{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nVarp (VPS9‐ankyrin‐repeat protein), originally characterized as a guanine nucleotide exchange factor (GEF) for Rab21, has been identified as an essential effector for the melanocyte‐specific small GTPases Rab32 and Rab38. In mouse melanocytes, the ANKR1 domain of Varp specifically binds to the active (GTP‐bound) forms of Rab32/38, a binding that is crucial for the proper delivery of tyrosinase-related protein 1 (Tyrp1) to melanosomes, thereby affecting melanogenic enzyme trafficking. This key role in pigment cell biology was revealed by experiments showing that knockdown or misexpression of the Varp ANKR1 domain led to dramatic reductions in peripheral melanosomal Tyrp1 localization."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nVarp’s activity is not limited to melanocytes; it also plays a significant role in regulating endosomal dynamics in other cell types. Acting as a guanine nucleotide exchange factor primarily toward Rab21, Varp localizes primarily to early endosomes, where its functional domains—the VPS9 domain and ankyrin repeats—are required for proper endosomal localization and activity. Disruption of Varp expression perturbs Rab21 function and results in notable alterations of endosome morphology."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nFurther dissection of Varp’s molecular functions in melanocytes has uncovered that the binding of Rab32/38, rather than the activation of Rab21 by its VPS9 domain, is essential for Tyrp1 trafficking. Detailed mutagenesis studies have pinpointed critical residues within both Rab32/38 and the ANKR1 domain of Varp that are indispensable for the formation of the functional complex necessary for melanosomal Tyrp1 distribution. Additionally, Varp was shown to interact with the vesicle SNARE VAMP7/TI-VAMP, suggesting that vesicle fusion events are integral to the process. In another regulatory layer, Varp stability is maintained in melanocytes by RACK1, which competes with Rab40C for binding to an alternate ankyrin repeat domain on Varp, ultimately promoting dendrite outgrowth."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": ""}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Kanako Tamura, Norihiko Ohbayashi, Yuto Maruta, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Varp is a novel Rab32/38-binding protein that regulates Tyrp1 trafficking in melanocytes."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Biol Cell (2009)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1091/mbc.e08-12-1161"}], "href": "https://doi.org/10.1091/mbc.e08-12-1161"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "19403694"}], "href": "https://pubmed.ncbi.nlm.nih.gov/19403694"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Xinjun Zhang, Xi He, Xin-Yuan Fu, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Varp is a Rab21 guanine nucleotide exchange factor and regulates endosome dynamics."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Cell Sci (2006)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1242/jcs.02810"}], "href": "https://doi.org/10.1242/jcs.02810"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "16525121"}], "href": "https://pubmed.ncbi.nlm.nih.gov/16525121"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Kanako Tamura, Norihiko Ohbayashi, Koutaro Ishibashi, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Structure-function analysis of VPS9-ankyrin-repeat protein (Varp) in the trafficking of tyrosinase-related protein 1 in melanocytes."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2011)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.M110.191205"}], "href": "https://doi.org/10.1074/jbc.M110.191205"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "21187289"}], "href": "https://pubmed.ncbi.nlm.nih.gov/21187289"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Soujiro Marubashi, Norihiko Ohbayashi, Mitsunori Fukuda "}, {"type": "b", "children": [{"type": "t", "text": "A Varp-Binding Protein, RACK1, Regulates Dendrite Outgrowth through Stabilization of Varp Protein in Mouse Melanocytes."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Invest Dermatol (2016)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.jid.2016.03.034"}], "href": "https://doi.org/10.1016/j.jid.2016.03.034"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "27066885"}], "href": "https://pubmed.ncbi.nlm.nih.gov/27066885"}]}]}]}