RTN3L and CALCOCO1 function in parallel to maintain proteostasis in the endoplasmic reticulum.

ABSTRACT

Reticulophagy is mediated by autophagy receptors that function in one of the two domains of the ER, tubules or flat sheets. Three different conserved mammalian receptors mediate autophagy in ER tubules RTN3L, ATL3 and CALCOCO1. Previous studies have shown that RTN3L maintains proteostasis by targeting mutant aggregation-prone proteins for autophagy at distinct foci in ER tubules that we named ERPHS (ER-reticulophagy sites). The role for ATL3 and CALCOCO1 in proteostasis has not been addressed. Here we analyzed three different misfolded disease-causing RTN3L substrates and show that ATL3 and CALCOCO1 target the same cargoes for autophagy. Colocalization and knock down studies revealed that RTN3L and ATL3 are both required for the formation of RTN3L-containing ERPHS, while CALCOCO1 is not. We propose that RTN3L, ATL3 and CALCOCO1 work in parallel to maintain proteostasis within the ER network by targeting cargoes at different sites in the tubules.Abbreviation ATL3 atlastin GTPase 3; Baf bafilomycin A1; CALCOCO1 calcium binding and coiled-coil domain 1; Epr1 ER-phagy receptor 1; ER endoplasmic reticulum; ERAD ER-associated protein degradation; ERPHS ER-reticulophagy sites; LAMP1 lysosomal associated membrane protein 1; PGRMC1 progesterone receptor membrane component 1; POMC proopiomelanocortin; Pro-AVP pro-arginine vasopressin; RETREG1 reticulophagy regulator 1; reticulophagy endoplasmic reticulum selective autophagy; RTN3L reticulon 3 long isoform; VAPA VAMP associated protein A.