Endoplasmic reticulum localized TMEM33 domain-containing protein is crucial for all life cycle stages of the malaria parasite

Endoplasmic reticulum (ER) plays a pivotal role in the regulation of stress responses in multiple eukaryotic cells. However, little is known about the effector mechanisms that regulate stress responses in ER of the malaria parasite. Herein, we aimed to identify the importance of a transmembrane protein 33 (TMEM33)-domain-containing protein in life cycle of the rodent malaria parasite Plasmodium berghei. TMEM33 is an ER membrane-resident protein that is involved in regulating stress responses in various eukaryotic cells. A C-terminal tagged TMEM33 was localized in the ER throughout the blood and mosquito stages of development. Targeted deletion of TMEM33 confirmed its importance for asexual blood stages and ookinete development, in addition to its essential role for sporozoite infectivity in the mammalian host. Pilot scale analysis shows that the loss of TMEM33 results in the initiation of ER stress response and induction of autophagy. Our findings conclude an important role of TMEM33 in the development of all life cycle stages of the malaria parasite, which indicates its potential as an antimalarial target.

Dynamic quality control machinery that operates across compartmental borders mediates the degradation of mammalian nuclear membrane proteins

Many human diseases are caused by mutations in nuclear envelope (NE) proteins. How protein homeostasis and disease etiology are interconnected at the NE is poorly understood. Specifically, the identity of local ubiquitin ligases that facilitate ubiquitin-proteasome-dependent NE protein turnover is presently unknown. Here, we employ a short-lived, Lamin B receptor disease variant as a model substrate in a genetic screen to uncover key elements of NE protein turnover. We identify the ubiquitin-conjugating enzymes (E2s) Ube2G2 and Ube2D3, the membrane-resident ubiquitin ligases (E3s) RNF5 and HRD1, and the poorly understood protein TMEM33. RNF5, but not HRD1, requires TMEM33 both for efficient biosynthesis and function. Once synthesized, RNF5 responds dynamically to increased substrate levels at the NE by departing from the endoplasmic reticulum, where HRD1 remains confined. Thus, mammalian protein quality control machinery partitions between distinct cellular compartments to address locally changing substrate loads, establishing a robust cellular quality control system.

MiR-103a-3p aggravates renal cell carcinoma by targeting TMEM33

OBJECTIVE

We investigated the mechanism of miR-103a-3p-mediated renal cell carcinoma (RCC) progression.

METHODS

The miR-103a-3p expressions were measured in clinical samples and in two RCC cell lines. MiR-103a-3p was inhibited or over-expressed in the 786-O and UO31 cell lines, respectively.

RESULTS

We found that miR-103a-3p is closely related to the development of RCC cells. A bioinformatics analysis and a dual-luciferase reporter gene assay revealed that there is a direct interaction between TMEM33 and miR-103a-3p. Moreover, a rescue assay further confirmed that TMEM33 overexpression can attenuate miR-103a-3p-induced RCC cell development.

CONCLUSION

miR-103a-3p exerts a carcinogenic function in RCC by regulating TMEM33, a finding that may provide new insights into the development of prognostic markers and therapeutic targets for RCC.

PKM2-TMEM33 axis regulates lipid homeostasis in cancer cells by controlling SCAP stability

The pyruvate kinase M2 isoform (PKM2) is preferentially expressed in cancer cells to regulate anabolic metabolism. Although PKM2 was recently reported to regulate lipid homeostasis, the molecular mechanism remains unclear. Herein, we discovered an ER transmembrane protein 33 (TMEM33) as a downstream effector of PKM2 that regulates activation of SREBPs and lipid metabolism. Loss of PKM2 leads to up-regulation of TMEM33, which recruits RNF5, an E3 ligase, to promote SREBP-cleavage activating protein (SCAP) degradation. TMEM33 is transcriptionally regulated by nuclear factor erythroid 2-like 1 (NRF1), whose cleavage and activation are controlled by PKM2 levels. Total plasma cholesterol levels are elevated by either treatment with PKM2 tetramer-promoting agent TEPP-46 or by global PKM2 knockout in mice, highlighting the essential function of PKM2 in lipid metabolism. Although depletion of PKM2 decreases cancer cell growth, global PKM2 knockout accelerates allografted tumor growth. Together, our findings reveal the cell-autonomous and systemic effects of PKM2 in lipid homeostasis and carcinogenesis, as well as TMEM33 as a bona fide regulator of lipid metabolism.

Identification and characterization of TMEM33 as a reticulon-binding protein

Endoplasmic reticulum (ER) is an organelle that has an elaborate and continuous membrane system composed of sheet-like cisternae and a network of interconnected tubules. The ER tubules are shaped by reticulons, a conserved ER membrane protein family. However, how the membrane-shaping activity is regulated remains to be elucidated. To understand the mode of action of reticulons, we isolated TMEM33, a conserved protein harboring three transmembrane domains, as a reticulon 4C-binding protein by affinity chromatography. In addition to reticulon 4C, TMEM33 binds to reticulon 1A, -2B, -3C and a reticulon homology domain-containing protein Arl6IP1. Exogenously expressed TMEM33 localizes at both the ER membrane and the nuclear envelope. Exogenously expressed TMEM33 co-localizes with exogenously expressed reticulon 4C well at the ER sheets and partially at the ER tubules. Exogenously expressed TMEM33 suppresses the exogenously expressed reticulon 4C-induced tubulation of ER. These results suggest that TMEM33 has a potency to suppress the membrane-shaping activity of reticulons, thereby regulating the tubular structure of ER.