The Ragulator-Rag complex is a regulator of lysosomal signalling and trafficking in eukaryotic cells, which plays an important role in regulating cell metabolism and growth in response to nutrient availability in the cell. [1] The Ragulator-Rag Complex is composed of five LAMTOR subunits, which work to regulate MAPK and mTOR complex 1. [2] The LAMTOR subunits form a complex with Rag GTPase and v-ATPase, which sits on the cell’s lysosomes and detects the availability of amino acids. [1] If the Ragulator complex receives signals for low amino acid count, it will start the process of catabolizing the cell. If there is an abundance of amino acids available to the cell, the Ragulator complex will signal that the cell can continue to grow. [1] Ragulator proteins come in two different forms: Rag A/Rag B and Rag C/Rag D. These interact to form heterodimers with one another.
Lamtor1 | |||||||
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Identifiers | |||||||
Symbol | 26068 | ||||||
Alt. symbols | p18 | ||||||
Alt. names | p18 | ||||||
NCBI gene | 55004 | ||||||
OMIM | 613510 | ||||||
RefSeq | NM_017907.2 | ||||||
UniProt | Q6IAA8 | ||||||
Other data | |||||||
Locus | Chr. 11 q13.4 | ||||||
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Lamtor2 | |||||||
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Identifiers | |||||||
Symbol | 29796 | ||||||
Alt. symbols | p14 | ||||||
NCBI gene | 28956 | ||||||
OMIM | 610389 | ||||||
RefSeq | NM_014017.3 | ||||||
UniProt | Q9Y2Q5 | ||||||
Other data | |||||||
Locus | Chr. 1 q22 | ||||||
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Lamtor3 | |||||||
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Identifiers | |||||||
Symbol | 15606 | ||||||
Alt. symbols | MP1 | ||||||
NCBI gene | 8649 | ||||||
OMIM | 603296 | ||||||
RefSeq | NM_021970.3 | ||||||
UniProt | Q9UHA4 | ||||||
Other data | |||||||
Locus | Chr. 4 q23 | ||||||
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Lamtor4 | |||||||
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Identifiers | |||||||
Symbol | 33772 | ||||||
Alt. symbols | c7orf59 | ||||||
NCBI gene | 389541 | ||||||
RefSeq | NM_001008395.3 | ||||||
UniProt | Q0VGL1 | ||||||
Other data | |||||||
Locus | Chr. 7 q22.1 | ||||||
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Lamtor5 | |||||||
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Identifiers | |||||||
Symbol | 17955 | ||||||
Alt. symbols | HBXIP | ||||||
NCBI gene | 10542 | ||||||
OMIM | 608521 | ||||||
RefSeq | NM_006402.2 | ||||||
UniProt | O43504 | ||||||
Other data | |||||||
Locus | Chr. 1 p13.3 | ||||||
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mTORC1 is a complex within the lysosome membrane that initiates growth when promoted by a stimulus, such as growth factors. A GTPase is a key component in cell signaling, and there were, in 2010, four RAG complexes discovered within the lysosomes of cells. In 2008, it was thought that these RAG complexes would slow down autophagy and activate cell growth by interacting with mTORC1. [3] However, in 2010, the Ragulator was discovered. Researchers determined that the function of this Ragulator was to interact with the RAG A, B, C, and D complexes to promote cell growth. This discovery also led to the first use of the term “Rag-Ragulator” complex, because of the interaction between these two. [4]
The amino acid level, cell growth, and other important factors are influenced by the mTOR Complex 1 pathway. On the lysosomal surface, the amino acids signal the activation of the four Rag proteins (RagA, RagB, RagC, and RagD) to translocate mTORC1 to the site of activation. [5]
A 2014 study noted that AMPK (AMP-activated protein kinase) and mTOR play important roles in managing different metabolic programs. It was also found that the protein complex v-ATPase-Ragulator was essential for activation of mTOR and AMPK. The v-ATPase-Ragulator complex is also used as an initiating sensor for energy stress, and serves as an endosomal docking site for LKB1-mediated AMPK activation by forming the v-ATPase-Ragulator-AXIN/LKB1-AMPK complex. This allows a switch between catabolism and anabolism. [6]
In 2016, it was established that RagA and Lamtor4 were key to microglia functioning and biogenesis regulation within the lysosome. Further studies also indicate that the Ragulator-Rag complex interacts with proteins other than mTORC1, including an interaction with v-ATPase, which facilitates functions within microglia of the lysosome. [7]
In 2017, the Ragulator was thought to regulate the position of the lysosome, and interact with BORC, a multi subunit complex located on the surface of the lysosomal membrane. [8] Both BORC and mTORC1 work together in activating the GTPases to change the position of the lysosome. It was concluded that BORC and GTPases compete for a binding site in the LAMTOR 2 protein to reposition the lysosome. [9]
While the intricate functions of the Ragulator-Rag Complex are not fully understood, it is known that the Ragulator-Rag Complex associates with the lysosome and plays a key role in mTOR (mammalian target of rapamycin) signaling regulation. [10] mTOR signaling is sensitive to amino acid concentrations in the cytoplasm of the cell, and the Ragulator complex works to detect amino acid concentration and transmit signals that activate, or inhibit, mTORC1. [11]
The Ragulator, along with the Rag GTPases and v-ATPases, are part of an amino acid identifying pathway, and are necessary for the localization of the mTORC1 to the lysosome surface. The Ragulator and v-ATPases reside on the lysosomal surface. The Rag GTPases cannot be directly bound to the lysosome because they lack the proteins necessary to bind to its lipid bilayer, so Rag GTPases must instead be anchored to the Ragulator. [12] The Ragulator is bound to the surface via the V-ATPase. [13] The Ragulator is a crystalized structure composed of five different subunits; LAMTOR 1, LAMTOR 2, LAMTOR 3, LAMTOR 4, LAMTOR 5. There are two sets of obligate heterodimers in the complex, LAMTOR 2/3, which sits right above LAMTOR 4/5. [12] The LAMTOR 1 dimer does not have the same structure as the other subunits. LAMTOR 1 surrounds most of the two heterodimers, providing structural support and keeping the heterodimers in place. When amino acids are present, the subunits are folded and positioned in such a way that allows for the Rag-GTPases to be anchored to its primary docking site of LAMTOR 2/3 on the Ragulator. [12] The Rag-GTPases consist of two sets of heterodimers; RAGs A/B and RAGs C/D. Before Rag-GTPases can bind to the Ragulator, Rag A/B must be GTP loaded via guanine nucleotide exchange factors (GEFs), and RAG C/D must be GDP loaded. [14] Once Rag-GTPases are bound to the regulator complex, the mTORC1 can be translocated to the surface of the lysosome. At the lysosomal surface, the mTORC1 will then bind to Rheb, but only if Rheb was first loaded to a GTP via GEFs. [13] If the amount of nutrients and the concentration of amino acids are sufficient, mTORC1 will be activated.
The lysosomal membrane is the main area in which mTORC1 is activated. However, some activation can occur in the Golgi apparatus and the peroxisome. [15] In mammalian cells, GTPase RagA and RagB are heterodimers with RagC and RagD, respectively. When enough amino acids are present, RagA/B GTPase becomes activated, which leads to the translocation of mTORC1 from the cytoplasm to the lysosome surface, via the Raptor. This process brings mTORC1 in close enough proximity to Rheb for Rheb to either (1) cause a conformational change to mTORC1, leading to and increase in substrate turnover, or (2) induce kinase activity of mTORC1. Rags do not contain membrane-targeting sequences, and as a result, depend on the entire Ragulator-Rag Complex to bind to the lysosome, activating mTORC1. [16]
While most amino acids indirectly activate mTORC1 in mammals, Leucine has the ability to directly activate mTORC1 in cells that are depleted of amino acids. Yeast contain LRS (leucyltRNA synthetase), which is a molecule that can interact with Rags, directly activating the molecule. [16]
The complex consists of five subunits, [2] named LAMTOR 1-5 (Late endosomal/lysosomal adaptor, mapk and mtor activator 1), however several have alternative names.
GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a protein domain common to many GTPases.
5' AMP-activated protein kinase or AMPK or 5' adenosine monophosphate-activated protein kinase is an enzyme that plays a role in cellular energy homeostasis, largely to activate glucose and fatty acid uptake and oxidation when cellular energy is low. It belongs to a highly conserved eukaryotic protein family and its orthologues are SNF1 in yeast, and SnRK1 in plants. It consists of three proteins (subunits) that together make a functional enzyme, conserved from yeast to humans. It is expressed in a number of tissues, including the liver, brain, and skeletal muscle. In response to binding AMP and ADP, the net effect of AMPK activation is stimulation of hepatic fatty acid oxidation, ketogenesis, stimulation of skeletal muscle fatty acid oxidation and glucose uptake, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipogenesis, inhibition of adipocyte lipolysis, and modulation of insulin secretion by pancreatic β-cells.
The mammalian target of rapamycin (mTOR), also referred to as the mechanistic target of rapamycin, and sometimes called FK506-binding protein 12-rapamycin-associated protein 1 (FRAP1), is a kinase that in humans is encoded by the MTOR gene. mTOR is a member of the phosphatidylinositol 3-kinase-related kinase family of protein kinases.
The bafilomycins are a family of macrolide antibiotics produced from a variety of Streptomycetes. Their chemical structure is defined by a 16-membered lactone ring scaffold. Bafilomycins exhibit a wide range of biological activity, including anti-tumor, anti-parasitic, immunosuppressant and anti-fungal activity. The most used bafilomycin is bafilomycin A1, a potent inhibitor of cellular autophagy. Bafilomycins have also been found to act as ionophores, transporting potassium K+ across biological membranes and leading to mitochondrial damage and cell death.
The tumor suppressor gene FLCN encodes the protein folliculin, also known as Birt–Hogg–Dubé syndrome protein, which functions as an inhibitor of Lactate Dehydrogenase-A and a regulator of the Warburg effect. Folliculin (FLCN) is also associated with Birt–Hogg–Dubé syndrome, which is an autosomal dominant inherited cancer syndrome in which affected individuals are at risk for the development of benign cutaneous tumors (folliculomas), pulmonary cysts, and kidney tumors.
Tuberous Sclerosis Complex 2 (TSC2), also known as Tuberin, is a protein that in humans is encoded by the TSC2 gene.
RHEB also known as Ras homolog enriched in brain (RHEB) is a GTP-binding protein that is ubiquitously expressed in humans and other mammals. The protein is largely involved in the mTOR pathway and the regulation of the cell cycle.
Ribosomal protein S6 kinase beta-1 (S6K1), also known as p70S6 kinase, is an enzyme that in humans is encoded by the RPS6KB1 gene. It is a serine/threonine kinase that acts downstream of PIP3 and phosphoinositide-dependent kinase-1 in the PI3 kinase pathway. As the name suggests, its target substrate is the S6 ribosomal protein. Phosphorylation of S6 induces protein synthesis at the ribosome.
5'-AMP-activated protein kinase catalytic subunit alpha-1 is an enzyme that in humans is encoded by the PRKAA1 gene.
Regulatory-associated protein of mTOR also known as raptor or KIAA1303 is an adapter protein that is encoded in humans by the RPTOR gene. Two mRNAs from the gene have been identified that encode proteins of 1335 and 1177 amino acids long.
DNA-damage-inducible transcript 4 (DDIT4) protein also known as protein regulated in development and DNA damage response 1 (REDD1) is a protein that in humans is encoded by the DDIT4 gene.
Transcription factor EB is a protein that in humans is encoded by the TFEB gene.
Tuberous sclerosis proteins 1 and 2, also known as TSC1 (hamartin) and TSC2 (tuberin), form a protein-complex. The encoding two genes are TSC1 and TSC2. The complex is known as a tumor suppressor. Mutations in these genes can cause tuberous sclerosis complex. Depending on the grade of the disease, intellectual disability, epilepsy and tumors of the skin, retina, heart, kidney and the central nervous system can be symptoms.
mTORC1, also known as mammalian target of rapamycin complex 1 or mechanistic target of rapamycin complex 1, is a protein complex that functions as a nutrient/energy/redox sensor and controls protein synthesis.
mTOR Complex 2 (mTORC2) is an acutely rapamycin-insensitive protein complex formed by serine/threonine kinase mTOR that regulates cell proliferation and survival, cell migration and cytoskeletal remodeling. The complex itself is rather large, consisting of seven protein subunits. The catalytic mTOR subunit, DEP domain containing mTOR-interacting protein (DEPTOR), mammalian lethal with sec-13 protein 8, and TTI1/TEL2 complex are shared by both mTORC2 and mTORC1. Rapamycin-insensitive companion of mTOR (RICTOR), mammalian stress-activated protein kinase interacting protein 1 (mSIN1), and protein observed with rictor 1 and 2 (Protor1/2) can only be found in mTORC2. Rictor has been shown to be the scaffold protein for substrate binding to mTORC2.
David M. Sabatini is an American scientist and a former professor of biology at the Massachusetts Institute of Technology. From 2002 to 2021, he was a member of the Whitehead Institute for Biomedical Research. He was also an investigator of the Howard Hughes Medical Institute from 2008 to 2021 and was elected to the National Academy of Sciences in 2016. He is known for his contributions in the areas of cell signaling and cancer metabolism, most notably the co-discovery of mTOR.
Nitrogen permease regulator-like 3 is a protein that in humans is encoded by the NPRL3 gene.
C12orf66 is a protein that in humans is encoded by the C12orf66 gene. The C12orf66 protein is one of four proteins in the KICSTOR protein complex which negatively regulates mechanistic target of rapamycin complex 1 (mTORC1) signaling.
Solute carrier family 38 member 9 is a protein that in humans is encoded by the SLC38A9 gene.
Late endosomal/lysosomal adaptor, MAPK and MTOR activator 1 is a protein that in humans is encoded by the LAMTOR1 gene.