The endoplasmic reticulum (ER) stands as a vital hub for the synthesis, folding, and quality control of proteins in eukaryotic cells. Among the diverse array of proteins orchestrating these processes, the protein disulfide isomerase (PDI) family holds particular significance. Within this family lies the thioredoxin-related transmembrane (TMX) protein family, a group of membrane-bound PDIs with distinctive roles in protein biogenesis and ER quality control mechanisms.
Related Products
The TMX protein family comprises five members: TMX1, TMX2, TMX3, TMX4, and TMX5. These proteins feature an N-terminal signal sequence targeting them to the ER and harbor a catalytically active thioredoxin-like domain, known as a type-a TRX-like domain. Structurally, they are characterized by their membrane-bound nature, with topology variations influencing their functional specificity and cellular localization.
Fig. 1 Schematic representation of the TMX protein family members (Guerra C., Molinari M., 2020).
Despite their shared features, each TMX protein exhibits unique characteristics, reflecting their diverse roles within the ER. TMX1, for instance, has emerged as a key player in both protein folding and ER-associated degradation (ERAD), while TMX2 showcases distinct functions associated with its cytosolic orientation. TMX3, TMX4, and TMX5, though less characterized, offer intriguing insights into their roles in oxidative folding, protein quality control, and the pathophysiology of specific diseases.
Understanding the TMX protein family's intricacies holds significant implications for unraveling the mechanisms governing protein homeostasis within the ER and addressing diseases linked to protein misfolding and ER stress. Through comprehensive characterization and functional analyses, researchers aim to elucidate the physiological roles of TMX proteins, paving the way for targeted therapeutic interventions in protein folding disorders and ER-related pathologies.
TMX1: Orchestrating Protein Folding and ERAD
TMX1, also known as TXNDC1, represents a paradigmatic example of a topology-specific redox catalyst involved in both protein folding and ERAD pathways. Its distinct architecture, characterized by an N-terminal luminal region containing a catalytically active thioredoxin-like domain, enables TMX1 to interact selectively with membrane-tethered client proteins. Through cooperative interactions with ER lectin calnexin (CNX), TMX1 facilitates the folding of nascent polypeptides by assisting in the formation of correct disulfide bonds.
Moreover, TMX1 plays a pivotal role in the disposal of misfolded proteins by participating in ERAD. By selectively targeting membrane-anchored folding-defective polypeptides, TMX1 aids in the reduction of disulfide bonds, a prerequisite for their retro-translocation across the ER membrane and subsequent degradation by cytosolic proteasomes. Additionally, TMX1's localization at ER-mitochondria contact sites, known as MAM (mitochondria-associated membranes), underscores its multifunctional nature, as it regulates calcium flux between ER and mitochondria, thereby impacting cellular metabolism and stress responses.
TMX2: A Cytosolic Active Site Protein with Diverse Localization
TMX2, also known as TXNDC14, stands out as the enigmatic member within the TMX protein family. Identified in 2003 from a fetal cDNA library, TMX2 is a 296-amino acid non-glycosylated protein initially categorized as a type I membrane protein. Recent investigations have revealed its complex topology, positioning both N- and C-terminal regions on the cytosolic side, unlike other TMX family members. This unique orientation places its SNDC catalytic site toward the cytosol, distinguishing it from its counterparts. Despite its ubiquitous expression across tissues, TMX2's physiological function remains elusive, except for its potential involvement in the importin-β:Ran complex, regulating nuclear targeting of specific cargo proteins. Localized in ER sub-compartments like the nuclear outer membrane and MAM, TMX2 plays a crucial role in calcium flux modulation, akin to TMX1. Notably, its high expression in the brain correlates with its association with brain developmental abnormalities and microlissencephaly. These conditions may arise from TMX2's protective role against ER stresses, which are implicated in neuronal death.
TMX3: The Classic PDI with Multiple TRX-like Domains
TMX3, also referred to as TXNDC10, is a glycoprotein with 454 amino acids, discovered in 2005 due to its TRX-like domain. It possesses two N-glycosylation sites and three TRX-like domains, including an active type-a domain and two inactive type-b domains. TMX3 is widely expressed, with higher levels in heart and skeletal muscle. Unlike other TMX proteins, it doesn't upregulate during ER stress. Functionally, TMX3 acts as an oxidase and may recruit substrates through its b' domain. Its role remains unclear, but it shows potential protection against neuronal atrophy in Huntington's disease models, possibly by mitigating ER stress induced by mutated huntingtin. Deletion or missense mutations in TMX3 are associated with coronary artery diseases and microphthalmia.
TMX4: Paralogue of TMX1 with ER-Restructuring Potential
TMX4, or TXNDC13, is a 349-amino acid single-pass type I glycoprotein discovered in 2010 during a search for TRX-like domain-containing proteins. It's identified as the paralog of TMX1, sharing high similarity in the N-terminal luminal regions despite differences in the C-terminal domain. TMX4 is expressed ubiquitously, with elevated levels in heart tissue. Unlike TMX1, TMX4 is not upregulated during ER stress due to the absence of an ERSE motif in its promoter region. It possesses a luminal type-a TRX-like domain with a CPSC active site, suggesting a role as an ER reductase. While its exact function remains unclear, TMX4 interacts with CNX and ERp57, indicating a potential role in protein folding pathways. However, its involvement in ERAD is debated, as it does not interact with known ERAD factors, yet recent discoveries suggest its topology-specific client selection akin to TMX1. TMX4's localization at the inner membrane of the nuclear envelope hints at a role in regulating NE structure through redox cycles, impacting protein complexes like the torsinA-LINC complex. Further research is needed to elucidate TMX4's precise cellular functions, especially regarding its role in protein quality control and potential involvement in ERAD pathways.
TMX5: The Natural Trapping Mutant Associated with Ciliopathies
TMX5, also known as TXNDC15, is a poorly understood TMX family member, identified in a large-scale protein screening in 2003. Structurally, it's a predicted single-span type I protein with a large N-terminal luminal domain and a short C-terminal cytoplasmic tail lacking ER retention signals. TMX5 features four putative N-glycosylation sites and a type-a TRX-like domain with a non-canonical CRFS active site. Its unique active site structure categorizes TMX5 as a natural trapping mutant protein. Its physiological roles remain unknown, but mutations in TMX5 are linked to Meckel-Gruber syndrome (MKS), characterized by defective ciliogenesis. Deletions and missense mutations result in truncated forms of TMX5, impacting its localization within primary cilia and correlating with MKS development. This suggests a potential role for TMX5 in ciliary function regulation, implicating it in the pathology of ciliopathies like MKS.
Conclusion
The TMX protein family represents a diverse group of ER-resident proteins with multifaceted roles in protein folding, quality control, and cellular homeostasis. While significant progress has been made in understanding their structural features and physiological implications, further research is warranted to elucidate their precise cellular functions and therapeutic potential in disease contexts.
References
- Matsuo Y. Introducing thioredoxin-related transmembrane proteins: emerging roles of human TMX and clinical implications. Antioxidants & Redox Signaling. 2022, 36(13): 984-1000.
- Guerra C. Molinari M. Thioredoxin-related transmembrane proteins: TMX1 and little brothers TMX2, TMX3, TMX4 and TMX5. Cells. 2020, 9(9): 2000.
.
