Recombinant Human Transmembrane protein 68 (TMEM68)

What is TMEM68 and what is its primary function in cellular metabolism?

TMEM68 (Transmembrane protein 68) is an evolutionarily conserved protein that functions as an acyltransferase in mammalian cells. It is primarily involved in triacylglycerol (TG) biosynthesis, which is an important metabolic process for intracellular storage of surplus energy, intestinal dietary fat absorption, attenuation of lipotoxicity, lipid transportation, lactation, and signal transduction .

Recent research has demonstrated that TMEM68 exhibits both monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT) activities, catalyzing essential steps in the glycerolipid synthesis pathway . When overexpressed in mammalian cells, TMEM68 promotes TG accumulation and lipid droplet (LD) formation in a conserved active sites-dependent manner .

Methodology for studying TMEM68 function:

• Overexpression and knockdown studies in cell lines

• In vitro enzymatic assays for MGAT and DGAT activities

• Lipidomic analysis to detect changes in lipid profiles

• Mutational studies of conserved active sites (particularly His129 and Asp135)

What is the subcellular localization of TMEM68 and how is this determined experimentally?

TMEM68 is primarily localized to the endoplasmic reticulum (ER). Live cell imaging demonstrates that TMEM68-GFP fusion proteins display a reticular staining pattern and co-localize extensively with recombinant ER markers .

Experimental methods to determine subcellular localization:

• Confocal microscopy - Co-localization studies using TMEM68-GFP fusion proteins with organelle-specific markers (like DsRed-ER)

• Subcellular fractionation - Separation of cytosolic (100,000g supernatant) and membrane fractions (100,000g pellet) followed by immunoblotting

• Membrane extraction assays - Treatment with various solubilizing agents (SDS, Triton X-100, sodium carbonate) to determine the mode of membrane association

Notably, TMEM68 is an integral membrane protein, as it remains in the pellet fraction when treated with sodium carbonate (which typically releases peripheral membrane proteins) but can be solubilized with detergents like SDS or Triton X-100 .

How is the membrane topology of TMEM68 characterized?

TMEM68 is a polytopic membrane protein with both N- and C-termini oriented toward the cytosol. This topology has been determined through protease protection assays on intact membrane vesicles from cells expressing tagged TMEM68 proteins .

Methodological approach:

• Protease protection assays - Membrane vesicles from cells expressing TMEM68 with either N-terminal His₆-tag or C-terminal FLAG tag are treated with proteinase K in the presence or absence of detergent

• Immunoblotting analysis - Detection of epitope tags after protease treatment reveals exposure to cytosol

• Control experiments - Using luminal ER proteins like PDI to confirm vesicle integrity

TMEM68 contains two predicted transmembrane domains (TMDs). Interestingly, the first TMD (TMD1, residues 51-75) is both necessary and sufficient for ER targeting, while the second TMD (TMD2, residues 121-145) is dispensable for ER localization .

What enzymatic activities does TMEM68 exhibit and how are they experimentally measured?

TMEM68 exhibits both monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT) activities. These activities are dependent on conserved active site residues, particularly His129 and Asp135 in the catalytic motif I with a HXXXXD signature .

Experimental methods for measuring enzymatic activity:

• In vitro acyltransferase assays using:

  • 2-oleoyl-glycerol as oleoyl acceptor (for MGAT activity)

  • 1-2-dioleoyl-sn-glycerol as oleoyl acceptor (for DGAT activity)

• Mutational analysis - Comparing wild-type TMEM68 with mutant TMEM68 (H129A and D135N) to demonstrate dependence on conserved active sites

• Selective inhibitors - Using DGAT1-specific inhibitors (e.g., T863) to distinguish TMEM68 activity from endogenous DGAT1

Results show that overexpression of wild-type TMEM68 leads to a ~3-fold increase in MGAT activity compared to control cells, with a smaller but significant increase in DGAT activity. Importantly, mutant TMEM68 lacking putative active site residues does not increase these acyltransferase activities .

How does TMEM68 expression impact cellular lipid profiles?

TMEM68 expression significantly alters cellular lipid composition beyond just increasing triacylglycerol levels. Quantitative targeted lipidomic analysis reveals that TMEM68 overexpression affects multiple lipid classes .

Key findings from lipidomic studies:

• Increased lipid storage components:

  • Diacylglycerol (DG) - particularly species with saturated and monounsaturated fatty acids

  • Free fatty acid (FFA) - primarily saturated and monounsaturated

  • Triacylglycerol (TG) - preferential increase in saturated and monounsaturated TAGs

• Altered membrane lipid composition:

  • Changes in glycerophospholipids including phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol

  • Profound reduction in ether-linked glycerophospholipids

  • Decreased prevalence of polyunsaturated glycerophospholipids

• Impact on fatty acid saturation:

  • TMEM68 overexpression preferentially increases lipids containing saturated and monounsaturated fatty acids

  • Decreases specific DG species containing polyunsaturated fatty acids (18:2 or 20:4)

This suggests TMEM68 acts not only as a TAG synthase but also as a multifaceted regulator of membrane lipid composition and polyunsaturated fatty acid homeostasis .

How do TMEM68 loss-of-function and gain-of-function approaches affect cellular lipid metabolism?

Combining genetic gain- and loss-of-function approaches with lipidomics has revealed TMEM68's specific contributions to cellular lipid metabolism .

Gain-of-function (Overexpression) effects:

• Promotes TAG synthesis and lipid droplet formation

• Increases storage lipids at the expense of membrane lipids

• Reduces ether-linked glycerophospholipids

• Shifts fatty acid composition toward saturated and monounsaturated species

• Upregulates expression of lipogenesis genes (including DGATs, fatty acid synthesis-related genes, and peroxisome proliferator-activated receptor γ)

Loss-of-function (Knockdown) effects:

• Reduces basal cellular TAG storage

• Alters diacylglycerol abundance (reduces approximately 50% of detected DAG species)

• Changes polyunsaturated fatty acid distribution in membrane lipids

• Does not completely eliminate TAG synthesis, suggesting redundancy with other acyltransferases

These complementary approaches demonstrate that TMEM68 contributes a discrete fraction of basal cellular TAG storage and functions independently of canonical DGAT1 and DGAT2 enzymes .

What structural features are critical for TMEM68 function and subcellular targeting?

Several structural elements are essential for TMEM68's enzymatic activity and proper localization:

• Conserved catalytic motif I (HXXXXD signature):

  • Contains putative active site residues His129 and Asp135

  • Forms a catalytic dyad typical of acyltransferase enzymes

  • Mutation of these residues (H129A and D135N) abolishes enzymatic activity and prevents TG accumulation and LD formation

• Transmembrane domains:

  • First TMD (residues 51-75) is critical for ER targeting and membrane association

  • Deletion of TMD1 results in cytosolic distribution of TMEM68

  • TMD1 alone is sufficient to target cytosolic GFP to the ER

  • Second TMD (residues 121-145) is dispensable for ER localization

• Acyltransferase domain (pfam03982):

  • Defines TMEM68 as part of the DG acyltransferase family

  • Contains substrate binding regions that differ from GPAT/AGPAT enzymes

  • May explain TMEM68's unique substrate specificity

Methodological approaches for structural analysis include deletion mutants, domain swapping, and site-directed mutagenesis followed by localization studies and activity assays .

How does TMEM68 relate to canonical lipid synthesis pathways and other acyltransferases?

TMEM68 functions as an acyltransferase that operates alongside but independently from canonical lipid synthesis enzymes:

• Relationship with DGAT1/2:

  • TMEM68 promotes TAG synthesis independently of canonical DGAT1 and DGAT2 enzymes

  • TMEM68 activity is maintained even in the presence of DGAT1-specific inhibitors

  • Overexpression of TMEM68 upregulates expression of DGATs, suggesting regulatory crosstalk

• Phylogenetic positioning:

  • Forms a distinct subgroup within the glycerophospholipid acyltransferase family

  • More closely related to DG acyltransferases than glycerophospholipid acyltransferases

  • Contains the pfam01553 acyltransferase domain shared with GPAT and AGPAT enzymes

  • Unlike GPAT/AGPAT enzymes, TMEM68 contains only motif I of the pfam01553 domain and lacks conserved substrate binding motifs II and III

• Functional comparison:

  • Exhibits both MGAT and DGAT activities, but may also have PDAT (phospholipid:diacylglycerol acyltransferase) activity according to some studies

  • Contributes a discrete fraction of cellular TAG synthesis capacity

  • Has broader effects on lipid metabolism than canonical enzymes, including regulation of membrane lipid composition

This unique positioning makes TMEM68 an interesting target for understanding alternative pathways in lipid metabolism.

What is the tissue distribution of TMEM68 and how does this inform its physiological relevance?

TMEM68 shows a tissue-specific expression pattern with highest expression in brain tissue:

• Expression analysis across tissues:

  • Highest expression levels reported in brain

  • Detectable in multiple murine tissues

  • Expression analysis typically performed using qPCR with specific primers for TMEM68 and a reference gene (like ribosomal protein, large, P0/36B4)

• Brain-specific relevance:

  • High expression suggests importance in neural lipid metabolism

  • May contribute to the specialized lipid composition of brain tissue

  • Could be involved in regulation of polyunsaturated fatty acids that are abundant in neural tissues

• Potential role in pathology:

  • Recent studies have investigated TMEM68 in breast cancer cells

  • TMEM68 has been identified as a potential modifier of human breast cancer risk and outcomes

  • Regulates TAG levels and alters diacylglycerol content in breast cancer cells

The tissue-specific expression pattern suggests that TMEM68 may have specialized functions in different tissues, with particularly important roles in the brain and potentially in certain disease states.

What experimental approaches are most effective for studying TMEM68 in different cellular contexts?

Based on the published research, several experimental approaches have proven effective for studying TMEM68:

• Genetic manipulation strategies:

  • Overexpression using plasmid vectors (pcDNA™ 4/HisMax C, pFLAG-CMV-5.1, pEGFP-N3)

  • Gene knockout/knockdown using siRNA or CRISPR-Cas9

  • Site-directed mutagenesis (particularly of H129A and D135N active sites)

  • Creation of deletion mutants (e.g., ΔTMD1, ΔTMD2, ΔTMD1+2)

• Cell models:

  • HEK293 cells - for general overexpression studies

  • COS-7 cells - for localization studies

  • MCF-7 cells - for breast cancer-related studies

  • Neuro- and glioblastoma cells - for brain-relevant contexts

• Analytical methods:

  • Confocal microscopy with fluorescent tags for localization

  • Lipid droplet visualization using specific dyes (e.g., BODIPY)

  • Targeted quantitative lipidomics for comprehensive lipid profiling

  • In vitro enzymatic assays for MGAT/DGAT activity

  • Gene expression analysis for lipogenic pathways

• Treatment conditions:

  • Oleic acid (OA) supplementation to stimulate TG synthesis and lipid droplet formation

  • Use of selective inhibitors (e.g., T863 for DGAT1) to distinguish from endogenous enzymes

  • Membrane disruption agents (detergents, proteases) for topology studies

These approaches can be combined and adapted based on the specific research question and cellular context being studied.