Elucidating the molecular mechanisms of ER-phagy regulation around the FAM134 protein family
Authors:   , Berkane Rayene 
Published: November 2024

The endoplasmic reticulum (ER) constitutes a sophisticated network of membranes that commands diverse molecular processes comprising protein folding, lipid synthesis, and calcium storage. Safeguarding the ER's integrity against insults, achieved through a process known as ER-phagy, is imperative for optimal functionality at both the individual protein and organelle levels. Dysregulation in ER-phagy has been associated with various human diseases, including hereditary sensory and autonomic neuropathy (HSAN). The exploration of ER-phagy gained momentum with the discovery of ER-phagy receptors, exemplified by FAM134B—the first identified mammalian ER-phagy receptor. FAM134B’s interaction with autophagy modifiers forms molecular tethers, connecting the ER with autophagic membranes. The clustering of FAM134B with other reticulon homology domain (RHD) -containing proteins amplifies the capacity, versatility, and variability of ER remodeling, thereby facilitating adaptive responses to diverse cellular requirements. Belonging to the FAM134 protein family, FAM134B shares an LC3-interacting region (LIR) amino acid sequence with FAM134A and FAM134C. However, the limited understanding of the coordinated functions of FAM134 family members necessitates a detailed investigation into the molecular mechanism and regulation of these proteins. In this work, I present findings highlighting the role of each of the individual FAM134 proteins as well as the molecular mechanisms governing their ER-phagy functions. To this end, an interdisciplinary approach was employed to explore the dynamics within FAM134 protein functions and how clustering and post-translational modification regulate FAM134-dependent ER remodeling and the selective degradation of ER components at both macro- and nanoscale resolutions. The activity of both FAM134B and FAM134C can be influenced by inhibiting mTOR, the master kinase of autophagy, using Torin1. Employing a chemical screening approach, several downstream kinases – in particular CK2, emerge as crucial for Torin1-induced ER-phagy driven by FAM134B and FAM134C. The investigation underscores the fundamental role of phosphorylation sites within the RHD of FAM134 proteins in inducing ER-phagy upon Torin1 treatment. Furthermore, we demonstrate that the phosphorylation of FAM134B and FAM134C is essential for their subsequent ubiquitination and the formation of high-density receptor clusters. This suggests that the phosphorylation-dependent ubiquitination of both FAM134 proteins acts as the driving force behind the formation of high-density clusters at active ER-phagy sites. These findings provide valuable insights into the regulatory mechanisms governing FAM134B and FAM134C in the context of ER-phagy, emphasizing the interplay between phosphorylation, ubiquitination, and cluster formation in the complex regulation of ER remodeling. Overall, these discoveries expand our understanding of the molecular principles behind ER- phagy activation and may pave the way for the development of novel strategies to modulate ER-phagy, offering new avenues for therapeutic exploration. Ultimately, the investigation of interconnected cellular processes and the intricate regulatory networks of FAM134 proteins' functions may serve as a cornerstone for future studies, fostering advancements in our understanding of cellular physiology and pathology.