title: Biochemical and functional characterization of Rab27a mutations occurring in Griscelli syndrome patients.
abstract: Rab27a is a member of the Rab family of small GTPase proteins, and thus far is the first member to be associated with a human disease (ie, the Griscelli syndrome type 2). Mutations in the Rab27a gene cause pigment as well as cytotoxic granule transport defects, accounting for the partial albinism and severe immune disorder characteristics of this syndrome. So far, 3 Rab27a missense mutations have been identified. They open a unique opportunity to designate critical structural and functional residues of Rab proteins. We show here that the introduction of a proline residue in the alpha 4 (Ala152Pro) or beta 5 (Leu130Pro) loop, observed in 2 of these spontaneous mutants, dramatically affects both guanosine triphosphate (GTP) and guanosine diphosphate (GDP) nucleotide-binding activity of Rab27a, probably by disrupting protein folding. The third mutant, Trp73Gly, is located within an invariant hydrophobic triad at the switch interface, and was previously shown in active Rab3A to mediate rabphilin3A effector interaction. Trp73Gly is shown to display the same nucleotide-binding and GTPase characteristics as the constitutively active mutant Gln78Leu. However, in contrast to Gln78Leu, Trp73Gly mutant construct neither interacts with the Rab27a effector melanophilin nor modifies melanosome distribution and cytotoxic granule exocytosis. Substitutions introduced at the 73 position, including the leucine residue present in Ras, did not restore Rab27a protein functions. Taken together, our results characterize new critical residues of Rab proteins, and identify the Trp73 residue of Rab27a as a key position for interaction with the specific effectors of Rab27a, both in melanocytes and cytotoxic cells.
authors.affiliation: Unité de Recherche sur le Développement Normal et Pathologique du Système Immunitaire Institut National de la Santé et de la Recherche Médicale U429, Hôpital Necker-Enfants Malades, Paris, France.
authors.forename: Gaël
authors.initials: G
authors.lastname: Menasche
authors.forename: Jérôme
authors.initials: J
authors.lastname: Feldmann
authors.forename: Anne
authors.initials: A
authors.lastname: Houdusse
authors.forename: Catherine
authors.initials: C
authors.lastname: Desaymard
authors.forename: Alain
authors.initials: A
authors.lastname: Fischer
authors.forename: Bruno
authors.initials: B
authors.lastname: Goud
authors.forename: Genèvieve
authors.initials: G
authors.lastname: de Saint Basile
mesh_terms: D000417:Albinism / Q000209:etiology / Q000235:genetics* / Q000473:pathology; D019943:Amino Acid Substitution; D000818:Animals; D015550:Cell Degranulation / Q000235:genetics; D002460:Cell Line; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D007153:Immunologic Deficiency Syndromes / Q000235:genetics* / Q000473:pathology; D008544:Melanocytes / Q000378:metabolism / Q000648:ultrastructure; D020460:Melanosomes / Q000235:genetics / Q000378:metabolism; D051379:Mice; D009154:Mutation; D011485:Protein Binding / Q000235:genetics; D013577:Syndrome; D020691:rab GTP-Binding Proteins / Q000235:genetics*; D000075745:rab27 GTP-Binding Proteins
chemical_list: D000075745:rab27 GTP-Binding Proteins; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; C066694:RAB27A protein, human; C512962:Rab27a protein, mouse; D020691:rab GTP-Binding Proteins