Alternative titles; symbols
HGNC Approved Gene Symbol: ARL8B
Cytogenetic location: 3p26.1 Genomic coordinates (GRCh38): 3:5,122,292-5,180,911 (from NCBI)
Small GTPases function as molecular switches and work in conjunction with a range of cellular targets to elicit or regulate biologic function. ARL8A (616597) and ARL8B form a distinct subfamily of small GTPases (Okai et al., 2004).
By database analysis to identify novel small GTPases, followed by PCR of a human brain cDNA library, Okai et al. (2004) cloned ARL8B, which they called GIE1. The deduced 186-amino acid GIE1 protein has 5 motifs conserved in small GTPases, including GTP-binding domains and a putative effector domain, but it lacks a putative lipid modification motif found in other small GTPases. GIE1 shares 91% identity with GIE2 (ARL8A), and the GIE proteins share about 30% identity with other small GTPases. Northern blot analysis of 12 human tissues detected ubiquitous expression of a major 3.5-kb GIE1 transcript and a minor 2.0-kb transcript. An antibody that did not differentiate between GIE1 and GIE2 detected an apparent 22-kD polypeptide in rat PC12 cells. Immunohistochemical analysis revealed that GIE localized to the cytoplasm along with microtubules during interphase, then redistributed to spindle midzone in anaphase and to the midbody in late telophase. Database analysis revealed 2 GIE genes in mammals, a single GIE ortholog in Drosophila and C. elegans, and no ortholog in yeast.
Bagshaw et al. (2006) found that epitope-tagged ARL8B localized to lysosomes in transfected HeLa and Vero green monkey kidney cells.
Using thin-layer chromatography, Okai et al. (2004) found both GTP- and GDP-bound forms of epitope-tagged GIE1 following expression in HeLa cells, suggesting that GIE1 cycles between both nucleotide-bound forms. Overexpression of a dominant-negative GIE1 mutant or a mutant lacking the effector domain induced abnormal chromosomes and appearance of micronuclei. Knockdown of Drosophila Gie resulted in formation of chromatin bridges at anaphase, with occasional lagging chromosomes and missegregation. Epitope-tagged GIE1 immunoprecipitated with beta-tubulin (TUBB; 191130) from HeLa cells. GIE1 also interacted with alpha-tubulin (see 191110), but not with gamma-tubulin (see 191135). GIE1 cosedimented with stabilized microtubules, but not with free tubulin. Mutation analysis revealed that the effector domain, but not the nucleotide-binding domains, was required for interaction of GIE1 with beta-tubulin.
Using human ARL8B mutants that exclusively bound GTP or GDP, Bagshaw et al. (2006) found that GTP-ARL8B localized to lysosomal membranes, whereas GDP-ARL8B distributed in a diffuse, perinuclear pattern in transfected HeLa cells. GDP-ARL8B likely associated with microtubular structures. Overexpression of GTP-ARL8B caused dispersal of lysosomes to the cell periphery and into membrane projections.
Pu et al. (2015) identified a protein complex in HeLa cells that they called BLOC1 (see 601444)-related complex, or BORC. BORC recruited ARL8B to lysosomes and mediated kinesin-dependent movement of lysosomes toward the cell periphery along the plus ends of microtubules. Knockdown of BORC subunits, but not BLOC subunits, caused dissociation of ARL8B from lysosomes and collapse of lysosomes into the pericentriolar region. In turn, loss of BORC reduced cell spreading and cell migration. Knockdown of ARL8B also reduced cell spreading and migration and caused clustering of lysosomes in the juxtanuclear region. Pu et al. (2015) concluded that BORC acts at an early stage of lysosome positioning, recruiting ARL8B to the lysosomal membrane and enabling coupling to the SKIP (SKIIP; 603055)-kinesin-1 (see 602809) complex that drives microtubule-guided movement of lysosomes toward the cell periphery.
By genomic sequence analysis, Okai et al. (2004) mapped the ARL8B gene to chromosome 3p26.1.
Bagshaw, R. D., Callahan, J. W., Mahuran, D. J. The Arf-family protein, Arl8b, is involved in the spatial distribution of lysosomes. Biochem. Biophys. Res. Commun. 344: 1186-1191, 2006. [PubMed: 16650381] [Full Text: https://doi.org/10.1016/j.bbrc.2006.03.221]
Okai, T., Araki, Y., Tada, M., Tateno, T., Kontani, K., Katada, T. Novel small GTPase subfamily capable of associating with tubulin is required for chromosome segregation. J. Cell Sci. 117: 4705-4715, 2004. [PubMed: 15331635] [Full Text: https://doi.org/10.1242/jcs.01347]
Pu, J., Schindler, C., Jia, R., Jarnik, M., Backlund, P., Bonifacino, J. S. BORC, a multisubunit complex that regulates lysosome positioning. Dev. Cell 33: 176-188, 2015. [PubMed: 25898167] [Full Text: https://doi.org/10.1016/j.devcel.2015.02.011]