Phosphorylation of FAM134C by CK2 controls starvation-induced ER-phagy

Abstract

Selective degradation of the endoplasmic reticulum (ER) via autophagy (ER-phagy) is initiated by ER-phagy receptors, which facilitate the incorporation of ER fragments into autophagosomes. FAM134 reticulon family proteins (FAM134A, FAM134B, and FAM134C) are ER-phagy receptors with structural similarities and nonredundant functions. Whether they respond differentially to the stimulation of ER-phagy is unknown. Here, we describe an activation mechanism unique to FAM134C during starvation. In fed conditions, FAM134C is phosphorylated by casein kinase 2 (CK2) at critical residues flanking the LIR domain. Phosphorylation of these residues negatively affects binding affinity to the autophagy proteins LC3. During starvation, mTORC1 inhibition limits FAM134C phosphorylation by CK2, hence promoting receptor activation and ER-phagy. Using a novel tool to study ER-phagy in vivo and FAM134C knockout mice, we demonstrated the physiological relevance of FAM134C phosphorylation during starvation-induced ER-phagy in liver lipid metabolism. These data provide a mechanistic insight into ER-phagy regulation and an example of autophagy selectivity during starvation.

Signal-Retaining Autophagy Indicator as a Quantitative Imaging Method for ER-Phagy

“…Importantly, unlike FAM134B where BafA1 treatment considerably reduced the basal flux, the basal FAM134C-associated TOLLES:YPet index was not affected by autophagy block. This highlights the low FAM134C-degradation levels in basal conditions (i.e., low TOLLES-positive and YPet-negative puncta), consistent with previously reported studies [ 41 , 42 , 43 , 44 ].…”

“…They cooperate with quality-control pathways such as ER-associated protein degradation (ERAD) or UPR [ 35 , 40 ], and these SRAI-based reporters could be used to explore these stimuli. In this study, we used different ER-phagy receptors to analyze ER-phagy receptor flux and we observed that they all responded to nutrient starvation as previously reported [ 24 , 31 , 36 , 41 , 42 , 43 , 44 ]. Furthermore, we also observed some differences between the receptors such as the low basal degradation levels of the ER-phagy receptor FAM134C in comparison to FAM134B as previously reported [ 43 ].…”

“…In this study, we used different ER-phagy receptors to analyze ER-phagy receptor flux and we observed that they all responded to nutrient starvation as previously reported [ 24 , 31 , 36 , 41 , 42 , 43 , 44 ]. Furthermore, we also observed some differences between the receptors such as the low basal degradation levels of the ER-phagy receptor FAM134C in comparison to FAM134B as previously reported [ 43 ]. We anticipate that studying individual forms of ER-phagy involving specific cargo receptors will help determine the different role of the receptors in ER-phagy and help develop our limited knowledge of ER-phagy.…”

“…In addition, although we have not addressed this in this study, this probe might be used to quantify the number of lysosomes versus the number of autophagosomes to provide additional information as on autophagosome formation versus lysosomal block. In addition, although not yet tested, this probe also has the potential to be used in vivo like similar reporters employing the mRFP-GFP probe (e.g., ss-mRFP-GFP-KDEL and mRFP-GFP-FAM134C [ 43 ]).…”

Membrane Curvature: The Inseparable Companion of Autophagy

“…FAM134B binds to ATG8 through its AIM domain to recruit the IM and promote ER-phagy ( Figure 3 b). The FAM134 reticulon family proteins (FAM134A/B/C) have a similar structure and function, and all act as receptors [ 116 ]. However, there are some differences.…”