Recombinant Saccharomyces cerevisiae Golgi apparatus membrane protein TVP18 (TVP18)

What is TVP18 and where is it localized in yeast cells?

TVP18 (YMR071C) is an integral membrane protein localized to late Golgi vesicles along with the v-SNARE Tlg2p in Saccharomyces cerevisiae. It was discovered through proteomic analysis of immunoisolated Golgi subcompartments . The protein consists of 167 amino acids and likely contains multiple transmembrane domains, allowing it to embed firmly within the Golgi membrane. TVP18 may interact with ribosomes based on co-purification experiments and is thought to play a role in intracellular sterol transport . It belongs to a family of proteins that includes TVP38, TVP23, and TVP15, which were all identified in Tlg2-containing compartments, suggesting related or cooperative functions within the Golgi apparatus .

What expression systems are optimal for producing recombinant TVP18?

E. coli has been successfully employed to express recombinant full-length S. cerevisiae TVP18 protein with an N-terminal His tag (1-167aa) . When designing expression systems for membrane proteins like TVP18, researchers should consider several critical factors:

For optimal results when expressing TVP18, consider codon optimization for the selected host and incorporate appropriate fusion tags to enhance solubility and facilitate purification. For proper folding, include 6% trehalose in storage buffer at pH 8.0, as indicated in the available recombinant protein protocols .

What purification and storage methods maintain TVP18 stability?

Purification of recombinant TVP18 requires careful consideration of its membrane protein nature. Based on established protocols, researchers should:

• Begin with affinity chromatography using the His-tag for initial capture (Ni-NTA resin)

• Follow with size exclusion chromatography to separate properly folded protein from aggregates

• Consider detergent selection carefully—mild detergents like DDM or LMNG help maintain native conformation

• Include glycerol (6-50%) in final storage buffers to enhance stability

For storage, lyophilization has proven effective, with the protein maintaining >90% purity as determined by SDS-PAGE . Reconstitution should be performed in deionized sterile water to a concentration of 0.1-1.0 mg/mL, with 5-50% glycerol added as a cryoprotectant. Store aliquots at -20°C/-80°C and avoid repeated freeze-thaw cycles, which significantly decrease activity. For working stocks, storage at 4°C is acceptable for up to one week .

What methods are effective for studying TVP18 localization in yeast cells?

Multiple complementary approaches have proven effective for determining TVP18 subcellular localization:

• Immunofluorescence microscopy: Using HA-tagged TVP18 and co-staining with markers for Golgi subcompartments (particularly myc-tagged tSNAREs) to confirm localization to Tlg2-containing compartments .

• Live-cell imaging: Employing fluorescent protein fusions (GFP/YFP) for dynamic tracking of TVP18, similar to approaches used for the plant homolog ECH-EYFP .

• Subcellular fractionation: Separating cellular components through differential centrifugation followed by immunoblotting to detect TVP18 in specific fractions.

• Immunoelectron microscopy: Providing high-resolution localization data with immunogold labeling to precisely map TVP18 within Golgi substructures.

• Co-immunoprecipitation: Identifying interaction partners that colocalize with TVP18, as demonstrated by studies showing TVP18 interaction with Yip1-family proteins .

For definitive localization studies, combining at least two independent methods is recommended to overcome the limitations inherent to each technique.

How does knockout of TVP18 affect Golgi structure and function?

Knockout studies of TVP18 have revealed important insights into its role in maintaining Golgi architecture. In Pichia pastoris, TVP18 knockout causes irreversible perturbations in the stacking of Golgi cisternae . Remarkably, despite these structural abnormalities, no significant variations were observed in:

• The secretory pathway function

• ER size

• Whole cell glycomics

• Recombinant protein glycan profiles

This suggests that while TVP18 plays a role in maintaining proper Golgi morphology, functional redundancy exists within the secretory pathway that preserves essential Golgi functions even when structure is compromised. In Saccharomyces cerevisiae, TVP18 deletion alone doesn't produce dramatic phenotypes under standard laboratory conditions , suggesting species-specific differences in the requirement for TVP18 or functional compensation by other proteins.

The finding that TVP18 knockout affects Golgi structure is particularly significant when considered alongside the observation that proteins with disruption in TVP18, RSN1, or CSC1-2 all show annotated function as or homology to calcium/calcium permeable ion channels . This suggests a potential role for calcium homeostasis in maintaining Golgi structure that warrants further investigation.

What protein interactions has TVP18 been shown to participate in?

TVP18 engages in several key protein-protein interactions that provide insight into its cellular functions:

• Yip1-family proteins: Immunoprecipitation studies have demonstrated that TVP18 interacts with Yip4 and Yip5 . These interactions suggest a collective role in maintaining and/or functioning of late Golgi/endosomal compartments. The Yip family proteins are known to be involved in membrane trafficking, particularly in ER-to-Golgi and intra-Golgi transport.

• v-SNARE Tlg2p: TVP18 colocalizes with this SNARE protein in late Golgi vesicles , suggesting a potential functional relationship in vesicle fusion events during membrane trafficking.

• Ribosomes: BioGRID database information indicates that TVP18 "may interact with ribosomes, based on co-purification experiments" . This unexpected interaction might suggest roles in:

  • Localized protein synthesis near the Golgi

  • Ribosome-associated degradation of membrane proteins

  • Quality control of secretory proteins

• TVP family network: TVP18 appears to function in coordination with other TVP family members (TVP38, TVP23, TVP15), as they were all identified in the same proteomic analysis of Tlg2-containing membranes .

These interaction data collectively suggest that TVP18 functions as part of larger protein complexes involved in maintaining Golgi structure and facilitating vesicular trafficking in the late secretory pathway.

How conserved is TVP18 across different species?

TVP18 displays notable evolutionary conservation across eukaryotes, suggesting fundamental roles in cellular function. According to comparative analyses, conserved sequences of TVP18 and related family members (TVP38, TVP23) are found in higher eukaryotes, although many homologs remain uncharacterized .

Evidence for functional conservation comes from cross-species complementation studies. The Arabidopsis ECHIDNA (ECH) protein, functionally related to yeast TVP23, can restore growth of the tvp23Δ ypt6Δ double mutant in yeast when expressed heterologously . This remarkable cross-kingdom functional conservation indicates that fundamental aspects of TVP protein function have been preserved over more than a billion years of evolutionary divergence between plants and fungi.

In mammals, TVP23B has been identified as a homolog that is "conserved from yeast to humans" . This human TVP23 homolog plays critical roles in intestinal homeostasis, controlling Paneth cell function and goblet cell secretion, which ultimately affects host-microbe interactions .

The conservation pattern suggests that TVP family proteins serve essential roles in membrane trafficking and organelle structure across diverse eukaryotic lineages, with specialized functions having evolved to meet the specific needs of different cell types and organisms.

What functional homologs of TVP18 exist in higher eukaryotes and plants?

Several functional homologs of TVP18 and related TVP family proteins have been identified across eukaryotic kingdoms:

The functional conservation of these proteins highlights their fundamental importance in cellular trafficking systems. The Arabidopsis ECHIDNA protein is particularly well-characterized, functioning in vesicular trafficking at the trans-Golgi network and influencing cell elongation during plant development . ECHIDNA mutants display reduced cell elongation and altered secretory trafficking, phenotypes that reveal its essential role in plant growth.

In humans, TVP23B influences the intestinal barrier by regulating antimicrobial peptide production and mucus layer integrity, demonstrating how these evolutionarily conserved proteins have been adapted for tissue-specific functions in complex multicellular organisms .

How does TVP18 contribute to vesicular trafficking in the late Golgi?

TVP18's role in vesicular trafficking can be inferred from its localization and functional studies of related proteins. TVP18 localizes specifically to late Golgi vesicles along with the v-SNARE Tlg2p , suggesting direct involvement in vesicle-mediated transport processes.

Studies on the related protein TVP23 provide insights into potential mechanisms. TVP23 "has been shown to have a role in the retrograde pathway between TGN and Golgi in budding yeast" . By extension, TVP18 may play a similar role in facilitating retrograde transport within the Golgi system. This is particularly significant as retrograde trafficking is essential for retrieving resident Golgi proteins and maintaining proper compartmentalization within the Golgi stack.

The functional relationship with Yip1-family proteins (Yip4 and Yip5) further supports TVP18's role in vesicular trafficking . Yip proteins are known to interact with Rab GTPases, which are master regulators of membrane trafficking events. The association of TVP18 with this network positions it as a potential effector or regulator in Rab-mediated trafficking pathways.

For researchers investigating the precise mechanism of TVP18 in vesicular trafficking, approaches should include:

• Cargo trafficking assays in TVP18 knockout strains

• Analysis of vesicle budding and fusion events using in vitro reconstitution systems

• Live-cell imaging with simultaneous tracking of TVP18 and vesicle markers

• Genetic interaction mapping with known trafficking components

Understanding TVP18's specific contribution to vesicular trafficking will advance our knowledge of the fundamental mechanisms governing protein and lipid distribution within eukaryotic cells.

How do mutations in TVP18 affect yeast cell growth under different stress conditions?

While TVP18 is "nonessential for growth under laboratory conditions" , its importance may become apparent under specific stress conditions or in combination with mutations in related pathways. The study of TVP18 function under stress provides valuable insights into its physiological roles.

The finding that disruptions of other TVP family members (tvp15 and tvp23) show synthetic aggravation with ypt6 or ric1 null mutations suggests potential genetic interactions between TVP18 and other trafficking components. This phenomenon, known as synthetic lethality or synthetic growth defects, reveals functional relationships that might be obscured under standard growth conditions.

To systematically investigate TVP18's role in stress responses, researchers should design experiments that include:

• Temperature stress analysis: Growth assays comparing wild-type and tvp18Δ strains at elevated (37-39°C) and reduced (16-18°C) temperatures.

• Membrane stress challenges: Testing growth in the presence of:

  • Cell wall stressors (Calcofluor White, Congo Red)

  • Membrane-perturbing agents (SDS, ethanol)

  • Sterol-binding compounds (nystatin, amphotericin B)

• Oxidative stress response: Comparing sensitivity to H₂O₂ or menadione between wild-type and mutant strains.

• ER stress induction: Monitoring growth in the presence of tunicamycin or DTT, which disrupt protein folding.

• Double mutant analysis: Creating combinations of tvp18Δ with mutations in other trafficking pathways to identify functional connections.

These approaches will help define the context-dependent functions of TVP18 and may reveal conditional phenotypes that are not apparent under optimal growth conditions.

What are the latest findings on TVP18 homologs in human disease?

Recent research has uncovered significant roles for TVP18 homologs in human health and disease. TVP23B, a human homolog of the TVP family, has emerged as a critical regulator of intestinal homeostasis .

A forward genetic screen identified a mutation in TVP23B that conferred susceptibility to chemically induced and infectious colitis . Subsequent investigation revealed that TVP23B controls the homeostasis of Paneth cells and function of goblet cells in the intestinal epithelium. These effects lead to decreased production of antimicrobial peptides and a more penetrable mucus layer when TVP23B is disrupted .

Mechanistically, TVP23B interacts with another Golgi protein, YIPF6, which is similarly critical for intestinal homeostasis. Proteomic analysis of YIPF6 and TVP23B-deficient colonocytes revealed a common deficiency of several critical glycosylation enzymes . This suggests that TVP23B influences intestinal barrier function through regulation of protein glycosylation in specialized secretory cells.

These findings establish a connection between fundamental cellular processes involving Golgi function and clinically relevant conditions like inflammatory bowel disease. The study of TVP18 in yeast thus provides a valuable model system for understanding the basic mechanisms that underlie the functions of its homologs in higher organisms, potentially contributing to the development of new therapeutic approaches for intestinal disorders.