Alternative titles; symbols
HGNC Approved Gene Symbol: LAMTOR5
Cytogenetic location: 1p13.3 Genomic coordinates (GRCh38): 1:110,401,253-110,407,924 (from NCBI)
LAMTOR5 is a subunit of the Ragulator/LAMTOR complex (see LAMTOR1, 613510) that resides on the cytosolic face of late endosomes and lysosomes and regulates nutrient availability via RAG small GTPases (see RRAGA, 612194) and mechanistic target of rapamycin complex-1 (mTORC1; see 601231) (Schweitzer et al. (2015)).
Using hepatitis B virus X protein (HBX) as bait in a yeast 2-hybrid screen of a hepatoma cell line cDNA library, Melegari et al. (1998) cloned HBXIP, which they called XIP. The deduced 91-amino acid protein has a calculated molecular mass of 9.6 kD. It contains a putative leucine zipper motif and may form an amphipathic helix. HBXIP also has 2 putative threonine phosphorylation sites. Northern blot analysis detected a 0.7-kb transcript in all tissues tested, with highest expression in skeletal and cardiac muscle.
Hartz (2015) mapped the LAMTOR5 gene to chromosome 1p13.3 based on an alignment of the LAMTOR5 sequence (GenBank AF029890) with the genomic sequence (GRCh38).
By in vitro binding and pull-down assays, Melegari et al. (1998) confirmed that HBXIP interacts with HBX. HBXIP with a mutation in one of its putative threonine phosphorylation sites, thr36 to ala, failed to interact with HBX, while HBXIP with a mutation of the other site, thr12 to ala, still bound HBX. Transient or stable expression of HBXIP in hepatocellular carcinoma cells abolished the transactivation properties of HBX on reporter constructs driven by AP1 (165160) and endogenous HBV enhancer/promoter elements. HBXIP expression in differentiated hepatocellular carcinoma cells reduced wildtype HBV replication to levels observed following infection with an HBX-minus virus. Melegari et al. (1998) hypothesized HBXIP negatively regulates HBX activity and thus alters the replication life cycle of the virus.
Marusawa et al. (2003) found that survivin (603352), an antiapoptotic protein that is overexpressed in most human cancers, interacts with HBXIP. Survivin-HBXIP complexes, but neither protein alone, bound procaspase-9 (602234) and prevented its recruitment to APAF1 (602233), thereby selectively suppressing apoptosis initiated via the mitochondria/cytochrome c (123970) pathway. Viral HBX protein also interacted with the survivin-HBXIP complex and suppressed caspase activation in a survivin-dependent manner. Marusawa et al. (2003) concluded that HBXIP functions as a cofactor for survivin and serves as a link between the cellular apoptosis machinery and a viral pathogen involved in hepatocellular carcinogenesis.
Using fragments of HBX in protein binding assays, Wen et al. (2008) identified amino acids 137 to 140 (CRHK) of HBX as necessary for binding HBXIP. Mutations in the CRHK motif in HBX abolished HBXIP binding and dysregulated centrosome dynamics, and the mutants failed to localize to centrosomes. Either overexpression of HBX or knockdown of HBXIP, but not overexpression of HBX nonbinding mutants, altered centrosome assembly and induced modification of pericentrin (PCNT1; 170285) and centrin-2 (CETN2; 300006), proteins required for centrosome formation and function. Wen et al. (2008) proposed that HBXIP is a critical target for viral HBX to promote genetic instability through formation of defective spindles and subsequent aberrations in chromosome segregation.
Heterodimeric RAGA (RRAGA; 612194)/RAGB (RRAGB; 300725)-RAGC (RRAGC; 608267)/RAGD (RRAGD; 608268) GTPases, the Ragulator complex, and the v-ATPase (see 607027) form a signaling system necessary for amino acid sensing by mTORC1 (see 601231). By coimmunoprecipitation analysis in HEK293T cells and in vitro binding assays, Bar-Peled et al. (2012) showed that LAMTOR4 (618834) and HBXIP were components of a pentameric Ragulator complex in which LAMTOR4 and HBXIP formed a heterodimer that interacted with the MP1 (LAMTOR3; 603296)-p14 (LAMTOR2; 610389) heterodimer through p18 (LAMTOR1; 613510) to form the pentamer. LAMTOR4 and HBXIP were required for the Ragulator complex to interact strongly with RAG GTPases. The Ragulator complex acted as a scaffold to tether RAG GTPases and mTORC1 at the lysosomal membrane and sensed the availability of amino acids through activation of the mTORC1 pathway, which required both HBXIP and LAMTOR4. Ragulator preferentially interacted with nucleotide-free RAG heterodimers in an amino acid-dependent manner and acted as a guanine nucleotide exchange factor (GEF) for RAGA or RAGB to convert them from a GDP-bound to GTP-bound state. Ragulator GEF activity was regulated through interaction with v-ATPase. Amino acid accumulation in the lysosomal lumen generated an activating signal that was transmitted in a v-ATPase-dependent fashion to trigger Ragulator GEF activity toward RAGA/RAGB for GTP binding. Upon GTP loading, the RAG-Ragulator interaction weakened, resulting in activation of mTORC1 signaling.
By mass spectrometric analysis of proteins that immunoprecipitated with HBXIP from HEK293T cells, Schweitzer et al. (2015) confirmed that HBXIP is a Ragulator subunit. The immunoprecipitates included Ragulator subunits p18 (LAMTOR1), p14 (LAMTOR2), MP1 (LAMTOR3), and C7ORF59 (LAMTOR4), as well as the lysosomal membrane protein C17ORF59 (BORCS6; 616599), but not RAG GTPases. Epitope-tagged C17ORF59 also immunoprecipitated all Ragulator subunits, but not RAG GTPases. Schweitzer et al. (2015) found that binding of C17ORF59 to Ragulator inhibited the interaction between Ragulator and RAG GTPases, thus inhibiting mTORC1 recruitment to lysosomes and activation in response to amino acid stimulation.
Rasheed et al. (2019) determined the crystal structure of the human HBXIP-C7ORF59 heterodimer at 2.9-angstrom resolution. The structure revealed that an unfolded N terminus of C7ORF59 transitioned to an alpha-helical conformation upon binding p18 and stabilized the complex. Interaction studies suggested that assembly of the Ragulator pentamer started with the HBXIP-C7ORF59-p18 trimer, followed by incorporation of the MP1-p14 heterodimer. Phosphorylation of C7ORF59 at the evolutionarily conserved ser67 residue by protein kinase A (PKA; see 601639) impaired its interaction with p18, indicating that polarity of this serine residue is essential for stability of the complex.
Bar-Peled, L., Schweitzer, L. D., Zoncu, R., Sabatini, D. M. Ragulator is a GEF for the RAG GTPases that signal amino acid levels to mTORC1. Cell 150: 1196-1208, 2012. [PubMed: 22980980] [Full Text: https://doi.org/10.1016/j.cell.2012.07.032]
Hartz, P. A. Personal Communication. Baltimore, Md. 10/19/2015.
Marusawa, H., Matsuzawa, S., Welsh, K., Zou, H., Armstrong, R., Tamm, I., Reed, J. C. HBXIP functions as a cofactor of survivin in apoptosis suppression. EMBO J. 22: 2729-2740, 2003. [PubMed: 12773388] [Full Text: https://doi.org/10.1093/emboj/cdg263]
Melegari, M., Scaglioni, P. P., Wands, J. R. Cloning and characterization of a novel hepatitis B virus x binding protein that inhibits viral replication. J. Virol. 72: 1737-1743, 1998. [PubMed: 9499022] [Full Text: https://doi.org/10.1128/JVI.72.3.1737-1743.1998]
Rasheed, N., Lima, T. B., Mercaldi, G. F., Nascimento, A. F. Z., Silva, A. L. S., Righetto, G. L., Bar-Peled, L., Shen, K., Sabatini, D. M., Gozzo, F. C., Aparicio, R., Smetana, J. H. C. C7orf59/LAMTOR4 phosphorylation and structural flexibility modulate Ragulator assembly. FEBS Open Bio. 9: 1589-1602, 2019. Note: Electronic Article. [PubMed: 31314152] [Full Text: https://doi.org/10.1002/2211-5463.12700]
Schweitzer, L. D., Comb, W. C., Bar-Peled, L., Sabatini, D. M. Disruption of the Rag-Ragulator complex by c17orf59 inhibits mTORC1. Cell Rep. 12: 1445-1455, 2015. [PubMed: 26299971] [Full Text: https://doi.org/10.1016/j.celrep.2015.07.052]
Wen, Y., Golubkov, V. S., Strongin, A. Y., Jiang, W., Reed, J. C. Interaction of hepatitis B viral oncoprotein with cellular target HBXIP dysregulates centrosome dynamics and mitotic spindle formation. J. Biol. Chem. 283: 2793-2803, 2008. [PubMed: 18032378] [Full Text: https://doi.org/10.1074/jbc.M708419200]