutase 1 (SOD1) was reported to follow the same pathway as Acb1, and both depend on a di-acidic motif (Cruz-Garcia et al, 2017). The release of IL-1ß, however, does not appear to have a unifying theme, and depending on the cell type or stimulus different mechanisms have been proposed that include pore formation via gasdermin D, autophagymediated, and via an intermediate membrane compartment (Rubartelli et al, 1990;Andrei et al, 2004;Dupont et al, 2011;Liu et al, 2016;Chiritoiu et al, 2019).…”

Exploring the context of diacidic motif D-E as a signal for unconventional protein secretion in eukaryotic proteins

“…By systematically comparing the superoxide dismutase 1 (SOD1) from human and mouse cells with their extracellular SOD1 counterparts from human, mouse and yeast cells, a diacidic motif (asp-glu or D-E) had been identified to be responsible for its unconventional secretion. Of the 6 amino acid insertion in SOD1, compared to its extracellular homologs, UPS was found to be sensitive to the mutation of two amino acids aspartate (D) and glutamate (E) [16]. As a first and concrete observation, this finding raises the possibility that DE could be a generic UPS signal motif.…”