RTN4B interacting protein FAM134C promotes ER membrane curvature and has a functional role in autophagy

Abstract

The endoplasmic reticulum (ER) is comprised of a controlled ratio of sheets and tubules, which are maintained by several proteins with multiple functions. Reticulons (RTNs), especially RTN4, and DP1/Yop1p family members are known to induce ER membrane curvature. RTN4B is the main RTN4 isoform expressed in non-neuronal cells. In this study, we identified FAM134C as a RTN4B interacting protein in mammalian, non-neuronal cells. FAM134C localized specifically to the ER tubules and sheet edges. Ultrastructural analysis revealed that overexpression of FAM134C induced formation of unbranched, long tubules or dense globular structures comprised of heavily branched narrow tubules. In both cases, tubules were non-motile. ER tubulation was dependent on the reticulon homology domain (RHD) close to the N-terminus. FAM134C plays a role in the autophagy pathway as its level elevated significantly upon amino acid starvation but not during ER stress. Moreover, FAM134C depletion reduced the number and size of autophagic structures and the amount of ER as a cargo within autophagic structures under starvation conditions. Dominant-negative expression of FAM134C forms with mutated RHD or LC3 interacting region (LIR) also led to the reduced number of autophagic structures. Our results suggest that FAM134C provides a link between regulation of ER architecture and ER turnover by promoting ER tubulation required for subsequent ER fragmentation and engulfment into autophagosomes. [Media: see text] [Media: see text] [Media: see text] [Media: see text]

ER-phagy: mechanisms, regulation, and diseases connected to the lysosomal clearance of the endoplasmic reticulum

“…Nutrient starvation substantially enhances FAM134B-driven macro-ER-phagy ( 200 ). More recently, FAM134A and FAM134C have also been involved in starvation-induced macro-ER-phagy ( 199 , 206 ). This has led the HUGO gene nomenclature committee to introduce the names Reticulophagy Regulator1 (RETREG1), RETREG2, and RETREG3 for FAM134B, FAM134A, and FAM134C, respectively.…”

“…The FAM134 proteins contain a reticulon-homology domain at amino acid residues 80–271 (FAM134B), 66–261 (FAM134A), and 53–244 (FAM134C), which is composed of two hydrophobic domains connected by a hydrophilic loop ( FIGURE 8 ). Each hydrophobic domain forms a hairpin within the membrane bilayer without spanning it, promoting high membrane curvature and resulting in both the NH 2 and COOH termini being exposed at the cytosolic leaflet of the ER membrane ( 200 , 206 , 210 , 211 ). The reticulon-homology domain may contribute to membrane deformation, and clustering of FAM134 proteins may facilitate ER fragmentation ( 212 ).…”

“…3.2.3.1) but not in S. cerevisiae ( 186 ). SEC62 lacks a reticulon-homology domain, but it interacts with putative reticulon-homology domain-containing ER-phagy receptors such as FAM134C ( 215 ), which may contribute to the membrane curvature required for ER fragmentation ( 206 ). The SEC62 LIR motif preferentially binds the lipidated form of Atg8/LC3 proteins ( 186 ), and, as in several other ER-phagy receptors, it is positioned at the end of a long intrinsically disordered domain ( 186 , 228 ).…”

ER remodeling via ER-phagy

“…The term ER-phagy was first coined in 2006 ( Bernales et al., 2006 ) to describe selective degradation of the ER through autophagy, but we started gaining a deeper understanding into the process with the discovery of ER-phagy receptors—mammalian FAM134A, B, and C ( Khaminets et al., 2015 ; Kumar et al., 2021 ; Reggio et al., 2021 ), Sec62 ( Fumagalli et al., 2016 ), RTN3 ( Grumati et al., 2017 ), CCPG1 ( Smith et al., 2018 ), ATL3 ( Chen et al., 2019 ), TEX264 ( An et al., 2019 ; Chino et al., 2019 ), CALCOCO1 ( Nthiga et al., 2020 ), and C53 ( Stephani et al., 2020 ); yeast Atg39, Atg40 ( Mochida et al., 2015 ), and Epr1 ( Zhao et al., 2020 ); and plant ATI1, ATI2, and ATI3 ( Honig et al., 2012 ; Michaeli et al., 2014 ; Zhou et al., 2018 ), Rtn1 and Rtn2 ( Zhang et al., 2020 ), Sec62 ( Hu et al., 2020 ), and C53 ( Stephani et al., 2020 ). All hitherto identified ER-phagy receptors contain an ATG8-interacting motif (AIM), or LIR in mammals, and are localized to the ER.…”

Advances in ER-Phagy and Its Diseases Relevance

“…FAM134C may be involved in ER-phagy under amino acid starvation. It may also use its LIR to recruit autophagy machinery, and use its RHD to promote the bending and fragmentation of ER [ 41 ]. But the involvement of FAM134A in ER-phagy has not been reported yet.…”