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An N‐terminal truncated isoform of the ER‐phagy receptor FAM134B (FAM134B‐2) regulates starvation‐induced ER‐to‐lysosome protein degradation

Autophagy is a conserved system that adapts to nutrient starvation, after which proteins and organelles are degraded to recycle amino acids in response to starvation. Recently, the endoplasmic reticulum (ER) was added to the list of targets of autophagic degradation. Autophagic degradation of the ER (ER‐phagy) is considered a key mechanism in maintaining ER homeostasis. Four ER‐resident proteins (FAM134B, CCPG1, SEC62, and RTN3) have been identified as ER‐phagy cargo receptors, which contain LC3‐interacting regions. In this study we identified an N‐terminal truncated isoform of FAM134B (FAM134B‐2) that contributes to starvation‐induced ER‐phagy. FAM134B‐2 but not full length FAM134B (FAM134B‐1) is expressed in a fed state. Starvation drastically induces FAM134B‐2 but no other ER‐phagy cargo receptors through transcriptional activation by C/EBPβ. C/EBPβ overexpression increases FAM134B‐2 recruitment into autophagosomes and lysosomal degradation. In addition, FAM134B‐2 is preferentially localized in the smooth ER. This study demonstrates that the C/EBPβ‐FAM134B‐2 axis regulates starvation‐induced smooth ER‐phagy.