Recombinant Human Transmembrane protein 55B (TMEM55B)

What is the general structure and cellular localization of TMEM55B?

TMEM55B is a lysosomal membrane protein containing a CX5R phosphatase motif in its catalytic domain and two transmembrane domains at the C-terminus. It primarily localizes to late endosomes and lysosomes, with trafficking that may involve transit through the plasma membrane. TMEM55B shares approximately 51% sequence identity with its paralog TMEM55A . The protein contains an acidic-dileucine motif (specifically involving Leucine10 and Leucine11) at its N-terminus that serves as a critical lysosomal targeting signal. Mutation of these residues causes accumulation of the protein at the cell surface rather than in lysosomal compartments .

How is TMEM55B regulated at the transcriptional and post-translational levels?

TMEM55B expression is transcriptionally regulated by sterol-dependent transcription factors SREBF1 and SREBF2 (also known as SREBP1 and SREBP2), as well as by TFEB and TFE3 activation during starvation or cholesterol-induced lysosomal stress . At the post-translational level, TMEM55B undergoes S-palmitoylation at multiple cysteine residues, which is critical for its proper trafficking. Mutation of all cysteines in TMEM55B prevents S-palmitoylation and causes retention of the protein in the Golgi apparatus, preventing its normal function in lysosomal positioning .

What are effective methods for studying TMEM55B localization and trafficking in cells?

For TMEM55B localization studies, researchers commonly use:

• Fluorescent protein tagging: GFP-TMEM55B or Halo-TMEM55B constructs for live-cell imaging

• Co-localization analysis: Confocal microscopy with lysosomal markers (LAMP-1) and early endosome markers (HRS) to determine subcellular localization

• FRB-FKBP rapamycin-induced heterodimerization system: For forced targeting of TMEM55B domains to specific organelles to study trafficking dynamics

• Correlative light electron microscopy (CLEM): For high-resolution visualization of TMEM55B at membrane contact sites

• Domain analysis: Testing truncated constructs (e.g., TMEM55B CD, TMEM55B TM) to identify functional regions

For trafficking studies, research has employed:

• Dominant-negative dynamin mutants (GFP-Dynamin-K44A) to inhibit endocytosis

• Mutational analysis of dileucine-based lysosomal sorting motifs

• 3D rendering of z-stacks through high-resolution live-cell microscopy

What techniques are commonly used to manipulate TMEM55B expression in experimental systems?

Researchers have successfully employed several approaches to modulate TMEM55B expression:

• RNA interference: siRNA-mediated knockdown in cell lines (HepG2, Huh7, A549)

• Antisense oligonucleotides (ASOs): For in vivo knockdown in mouse models

• CRISPR-Cas9: Generation of TMEM55B knockout cell lines (e.g., A549 cells)

• Overexpression systems: Transient transfection of GFP-TMEM55B, Halo-TMEM55B, or other tagged variants to study gain-of-function effects

• Mouse models: TMEM55B-deficient mice to study physiological roles, including combined knockouts with related proteins (e.g., TMEM106B for neurodegeneration studies)

• Zebrafish models: Knockout of tmem55 genes to study responses to stress and toxicity

How does TMEM55B regulate cellular cholesterol homeostasis?

TMEM55B has emerged as a novel regulator of cellular cholesterol metabolism through multiple mechanisms:

• LDLR regulation: TMEM55B knockdown promotes the decay rate of the low-density lipoprotein receptor (LDLR), reduces cell surface LDLR protein, impairs LDL uptake, and reduces intracellular cholesterol .

• PI(4,5)P2 modulation: As a phosphatidylinositol-4,5-bisphosphate-4-phosphatase, TMEM55B converts PI(4,5)P2 to PI5P. TMEM55B knockdown increases PI(4,5)P2 levels, particularly in lysosomes, which promotes LDLR lysosomal degradation .

• Lysosomal clustering: TMEM55B affects lysosomal positioning, which impacts receptor trafficking and degradation processes. Knockdown increases LAMP1 and lysotracker staining, stimulates lysosomal clustering, and increases LDLR-lysosome colocalization .

Importantly, TMEM55B's effect appears specific to LDLR, as knockdown does not affect other recycling receptors like transferrin receptor (TFR) or LRP1, despite their similar trafficking pathways .

What experimental approaches can be used to investigate TMEM55B's effects on LDLR and plasma cholesterol levels?

Researchers investigating TMEM55B's role in cholesterol metabolism can employ:

• In vivo measurement techniques:

  • Fast performance liquid chromatography (FPLC) to analyze lipoprotein fractions

  • Ion mobility analysis to quantify changes in different-sized lipoprotein particles

  • Western blot analysis of apolipoproteins (ApoB, ApoE) in lipoprotein fractions

• Cell-based assays:

  • LDLR protein decay assays using cycloheximide treatment

  • Cell surface LDLR quantification using flow cytometry or biotinylation approaches

  • LDL uptake assays using fluorescently labeled LDL particles

  • PI(4,5)P2 measurement by confocal microscopy and flow cytometry

• Mechanistic approaches:

  • Comparison of effects in wild-type versus LDLR-knockout (Ldlr−/−) animals

  • Co-immunoprecipitation to test for protein-protein interactions

  • Exogenous PI(4,5)P2 addition to test for rescue of TMEM55B overexpression effects

  • Inhibition of lysosomal function to determine dependence on lysosomal activity

How does TMEM55B coordinate lysosomal positioning and what protein complexes are involved?

TMEM55B plays a crucial role in lysosomal positioning by:

• Recruiting JIP4 to lysosomal membranes: TMEM55B directly interacts with the scaffold protein JIP4 (C-Jun-amino-terminal kinase-interacting protein 4), which then connects to the dynein-dynactin motor complex .

• Promoting retrograde transport: This interaction drives dynein-dependent movement of lysosomes toward the microtubule minus-end (perinuclear region) .

• Domain-specific functions: The cytoplasmic domain of TMEM55B (TMEM55B CD) is both necessary and sufficient for retrograde trafficking. When artificially targeted to other organelles like mitochondria, TMEM55B CD can induce their clustering in the cell center .

The effect is specific to TMEM55B, as TMEM55B depletion by RNAi causes lysosomes to disperse toward the cell periphery, while overexpression causes dramatic perinuclear clustering .

Sequence analysis suggests that JIP4 contains a conserved motif (residues 887-898) that is similar to the TMEM binding motif found on RILPL1, potentially explaining the interaction mechanism .

What is known about TMEM55B's role at membrane contact sites between organelles?

TMEM55B functions at specialized contact sites between organelles:

• ER-lysosome membrane contact sites (MCSs): TMEM55B interacts with and recruits the tubular ER protein Tex2 to form contacts between the ER and late endosomes/lysosomes .

• Recruitment mechanism: Live-cell confocal microscopy demonstrates that overexpression of TMEM55B greatly recruits Tex2 to TMEM55B-positive LE/lys membranes. This recruitment appears specific, as TMEM55B fails to recruit other ER proteins like E-Syt1 .

• Domain requirements: The Tex2 N-terminal region (residues 1-517) is the minimal functional module required for TMEM55B interaction, with residues 1-276 being essential but not sufficient. The SMP domain of Tex2 is not required for interaction with TMEM55B .

• Functional significance: These contact sites are critical for lysosomal functions. Tex2 is required for proper lysosomal activity, and disruption of these contacts impairs lysosomal processes, potentially affecting lipid transfer between organelles .

Both TMEM55A and TMEM55B can recruit Tex2 to these contact sites, suggesting potential redundancy in their function .

How does TMEM55B contribute to cellular responses to oxidative stress and lysosomal damage?

TMEM55B serves as a molecular sensor that coordinates multiple cellular responses to stress:

• Autophagy regulation: TMEM55B mediates NEDD4-dependent PLEKHM1 ubiquitination, causing PLEKHM1 proteasomal degradation and halting autophagosome/lysosome fusion during stress conditions .

• Lysosomal repair: TMEM55B promotes recruitment of components of the ESCRT machinery to lysosomal membranes, stimulating membrane repair in response to damage .

• Transcriptional activation: TMEM55B sequesters the FLCN/FNIP complex, facilitating translocation of the transcription factor TFE3 to the nucleus. This allows expression of transcriptional programs that enable cellular adaptation to stress .

• Stress resistance in vivo: Knockout of tmem55 genes in zebrafish embryos increases their susceptibility to oxidative stress, causing early death in response to arsenite toxicity .

These multiple mechanisms allow TMEM55B to serve as a coordinating factor in the cellular response to stress, particularly oxidative stress that can damage lysosomal membranes.

What connections exist between TMEM55B and neurodegenerative diseases?

TMEM55B has been implicated in neurodegenerative mechanisms through several pathways:

• Lysosomal transport in neurons: TMEM55B mediates retrograde transport of lysosomes, which is critical for neuronal function. Analysis of TMEM55B and TMEM106B double deficiency models indicates that multiple proteins mediate lysosomal transport in neurons, and disruption of multiple pathways increases neurodegeneration risk .

• Complementary transport mechanisms: Loss of a single protein (like TMEM55B) might not severely impact neuron survivability, while impairment of multiple retrograde transport pathways increases neurodegeneration. This suggests that in neurodegenerative diseases with lysosomal accumulation phenotypes (such as Alzheimer's Disease), more than one trafficking pathway is likely disturbed .

• Connection to Parkinson's disease: TMEM55B may play a role in sensing lysosomal dysfunction relevant to Parkinson's disease. There are potential connections between TMEM55B regulation and PD-associated pathways, including possible regulation by mTORC1 and phosphorylation-dependent mechanisms .

• Potential as biomarker: If TMEM55B and its regulatory components are controlled by lysosomal stress, this could point toward the development of biomarkers to detect lysosomal stress pathways relevant to Parkinson's disease .

How do TMEM55A and TMEM55B differ in their functions and what is known about their potential redundancy?

Despite structural similarities, TMEM55A and TMEM55B show both overlapping and distinct roles:

• Structural homology: Human TMEM55A and TMEM55B share 51% identity in amino acid sequences. Both contain a CX5R motif in their phosphatase domains and two transmembrane domains at the C-terminus .

• Similar recruitment ability: Both TMEM55A and TMEM55B can recruit Tex2 to ER-lysosome contact sites with similar efficiency, suggesting functional overlap in this mechanism .

• S-palmitoylation: Both proteins undergo S-palmitoylation at multiple cysteine residues, which is critical for their trafficking and function .

• In vivo redundancy: Recent evidence suggests that TMEM55A fulfills redundant or additive functions to TMEM55B in vivo in mice, contradicting earlier published data that suggested more distinct roles .

• Differential regulation: While both can be regulated by similar mechanisms, there is evidence of differential expression patterns. For instance, in studies of VPS35[D620N] mutation effects, researchers observed no change in TMEM55A levels while examining TMEM55B regulation .

The functional redundancy between these paralogs may provide cellular resilience in certain contexts, but the specific conditions under which one may compensate for the other remain an active area of investigation.

What is the relationship between TMEM55B, lipophagy, and metabolic dysfunction-associated steatotic liver disease (MASLD)?

Recent research has revealed complex connections between TMEM55B and liver lipid metabolism:

• Dual roles in lipid metabolism: TMEM55B deficiency has been shown to enhance lipophagy (selective autophagy targeting lipid droplets for lysosomal decay) while simultaneously impairing mitophagy (selective autophagy of mitochondria) .

• Metabolic consequences: This imbalance leads to increased fatty acid release from lysosomes to mitochondria but also causes accumulation of dysfunctional mitochondria, resulting in increased lipid accumulation and oxidative stress .

• Disease progression: Inhibition of TMEM55B in murine models accelerates the onset and progression of metabolic dysfunction-associated steatotic liver disease (MASLD) .

• Therapeutic implications: While factors that augment lipophagy have been identified as targets for MASLD therapeutic development, the case of TMEM55B illustrates the complexity of targeting such pathways—enhancing one form of selective autophagy (lipophagy) while impairing another (mitophagy) may ultimately worsen disease outcomes .