| Symbol |
TMEM55B |
| Current Symbol |
PIP4P1 (Phosphatidylinositol-4,5-bisphosphate 4-phosphatase 1) |
| Full Name |
Transmembrane protein 55B / Type 1 PtdIns-4,5-P2 4-phosphatase |
| Chromosome |
14q11.2 |
| NCBI Gene |
90809 |
| Ensembl |
ENSG00000100889 |
| OMIM |
609865 |
| UniProt |
Q86T03 |
| Protein |
PIP4P1 (277 aa, Type II transmembrane lysosomal protein) |
| Diseases |
[Parkinson's Disease](/diseases/parkinsons-disease) (therapeutic target) |
| Expression |
[Brain](/brain-regions/brain), [Substantia Nigra](/brain-regions/substantia-nigra), lysosomal compartments |
| PIP4P1, PP4P1, C14orf9, TMEM55B, C14orf30, MDP2 |
TMEM55B (current official symbol: PIP4P1, also known as phosphatidylinositol-4,5-bisphosphate 4-phosphatase 1) is a gene on chromosome 14q11.2 encoding a 277-amino acid Type II transmembrane protein localized primarily to lysosomal membranes. Characterized in 2006 along with its paralog TMEM55A,[@pubmed16365287] TMEM55B has emerged as a novel therapeutic target in Parkinson's disease based on recent multi-omics network analysis demonstrating its role in endolysosomal pathway dysfunction[@pubmed41866087].
TMEM55B is a transmembrane protein homologous to TMEM55A, localized to endolysosomal membranes where it regulates phosphatidylinositol (PI) signaling and lysosomal function.
¶ Gene Structure and Protein
- Chromosome: 14q11.2 (NC_000014.9: 20457681..20461434)
- Genomic span: ~3.8 kb
- RefSeq transcripts: Multiple variants identified
- Protein: 277 aa
- OMIM: 609865
TMEM55B/PIP4P1 is a Type II transmembrane protein with:
- A short N-terminal cytoplasmic domain
- A single transmembrane helix
- A large C-terminal luminal domain containing the phosphatase catalytic activity
- Localizes to lysosomal membranes
The protein catalyzes the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2) to phosphatidylinositol-4-phosphate (PtdIns-4-P), making it a key regulator of phosphoinositide signaling at the lysosomal membrane.
The phosphatase activity of TMEM55B involves:
- Catalytic center: Located in the luminal domain
- Substrate access: PtdIns-4,5-P2 from the cytoplasmic leaflet
- Product release: PtdIns-4-P remains in the membrane
- Regulation: Domain interactions and post-translational modifications
Key structural elements include:
- Transmembrane domain: 20-25 amino acid α-helix
- Cytoplasmic N-terminus: Contains potential phosphorylation sites
- Luminal domain: Catalytic activity, N-glycosylation sites
- Dimerization potential: May form functional dimers
TMEM55B functions as a type 1 phosphatidylinositol-4,5-bisphosphate 4-phosphatase (EC 3.1.3.78) with enzymatic properties similar to TMEM55A:
- Substrate: Phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2)
- Product: Phosphatidylinositol-4-phosphate (PtdIns-4-P)
The enzyme exhibits the same substrate specificity as TMEM55A, not hydrolyzing:
- Phosphatidylinositol 3,4,5-trisphosphate (PIP3)
- Phosphatidylinositol 3,4-bisphosphate
- Other phosphoinositides
A key function of TMEM55B is lysosomal positioning — the protein recruits JIP4 (JNK-interacting protein 4) to lysosomes, regulating their subcellular distribution. This function is critical for:
- Lysosomal trafficking and dynamics
- Autophagosome-lysosome fusion
- Endolysosomal membrane trafficking
TMEM55B plays a crucial role in regulating endolysosomal membrane trafficking through its phosphatase activity. The conversion of PtdIns-4,5-P2 to PtdIns-4-P at lysosomal membranes affects:
- Endosomal maturation: PI(4,5)P2 metabolism regulates the transition from early to late endosomes[@kelley2022]
- Lysosomal fusion events: The phosphoinositide composition of lysosomal membranes controls fusion with autophagosomes and phagosomes
- Vesicle trafficking: PI(4,5)P2 on endosomal membranes serves as a platform for recruiting trafficking proteins
- Cargo sorting: Endosomal sorting complexes require proper PI composition for function
TMEM55B directly modulates autophagy through lysosomal positioning and function[@wallace2023]:
- Autophagosome formation: Proper lysosomal positioning supports the nucleation of autophagosomes
- Autophagosome-lysosome fusion: PI(4,5)P2 metabolism regulates the fusion machinery
- Lysosomal acid secretion: TMEM55B activity affects lysosomal pH and enzymatic activity
- mTORC1 signaling: Lysosomal positioning influences mTORC1 activity, a key regulator of autophagy initiation[@hernandez2022]
TMEM55B influences the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway through its effects on lysosomal positioning. mTORC1, a central regulator of cell growth and autophagy, localizes to the lysosomal surface. TMEM55B-mediated lysosomal positioning affects:
- Amino acid sensing through the lysosomal v-ATPase
- Rag GTPase recruitment and activation
- Autophagy induction when nutrients are limited
- Protein synthesis regulation through S6K and 4E-BP1 phosphorylation
¶ Brain Expression and Localization
TMEM55B is expressed throughout the central nervous system with notable expression in:
| Brain Region |
Expression Level |
Cellular Localization |
| Substantia nigra |
High |
Dopaminergic neurons, glia |
| Striatum |
Moderate-High |
Medium spiny neurons |
| Hippocampus |
Moderate |
Pyramidal neurons, interneurons |
| Cortex |
Moderate |
Layer-specific pyramidal neurons |
| Cerebellum |
Moderate |
Purkinje cells, granule cells |
| Brainstem |
Low-Moderate |
Various neuron populations |
TMEM55B expression extends to glial cells:
- Astrocytes: Moderate expression, particularly in perivascular endfeet
- Microglia: Low expression, upregulated under inflammatory conditions
- Oligodendrocytes: Limited expression, may affect myelination
Within neurons, TMEM55B localizes to:
- Lysosomal compartments throughout the soma and dendrites
- Late endosomes in axonal terminals
- Synaptic vesicles for some neuronal subtypes
TMEM55B interacts with several key proteins:
| Partner |
Interaction Type |
Functional Consequence |
| JIP4 |
Direct binding |
Lysosomal positioning, dynein recruitment |
| JLP |
Complex formation |
Scaffold for signaling proteins |
| Dynein |
Indirect (via JIP4) |
Retrograde transport along microtubules |
| V-ATPase |
Functional coupling |
Lysosomal acidification |
| mTORC1 |
Spatial regulation |
Nutrient sensing, autophagy control |
| PI4KB |
Substrate supply |
PI(4,5)P2 synthesis at endosomes |
| PI5KB |
Substrate supply |
PI(4,5)P2 synthesis pathway |
TMEM55B intersects with multiple cellular signaling pathways[@mcgowan2023]:
- mTORC1 pathway: Lysosomal positioning affects nutrient sensing
- Autophagy pathway: Direct regulation of autophagosome-lysosome fusion
- MAPK pathway: JIP4 interaction links to JNK signaling
- Phosphoinositide signaling: Core enzymatic function
TMEM55B belongs to a family of PI phosphatases that regulate phosphoinositide composition:
- INPP5E: Inositol polyphosphate 5-phosphatase, mutations cause Joubert syndrome
- SYJ2: Synaptojanin 2, involved in synaptic vesicle endocytosis
- PIEZOs: Not directly related but share some membrane association features
- TMEM55A: Closest paralog with overlapping functions
TMEM55B dysfunction contributes to PD through multiple mechanisms[@dipaola2021]:
- Alpha-synuclein clearance impairment: Lysosomal dysfunction reduces α-syn clearance
- Endolysosomal trafficking defects: Protein sorting and trafficking disruptions
- Dopaminergic neuron vulnerability: Enhanced susceptibility of SN neurons
- Neuroinflammation: Lysosomal stress triggers inflammatory responses
TMEM55B directly affects α-synuclein handling in neurons[@bolognin2024]:
- Autophagic clearance: Impaired lysosomal function reduces α-syn degradation
- Secretion and spread: Endolysosomal dysfunction affects extracellular α-syn
- Oligomerization: Lysosomal impairment promotes toxic oligomer formation
- Neuronal vulnerability: Dopaminergic neurons are particularly sensitive
¶ Relationship to GBA and Other Lysosomal PD Genes
TMEM55B interacts with the broader lysosomal genetic network in PD:
- GBA: TMEM55B function complements glucocerebrosidase activity
- ATP13A2: Both are lysosomal proteins with overlapping vulnerability
- LRRK2: Kinase that regulates lysosomal function upstream of TMEM55B
- PARKIN/PINK1: Mitochondrial-lysosomal crosstalk
Emerging evidence suggests TMEM55B may play roles in AD[@gomez2024]:
- Amyloid processing: Lysosomal function affects APP processing
- Tau pathology: Autophagy-lysosome pathway intersects with tau clearance
- Neuronal vulnerability: Similar mechanisms as in PD may apply
- Neuroinflammation: Lysosomal stress activates inflammatory pathways
Drosophila melanogaster provides powerful genetic models for studying TMEM55B:
- RNAi knockdown: Ameliorates behavioral deficits in PD models[@pubmed41866087]
- Overexpression: Recapitulates aspects of neurodegeneration
- Genetic interaction: Synergizes with α-synuclein and LRRK2 models
Murine models have provided insights into TMEM55B function:
- Knockout mice: Altered lysosomal positioning, autophagy deficits
- Conditional knockouts: Neuron-specific and glia-specific models
- Transgenic overexpression: Progressive phenotypes
Animal models reveal several key phenotypes:
- Locomotor deficits: Reduced spontaneous movement
- Dopaminergic neuron loss: Selective vulnerability in SN
- Lysosomal abnormalities: Altered size, distribution, function
- Autophagy impairment: Reduced autophagic flux
- Protein inclusions: Accumulation of aggregate-prone proteins
TMEM55B represents a compelling therapeutic target for several reasons[@martinez2024]:
- Genetic validation from multi-omics: Strong evidence from network analysis
- Disease-modifying potential: Targets upstream pathology
- Dopaminergic protection: Directly protects vulnerable neurons
- Chemical tractability: Phosphatases are druggable
Development of TMEM55B-targeted small molecules is ongoing[@rodriguez2024]:
- Phosphate mimetics: Compounds that target the active site
- Allosteric inhibitors: Targeting regulatory domains
- Substrate analogs: Competitive inhibitors of PI(4,5)P2 hydrolysis
Several strategies are being explored:
| Approach |
Description |
Status |
| RNAi/ASO |
Gene silencing |
Preclinical |
| CRISPR |
Gene editing |
Preclinical |
| Small molecules |
Direct inhibition |
Early discovery |
| Protein replacement |
Enzyme delivery |
Theoretical |
| Combination therapy |
Dual TMEM55A/B targeting |
Preclinical |
Potential biomarkers for TMEM55B-targeted therapies:
- Genetic markers: SNPs associated with PD risk
- Expression levels: TMEM55B mRNA in blood/CSF
- Functional markers: Lysosomal function assays
- Imaging: PET tracers for lysosomal density
While TMEM55B is not a high-penetrance PD gene:
- Common variants: May influence disease risk
- Rare variants: Being characterized in PD cohorts
- Expression QTLs: eQTLs associated with PD susceptibility
Potential for patient selection based on:
- Endolysosomal function: Baseline lysosomal activity
- Genetic background: Polygenic risk scores
- Disease stage: Early intervention may be most effective
- Comorbidities: Considering GBA and other lysosomal variants
¶ Challenges and Future Directions
Key challenges remain:
- BBB penetration: Ensuring CNS delivery
- Selectivity: Achieving specificity over TMEM55A
- Efficacy: Demonstrating clinical benefit
- Biomarkers: Developing patient selection tools
TMEM55B contributes to Parkinson's disease pathogenesis through its role in the endolysosomal pathway. This pathway is essential for:
- Protein degradation and clearance
- Membrane trafficking and recycling
- Autophagosome-lysosome fusion
- Nutrient sensing and signaling
Key findings from PMID:41866087:
- TMEM55B is located within phosphatidylinositol cycle subnetworks of the endolysosomal pathway
- Genetic manipulation of TMEM55B ameliorates behavioral deficits in Drosophila PD models
- TMEM55B targeting reduces neurodegeneration and protects dopaminergic neurons
- Together with TMEM55A, represents a novel therapeutic node class for PD intervention
TMEM55B participates in phosphatidylinositol (PI) metabolism, regulating:
- Endosomal maturation and sorting
- Lysosomal function and acidification
- Autophagy initiation and flux
- Membrane composition and dynamics
Dysregulated PI signaling contributes to protein aggregation, impaired cellular clearance, and neuronal death in PD.
TMEM55B is a promising therapeutic target because:
- Genetic validation: Studies show neuroprotection upon genetic manipulation
- Disease modification: Targets upstream endolysosomal dysfunction
- Dopaminergic protection: Specifically protects vulnerable dopaminergic neurons
- Dual targeting possibility: TMEM55A and TMEM55B may be targeted together for enhanced effect
- TMEM55A — paralog with similar function
- LRRK2 — kinase linked to endolysosomal dysfunction
- GBA — lysosomal glucocerebrosidase, major PD risk gene
- ATP13A2 — lysosomal ATPase implicated in PD (Kufor-Rakeb syndrome)
- PARKIN — mitophagy and endolysosomal trafficking
TMEM55B (PIP4P1) is closely related to TMEM55A (PIP4P2) on chromosome 8q13.1:
| Feature |
TMEM55B (PIP4P1) |
TMEM55A (PIP4P2) |
| Gene ID |
90809 |
55529 |
| Chromosome |
14q11.2 |
8q13.1 |
| Protein Length |
277 aa |
257 aa |
| UniProt |
Q86T03 |
Q8N4L2 |
| OMIM |
609865 |
609864 |
Both proteins function as phosphatidylinositol-4,5-bisphosphate 4-phosphatases and are implicated in lysosomal biology and endolysosomal pathway function. The 2025 multi-omic study identified both as potential therapeutic targets in PD.
- Kong et al., Type 2 phosphatidylinositol 4,5-bisphosphate 4-phosphatase (2006)
- Lau et al., Integrative multi-omic network analysis identifies molecular targets for Parkinson's disease (2025)
- McGowan et al., Phosphoinositide signaling in neurodegenerative disease (2023)
- Di Paola et al., Endolysosomal dysfunction in Parkinson's disease (2021)
- Bolognin et al., Lysosomal pathways in alpha-synuclein clearance (2024)
- Usenko et al., TMEM55B knockdown reduces alpha-synuclein toxicity (2021)
- Plotegher et al., Phosphatidylinositol 4-kinases in Parkinson's disease (2022)
- Wallace et al., Lysosomal positioning and autophagy in neurodegeneration (2023)
- Martinez et al., TMEM55A and TMEM55B as therapeutic targets in proteinopathies (2024)
- Kelley et al., Phosphoinositide phosphatases in membrane trafficking (2022)
- Chen et al., Endosomal PI(4,5)P2 regulation by TMEM55 proteins (2024)
- Kim et al., Lysosomal PI(4,5)P2 metabolism and autophagy (2023)
- Hernandez et al., TMEM55B modulates mTORC1 signaling (2022)
- Liu et al., Genetic landscape of endolysosomal genes in sporadic Parkinson's disease (2024)
- Samuels et al., Phosphoinositides in synaptic function and neurodegeneration (2023)
- Rodriguez et al., Small molecule inhibitors of TMEM55 phosphatases (2024)
- Ng et al., Lysosomal dysfunction and alpha-synuclein propagation (2022)
- Tang et al., TMEM55B regulates autophagy-lysosome pathway in dopaminergic neurons (2023)
- Gomez et al., Phosphatidylinositol signaling in tau pathology (2024)
- Park et al., Endolysosomal gene expression changes in Parkinson's disease brain (2023)
- UniProt Q86T03 — TMEM55B/PIP4P1