LYSET is an important Golgi membrane protein that maintains the correct trafficking of lysosomal enzymes by regulating GNPT and the mannose-6-phosphate pathway. Under nutrient-limited conditions, some cancer cells depend on lysosomal degradation of extracellular proteins to obtain amino acids, making LYSET an important research target connecting lysosomal function with cancer metabolic adaptation.

Keywords: LYSET; TMEM251; GNPTAB; mannose-6-phosphate; M6P; lysosome; macropinocytosis; tumor metabolism

1 Overview of LYSET and Lysosomal Enzyme Trafficking

1.1 Lysosomal Enzymes and the M6P Trafficking System

Lysosomes contain multiple acid hydrolases responsible for degrading macromolecules such as proteins, lipids, carbohydrates, and nucleic acids. Most soluble lysosomal enzymes are synthesized in the endoplasmic reticulum and enter the Golgi apparatus, where they must acquire mannose-6-phosphate (M6P) modifications before being recognized by M6P receptors and transported to endosomes and lysosomes.

1.2 Formation of the M6P Modification

(1) GNPT-Mediated Catalysis

In the cis-Golgi, GlcNAc-1-phosphotransferase (GNPT) recognizes lysosomal hydrolases and transfers GlcNAc-1-phosphate to their high-mannose N-glycans. GNPT consists of α/β subunits encoded by GNPTAB and a γ subunit encoded by GNPTG and is the key enzyme complex responsible for initiating M6P modification of lysosomal enzymes.

(2) NAGPA-Mediated Exposure of M6P

The uncovering enzyme encoded by NAGPA removes the terminal GlcNAc group, exposing the M6P moiety and generating a lysosomal sorting signal that can be recognized by M6P receptors.

(3) M6P Receptor-Mediated Sorting

Cation-dependent and cation-independent M6P receptors recognize M6P-tagged lysosomal enzymes in the trans-Golgi network and transport them to endosomes. As the endosomal environment becomes acidic, hydrolases dissociate from the receptors. The receptors recycle to the Golgi apparatus, whereas the hydrolases continue toward mature lysosomes.

1.3 Molecular Localization of LYSET

LYSET (lysosomal enzyme trafficking factor), also known as TMEM251 or GCAF, is a transmembrane protein located primarily in the Golgi apparatus. LYSET is neither a lysosomal hydrolase nor an M6P receptor. Instead, it is an important regulatory factor that maintains GNPT stability, maturation, and Golgi localization and acts upstream of M6P modification during lysosomal enzyme trafficking.

2 Mechanisms by Which LYSET Regulates Lysosomal Enzyme Trafficking

2.1 The LYSET–GNPT Complex

LYSET can form a complex with the GNPTα/β precursor encoded by GNPTAB. GNPTα/β must be retained in the Golgi apparatus and cleaved by Site-1 Protease (S1P/MBTPS1) to generate mature α and β subunits with normal catalytic activity. LYSET contributes to GNPT stability, proteolytic processing, and functional maintenance, enabling lysosomal enzymes to acquire normal M6P modifications.

2.2 Golgi Localization of GNPT

LYSET influences GNPT activity and participates in its Golgi localization and recycling. Studies indicate that the cytoplasmic region of LYSET can connect GOLPH3 with retromer-associated trafficking machinery, helping retain the LYSET–GNPT complex in the Golgi apparatus and reducing GNPT degradation caused by its aberrant entry into the endosomal–lysosomal pathway.

2.3 Impaired M6P Trafficking Caused by LYSET Loss

(1) Aberrant Secretion of Lysosomal Enzymes

Loss of LYSET reduces GNPT stability, maturation, and catalytic function, preventing multiple soluble lysosomal hydrolases from acquiring normal M6P modifications. These enzymes cannot be efficiently sorted to lysosomes by M6P receptors and instead enter the constitutive secretory pathway, decreasing intracellular lysosomal hydrolase levels.

(2) Reduced Lysosomal Degradative Capacity

When hydrolases such as Cathepsins fail to reach lysosomes in sufficient amounts, substrates delivered through endocytosis or autophagy cannot undergo efficient terminal degradation. This defect may lead to the accumulation of undegraded material, altered lysosomal morphology, and impaired clearance of autophagic substrates.

3 Metabolic Dependence of Cancer Cells on LYSET

3.1 Extracellular Proteins as Alternative Nutrient Sources

Solid tumors frequently contain regions of low perfusion, nutrient deficiency, and metabolic competition. Some cancer cells can internalize extracellular proteins such as albumin through macropinocytosis, deliver them to lysosomes, and degrade them into free amino acids, thereby supplementing the nutrients required for cell growth and biosynthesis.

3.2 LYSET Maintains Extracellular Protein Utilization

After extracellular proteins are internalized, they must be degraded by lysosomes containing a functional hydrolase system to release usable amino acids. Loss of LYSET disrupts the trafficking of soluble lysosomal enzymes such as Cathepsins, preventing efficient hydrolysis of extracellular proteins delivered to the endolysosomal system and reducing the ability of cancer cells to use extracellular proteins as nutrient sources.

3.3 Dependence on Autophagic Substrate Degradation

Autophagy transports intracellular proteins and organelles to lysosomes, where their degradation enables the recycling of amino acids, lipids, and other metabolites. The lysosomal hydrolase deficiency caused by LYSET loss can restrict the terminal degradation of autophagic substrates, further impairing tumor cells that depend on intracellular material recycling to maintain proteostasis and nutrient availability.

3.4 Selective Dependence Under Nutrient Stress

LYSET is not essential for cancer cell growth under all culture conditions. When sufficient free amino acids are directly available, some LYSET-deficient cells can continue to proliferate. When free amino acids are limited and extracellular proteins are required as alternative nutrient sources, the growth defects caused by LYSET loss become substantially more pronounced.

4 LYSET-Associated Metabolic Vulnerabilities in Cancer

4.1 Lysosomal Acquisition of Protein-Derived Nutrients

Macropinocytosis delivers extracellular proteins into cells, whereas lysosomal proteolysis converts these macromolecules into usable nutrients. LYSET acts at the M6P trafficking step before hydrolases enter lysosomes. Targeting LYSET may therefore simultaneously affect multiple soluble lysosomal hydrolases rather than restricting the function of a single Cathepsin.

4.2 The Lysosomal Degradation Network

Loss of LYSET can simultaneously disrupt the degradation of endocytic substrates, autophagic substrates, and other materials that depend on lysosomal hydrolases. In cancer cells with high demands for nutrient recycling and protein degradation, impaired M6P-dependent enzyme trafficking may create a metabolic vulnerability spanning multiple lysosomal degradation processes.

4.3 Normal Tissue Dependence and the Therapeutic Window

LYSET is also an essential component of the normal M6P-dependent lysosomal enzyme trafficking system. Pathogenic defects in TMEM251 can cause severe lysosomal storage disease-like phenotypes, indicating that prolonged, systemic, and complete inhibition of LYSET may also damage normal tissues. The key to targeting LYSET in cancer lies in exploiting the increased lysosomal dependence of cancer cells under nutrient stress rather than completely disabling lysosomal enzyme trafficking in normal cells.

5 Major LYSET-Associated Intervention Strategies

5.1 Direct Targeting of LYSET

Current studies of LYSET function mainly rely on genetic approaches such as CRISPR/Cas9 knockout, CRISPR interference, and RNA interference. Selective small-molecule LYSET inhibitors that have been fully validated and advanced into mature development are not yet available. The LYSET–GNPT interaction interface, transmembrane regions, and Golgi-localization mechanisms provide a foundation for future direct pharmacological intervention.

5.2 Intervention in the LYSET–GNPT Trafficking Complex

LYSET maintains the M6P pathway through interactions with GNPT, GOLPH3, and retromer-associated trafficking machinery. Disrupting LYSET–GNPT binding, GNPT maturation, or LYSET-mediated Golgi recycling may reduce the efficiency of M6P modification and lysosomal enzyme trafficking, although these strategies currently remain mainly at the mechanistic research stage.

5.3 Inhibition of Lysosomal Acidification and Proteolysis

(1) Inhibition of Lysosomal Acidification

V-ATPase maintains the acidic lysosomal environment required for the catalytic activity of most acid hydrolases. Bafilomycin A1 inhibits V-ATPase, whereas Chloroquine perturbs the environment of acidic organelles. Both tools can reduce terminal lysosomal degradation and are used for functional comparison with the enzyme-trafficking defects caused by LYSET loss.

(2) Cathepsin Inhibition

E-64d, VBY-825, and other Cathepsin inhibitors can directly reduce lysosomal proteolytic activity. These compounds are used to determine whether extracellular protein utilization, autophagic substrate degradation, and cancer cell adaptation to nutrient stress depend on Cathepsin-mediated terminal hydrolysis.

5.4 Intervention in Extracellular Protein Uptake and Autophagy

EIPA is commonly used to inhibit macropinocytosis-associated processes and reduce extracellular protein uptake, whereas tools such as 3-Methyladenine can interfere with autophagy-associated signaling. Blocking exogenous protein entry and intracellular material recycling separately can help identify the nutrient sources responsible for cancer cell dependence on the LYSET–lysosomal degradation system.

6 Research Progress on LYSET

6.1 LYSET and the Nutritional Utilization of Extracellular Proteins

(1) Genetic Screening Under Nutrient-Limited Conditions

In 2022, researchers identified TMEM251 through genome-wide genetic screening under nutrient-limited conditions. When cancer cells lacked free essential amino acids and depended on extracellular proteins to sustain growth, loss of TMEM251 substantially restricted cell proliferation. The protein was consequently named lysosomal enzyme trafficking factor (LYSET).

(2) Defective Lysosomal Enzyme Trafficking

Loss of LYSET markedly reduces the M6P modification of lysosomal enzymes and causes the aberrant secretion of multiple hydrolases, impairing lysosomal degradation of extracellular proteins and autophagic substrates. These findings established a connection between LYSET-mediated lysosomal enzyme trafficking and cancer cell adaptation to nutrient stress.

6.2 LYSET and M6P-Dependent Enzyme Trafficking

(1) Independent Genetic Screening

Independent genetic studies conducted during the same period also identified LYSET and demonstrated that its loss causes widespread abnormalities in lysosomal hydrolase trafficking and reduced Cathepsin function. These results further established LYSET as an important component of M6P-dependent lysosomal enzyme trafficking.

(2) Evidence from Human Genetics

Pathogenic variants in TMEM251 can cause defects in lysosomal hydrolase trafficking and lysosomal storage disease-like manifestations. These observations extend the function of LYSET from cell-based genetic screening to human genetic disease and demonstrate its fundamental role in maintaining normal lysosomal homeostasis.

6.3 GCAF/TMEM251 and GNPT Activation

(1) Regulation of GNPT Processing

TMEM251 has also been identified as GNPTAB cleavage and activity factor (GCAF). LYSET/GCAF participates in S1P-mediated processing of the GNPTAB precursor and maintains normal GNPT catalytic function, enabling lysosomal hydrolases to acquire M6P targeting modifications.

(2) Alterations in Lysosomal Homeostasis

Loss of TMEM251 can lead to reduced M6P modification, abnormal hydrolase secretion, accumulation of undegraded substrates, and altered lysosomal morphology, linking impaired GNPT processing and activity with broader lysosomal dysfunction.

6.4 GOLPH3–Retromer–LYSET Regulation

(1) Mechanism of Golgi Localization

Further mechanistic studies indicate that the cytoplasmic region of LYSET helps connect GOLPH3 with retromer-associated trafficking machinery, retaining the LYSET–GNPT complex in the Golgi apparatus or recycling it from the endosomal pathway and limiting aberrant GNPT entry into lysosomal degradation pathways.

(2) GNPT Stability and Maturation

Impaired LYSET function can affect GNPT Golgi localization, stability, and proteolytic maturation, ultimately reducing M6P modification and lysosomal hydrolase trafficking. LYSET therefore extends beyond a simple GNPT accessory factor and functions as a trafficking regulatory node connecting GNPT processing, localization, and recycling.

7 Products

7.1 Products for Lysosomal Acidification and Proteolysis Research

7.2 Products for Extracellular Protein Utilization and Autophagy Research

7.3 Products for Lysosome, Golgi, and Cathepsin Research

LYSET enables cancer cells to use extracellular proteins and recycled intracellular substrates under nutrient stress by maintaining GNPT function and M6P-dependent lysosomal enzyme trafficking. Its research value centers on the lysosomal metabolic dependence of tumor cells, while direct pharmacological targeting of LYSET remains at an early target-validation stage.

References

[1] Pechincha C, Groessl S, Kalis R, et al. Lysosomal enzyme trafficking factor LYSET enables nutritional usage of extracellular proteins. Science. 2022;378(6615):eabn5637.

[2] Richards CM, Jabs S, Qiao W, et al. The human disease gene LYSET is essential for lysosomal enzyme transport and viral infection. Science. 2022;378(6615):eabn5648.

[3] Zhang W, Yang X, Li Y, et al. GCAF(TMEM251) regulates lysosome biogenesis by activating the mannose-6-phosphate pathway. Nat Commun. 2022;13:5351.

[4] Jabs S, Groessl S, Pechincha C, et al. LYSET/TMEM251—A novel key component of the mannose 6-phosphate pathway. Autophagy. 2023.

[5] Lemberg MK, Jabs S. Lysosomal enzyme trafficking: from molecular mechanisms to human diseases. Trends Cell Biol. 2024;34(3):198–210.

For more related articles, please see below:

[1] The Role of Lysosomes in Metabolic and Autoimmune Diseases

[2] Animal Modeling—Tumor Disease Models