The ortholog of human REEP1-4 is required for autophagosomal enclosure of ER-phagy/nucleophagy cargos in fission yeast

Reveal the autophagic degradation of glutaredoxin Grx4 in Schizosaccharomyces pombe

Glutaredoxins (Grxs) are small, heat-stable proteins that serve as multi-functional glutathione-dependent thiol transferases. Recent studies have elucidated their role in regulating cellular iron and copper homeostasis. To further elucidate their functions, we employed a combination of bioinformatics and experimental analyses. In S. pombe, five Grxs have been identified. Our study utilized multiple sequence alignment and conserved domain prediction, revealing that Grx4 and its homologs possess a glutaredoxin domain (GRX domain) at the C-terminal and a thioredoxin-like domain (TRX domain) exclusively at the N-terminal. The functional roles of the GRX domain and TRX domain were investigated by constructing strains that express a truncated Grx4 under the regulation of either a constitutive cam1 promoter or its native promoter. Our findings indicated that two Atg8 interacting motifs (AIM), FLKI and FQEI, located within the TRX domain of Grx4, are sufficient to induce autophagic degradation under nitrogen- or iron-starvation conditions, respectively. This represents a significant advancement in understanding TRX domain function within Grxs for the first time. Moreover, the altered expression level of Pcl1 in Δatg5 or Δatg8 strains under iron starvation suggests that autophagy is essential for maintaining iron homeostasis. Further investigations revealed that Grx4 is required for cellular survival and endoplasmic reticulum autophagy (ER-phagy) during DTT treatment, implying a potential correlation between Grxs and the endoplasmic reticulum (ER). Additionally, the loss of Grx4 disrupted nuclear integrity during ER stress, highlighting the versatility and importance of further investigations into the functions of Grx4.

3,3'-Diindolylmethane disrupts the endoplasmic reticulum and nuclear envelope in Schizosaccharomyces pombe

3,3'-Diindolylmethane is recognized for its anti-cancer activities in various pathways, though its mechanism remains to be fully elucidated. Previous studies have shown that 3,3'-Diindolylmethane disturbed the localization of Cut11, a nuclear pore complex subunit in Schizosaccharomyces pombe. This study further reveals that in Schizosaccharomyces pombe, 3,3'-Diindolylmethane also disrupts other components of nuclear envelope, causing GFP-NLS leakage, making it evident that 3,3'-Diindolylmethane disrupts the nuclear envelope. 3,3'-Diindolylmethane also disturbs the localization of GFP-ADEL and Ost4, which are endoplasmic reticulum lumen proteins and membrane proteins respectively, suggesting the function of 3,3'-Diindolylmethane on endoplasmic reticulum disturbance. The nuclear envelope repairment, normal nuclear envelope physical properties, and lipid metabolism homeostasis are crucial for cell survival in the presence of 3,3'-Diindolylmethane. These findings provide new insights into the understanding and development of 3,3'-Diindolylmethane as an anti-cancer agent.

Yop1 stability and membrane curvature generation propensity are controlled by its oligomerisation interface

The DP1 family of integral membrane proteins stabilize high membrane curvature in the endoplasmic reticulum and phagophores. Mutations in the human DP1 gene REEP1 are associated with Hereditary Spastic Paraplegia type 31 and distal hereditary motor neuropathy. Four missense mutations map to a putative dimerization interface but the impact of these mutations on DP1 structure and tubule formation are unknown. Combining biophysical measurements, functional assays, and computational modeling in the context of the model protein Yop1, we found that missense mutations have variable effects on DP1 dimer structure and in vitro tubulation activity, and provide mechanistic insights into the role of DP1 oligomerisation on membrane curvature stabilization. Whereas the mutations P71L and S75F decreased dimer homogeneity and led to polydisperse oligomerization and impaired membrane curving activity, A72E introduced new polar interactions between subunits that stabilized the Yop1 dimer and allowed robust tubule formation but prevented formation of more highly-curved lipoprotein particles (LPP). The introduction of a BRIL domain to the cytoplasmic loop of A72E rescued LPP formation, consistent with a requirement for dimer splaying in highly curved membranes. These results suggest that the membrane curving activity of DP1 proteins requires both dimer stability and conformational plasticity at the intermolecular interface.

The membrane curvature-inducing REEP1-4 proteins generate an ER-derived vesicular compartment

The endoplasmic reticulum (ER) is shaped by abundant membrane curvature-generating proteins that include the REEP family member REEP5. The REEP1 subfamily, consisting of four proteins in mammals (REEP1-4), is less abundant and lack a N-terminal region. Mutations in REEP1 and REEP2 cause Hereditary Spastic Paraplegia, but the function of these four REEP proteins remains enigmatic. Here we show that REEP1-4 reside in a unique vesicular compartment and identify features that determine their localization. Mutations in REEP1-4 that compromise curvature generation, including those causing disease, relocalize the proteins to the bulk ER. These mutants interact with wild-type proteins to retain them in the ER, consistent with their autosomal-dominant disease inheritance. REEP1 vesicles contain the membrane fusogen atlastin-1, but not general ER proteins. We propose that REEP1-4 generate these vesicles themselves by budding from the ER, and that they cycle back to the ER by atlastin-mediated fusion. The vesicles may serve to regulate ER tubule dynamics.

REEPing the harvest of reticulophagy and nucleophagy

Under stress conditions, the endoplasmic reticulum and nucleus undergo turnover through selective macroautophagy/autophagy processes termed reticulophagy and nucleophagy, respectively. Our recent study has identified the protein Hva22/Rop1/Yep1, a member of the REEP1-REEP4 subfamily of the REEP protein family, as an essential factor for both processes in the fission yeast Schizosaccharomyces pombe. In the absence of Hva22/Yep1, reticulophagy and nucleophagy cargos without surrounding autophagic membranes accumulate in the cytoplasm. Interestingly, human proteins in the REEP1-REEP4 subfamily can functionally substitute for Hva22/Yep1 to facilitate reticulophagy. Phylogenetic and synteny analyses further reveal that the budding yeast reticulophagy receptor Atg40 is also a REEP1-REEP4 subfamily member. Similar to human REEP1-REEP4 subfamily proteins, Atg40 can functionally replace Hva22/Yep1. Based on our findings, we propose that promoting reticulophagy is a conserved function of REEP1-REEP4 subfamily proteins.