The intrinsically disordered regions of organellophagy receptors control organelle fragmentation, ER-phagy and mitophagy flux

Organelle fragmentation is crucial for the onset of autophagic programs that control lysosomal clearance of portions of organelles to be removed from cells. It is driven by membrane-bound organello-phagy receptors that display cytoplasmic intrinsically disordered modules (IDRs) containing short LC3-interacting regions (LIRs). Studies on ER-phagy receptors of the FAM134 family revealed the importance of transcriptional induction, and receptors phosphorylation, ubiquitylation and clustering for execution of the ER-phagy programs. In this model, ER fragmentation is promoted by the membrane-remodeling function of FAM134 reticulon homology domains (RHDs)18,19. However, RHDs are not conserved in ER-phagy receptors, nor in receptors for autophagy of other organelles that also require fragmentation such as the mitochondria. Thus, membrane remodeling by RHDs is unlikely to be a conserved mechanism to regulate organelle turnover. Here, we show that the membrane-tethering modules of ER-phagy receptors (RHDs for FAM134B, single/multi spanning transmembrane domains for TEX264 and SEC62) determine the sub-compartmental distribution of the receptors but are dispensable for ER fragmentation, regardless of their propensity to remodel the ER membrane. Our experiments reveal that the information encoding for ER fragmentation is contained in the cytoplasmic IDR modules of the ER-phagy receptors, which also control delivery of the ER fragments within degradative acidic compartments upon engagements of lipidated LC3/GABARAP proteins via their LIRs. Notably, the transplantation of ER-phagy receptors IDRs at the mitochondrial membrane induces DRP1-driven mitochondrial fragmentation and mitophagy, and the transplantation of mitophagy receptors IDRs at the ER membrane induces ER fragmentation and ER-phagy. Our work reveals the functional conservation of membrane-exposed IDRs in promoting organelle fragmentation and offers a method to control integrity and activity of intracellular organelles by surface activation of IDR modules with net negative charges.

细胞器碎裂（organelle fragmentation）对于启动自噬程序至关重要，此类程序负责调控溶酶体清除待从细胞中移除的细胞器片段。该过程由膜结合的细胞器自噬受体所驱动，这类受体携带有包含短LC3互作区域（LC3-interacting regions, LIRs）的细胞质内在无序结构模块（intrinsically disordered modules, IDRs）。针对FAM134家族的ER自噬（ER-phagy）受体的研究揭示了转录诱导、受体磷酸化、泛素化与聚集对于执行ER自噬程序的必要性。在该模型中，FAM134的网状蛋白同源结构域（reticulon homology domains, RHDs）的膜重塑功能可促进内质网（endoplasmic reticulum, ER）碎裂[18,19]。然而，RHDs在ER自噬受体中并不保守，在其他同样依赖碎裂过程的细胞器自噬受体（如线粒体自噬（mitophagy）受体）中亦无保守性。因此，由RHDs介导的膜重塑不太可能是调控细胞器周转的保守机制。本研究证实，ER自噬受体的膜锚定模块——对于FAM134B为RHDs，对于TEX264与SEC62则为单次/多次跨膜结构域——可决定受体的亚细胞区室分布，但对于ER碎裂并非必需，且与其重塑ER膜的倾向无关。实验结果表明，编码ER碎裂的信息蕴藏于ER自噬受体的细胞质IDR模块中；此类模块还可通过其LIRs与脂化LC3/GABARAP蛋白结合，进而调控ER片段被递送至降解性酸性区室。值得注意的是，将ER自噬受体的IDR模块移植至线粒体膜上，可诱导DRP1介导的线粒体碎裂与线粒体自噬；而将线粒体自噬受体的IDR模块移植至ER膜上，则可诱导ER碎裂与ER自噬。本研究揭示了膜暴露IDR模块在促进细胞器碎裂方面的功能保守性，并提供了一种通过激活带有净负电荷的表面IDR模块来调控细胞内细胞器完整性与活性的方法。