Biology:RALB

Generic protein structure example

Ras-related protein Ral-B (RalB) is a protein that in humans is encoded by the RALB gene on chromosome 2. This protein is one of two paralogs of the Ral protein, the other being RalA, and part of the Ras GTPase family. RalA functions as a molecular switch to activate a number of biological processes, majorly cell division and transport, via signaling pathways. Its biological role thus implicates it in many cancers.

Structure

The Ral isoforms share an 80% overall match in amino acid sequence and 100% match in their effector-binding region. The two isoforms mainly differ in the C-terminal hypervariable region, which contains multiple sites for post-translational modification, leading to diverging subcellular localization and biological function. For example, phosphorylation of Serine 194 on RalA by the kinase Aurora A results in the relocation of RalA to the inner mitochondrial membrane, where RalA helps carry out mitochondrial fission; whereas phosphorylation of Serine 198 on RalB by the kinase PKC results in the relocation of RalB to other internal membranes and activation of its tumorigenic function.

Function

RalB is one of two proteins in the Ral family, which is itself a subfamily within the Ras family of small GTPases. As a Ras GTPase, RalB functions as a molecular switch that becomes active when bound to GTP and inactive when bound to GDP. RalB can be activated by RalGEFs and, in turn, activate effectors in signal transduction pathways leading to biological outcomes. For instance, RalB interacts with two components of the exocyst, Exo84 and Sec5, to promote autophagosome assembly, secretory vesicle trafficking, and tethering. Other downstream biological functions include exocytosis, receptor-mediated endocytosis, tight junction biogenesis, filopodia formation, mitochondrial fission, and cytokinesis.

While the above functions appear to be shared between the two Ral isoforms, their differential subcellular localizations result in their differing involvement in certain biological processes. In particular, RalB is more involved in apoptosis and cell motility. Moreover, RalB specifically interacts with Exo84 to assemble the beclin-1–VPS34 autophagy initiation complex, and with Sec5 to activate the innate immune response via the Tank-binding kinase 1 (TBK1).

Clinical significance

Ral proteins have been associated with the progression of several cancers, including bladder cancer and prostate cancer. Though the exact mechanisms remain unclear, studies reveal that RalB promotes tumor invasion and metastasis. As a result, inhibition of RalB inhibits further progression of cancer. In addition, RalB regulates p53 levels in a K-Ras-independent manner during cancer development. RalB also promotes cell survival during infection by double-stranded DNA viruses by activating TBK1 to carry out an immune response.

Interactions

RalB has been shown to interact with:

• CDC42,
• EXOC8,
• RALBP1, and
• Sec5.

Further reading

• "Chromosomal localization and cDNA sequence of human ralB, a GTP binding protein.". Somat. Cell Mol. Genet. 16 (4): 407–10. 1990. doi:10.1007/BF01232469. PMID 2120779. 
• "Coding sequences of human ralA and ralB cDNAs". Nucleic Acids Res. 17 (11): 4380. 1989. doi:10.1093/nar/17.11.4380. PMID 2662142. 
• "Bridging Ral GTPase to Rho pathways. RLIP76, a Ral effector with CDC42/Rac GTPase-activating protein activity". J. Biol. Chem. 270 (38): 22473–7. 1995. doi:10.1074/jbc.270.38.22473. PMID 7673236. 
• "Generation of antibodies specific for the RalA and RalB GTP-binding proteins and determination of their concentration and distribution in human platelets". Biochim. Biophys. Acta 1314 (1–2): 157–66. 1997. doi:10.1016/s0167-4889(96)00073-0. PMID 8972729. 
• "Identification and characterization of a novel protein interacting with Ral-binding protein 1, a putative effector protein of Ral". J. Biol. Chem. 273 (2): 814–21. 1998. doi:10.1074/jbc.273.2.814. PMID 9422736. 
• "The exocyst complex binds the small GTPase RalA to mediate filopodia formation". Nat. Cell Biol. 4 (1): 73–8. 2002. doi:10.1038/ncb720. PMID 11744922. 
• "Calmodulin binds RalA and RalB and is required for the thrombin-induced activation of Ral in human platelets". J. Biol. Chem. 277 (32): 28972–80. 2002. doi:10.1074/jbc.M201504200. PMID 12034722. 
• "RAL GTPases are linchpin modulators of human tumour-cell proliferation and survival". EMBO Rep. 4 (8): 800–6. 2004. doi:10.1038/sj.embor.embor899. PMID 12856001. 
• "rgr oncogene: activation by elimination of translational controls and mislocalization". Cancer Res. 63 (14): 4188–95. 2003. PMID 12874025. 
• "Ral GTPases regulate exocyst assembly through dual subunit interactions". J. Biol. Chem. 278 (51): 51743–8. 2004. doi:10.1074/jbc.M308702200. PMID 14525976. 
• "Regulation of phospholipase C-delta1 through direct interactions with the small GTPase Ral and calmodulin". J. Biol. Chem. 280 (23): 21933–41. 2005. doi:10.1074/jbc.M412966200. PMID 15817490. 
• "Towards a proteome-scale map of the human protein-protein interaction network". Nature 437 (7062): 1173–8. 2005. doi:10.1038/nature04209. PMID 16189514. Bibcode: 2005Natur.437.1173R. 
• "RalB GTPase-mediated activation of the IkappaB family kinase TBK1 couples innate immune signaling to tumor cell survival". Cell 127 (1): 157–70. 2006. doi:10.1016/j.cell.2006.08.034. PMID 17018283. 
• "Divergent roles for RalA and RalB in malignant growth of human pancreatic carcinoma cells". Curr. Biol. 16 (24): 2385–94. 2007. doi:10.1016/j.cub.2006.10.023. PMID 17174914. 
• "Expression of ral GTPases, their effectors, and activators in human bladder cancer". Clin. Cancer Res. 13 (13): 3803–13. 2007. doi:10.1158/1078-0432.CCR-06-2419. PMID 17606711. 
• "Activation of the RalGEF/Ral Pathway Promotes Prostate Cancer Metastasis to Bone". Mol. Cell. Biol. 27 (21): 7538–50. 2007. doi:10.1128/MCB.00955-07. PMID 17709381.