This entry includes Rabphilin and Doc2. Rabphilin and Doc2s share highly homologous tandem C2 domains, although their N-terminal structures are completely different: Rabphilin contains an N-terminal Rab-binding domain (RBD), whereas Doc2 contains an N-terminal Munc13-1-interacting domain (MID) []. This entry also includes Noc2 (also known as Rabphilin-3A-like protein), which is a potential effector of Ras-associated binding proteins Rab3A and Rab27A. Noc2 contains an N-terminal Rab3A effector domain which harbors a conserved zinc finger, but lacks tandem C2 domains [].
The Rab3 subfamily contains Rab3A, Rab3B, Rab3C, and Rab3D. All four isoforms were found in mouse brain and endocrine tissues, with varying levels of expression. Rab3A, Rab3B, and Rab3C localized to synaptic and secretory vesicles; Rab3D was expressed at high levels only in adipose tissue, exocrine glands, and the endocrine pituitary, where it is localized to cytoplasmic secretory granules []. Rab3 appears to control Ca2+-regulated exocytosis. The appropriate GDP/GTP exchange cycle of Rab3A is required for Ca2+-regulated exocytosis to occur, and interaction of the GTP-bound form of Rab3A with effector molecule(s) is widely believed to be essential for this process [].
Noc2, also known as Rabphilin-3A-like protein, is a potential effector of Ras-associated binding proteins Rab3A and Rab27A [, ]that is involved in the regulation of exocytosis in endocrine and exocrine cells [, , ]. Noc2 may regulate insulin secretion from pancreatic beta-cells and amylase secretion from pancreatic acini [].
Rab effector Noc2 (also known as RPH3AL) is a Rab3 effector that mediates the regulation of secretory vesicle exocytosis in neurons and certain endocrine cells []. It also functions as a Rab27 effector and is involved in isoproterenol (IPR)-stimulated amylase release from acinar cells [, ]. Noc2 contains an N-terminal Rab3A effector domain which harbors a conserved zinc finger, but lacks tandem C2 domains. The FYVE domain of Noc2 resembles a FYVE-related domain that is structurally similar to the canonical FYVE domains but lacks the three signature sequences: an N-terminal WxxD motif (x for any residue), the central basic R(R/K)HHCRxCG patch, and a C-terminal RVC motif [].
This entry represents the Rab-binding domain.Rab are small GTPases implicated in vesicle trafficking. Like the other smallGTPases, Rab proteins act as molecular switches, with an active GTP-bound formthat interacts with its target or effector protein and an inactive GDP-boundform. A subgroup of Rab effectors contain in their N-terminal part a conservedregion of around 70 amino acid residues, the Rab-binding domain (RabBD). Insome Rab effector domains an atypical FYVE-type zinc finger is inserted in thecentral part [].The crystal structure of the Rab effector domain of Rabphilin-3A in complexwith Rab3A has been solved []. The structure consists of twolong helices separated by an atypical FYVE-type zinc finger which adopts aconformation similar to classical ones. The central zincfinger does not directly interact with Rab3A. The amino acids important forthis interaction are located around a short C-terminal motif (SGAWFF) and anacidic cluster in the N-terminal area.
Rab proteins constitute a family of small GTPases that serve a regulatory role in vesicular membrane traffic [, ]; C-terminal geranylgeranylation is crucial for their membrane association and function. This post-translational modification is catalysed by Rab geranylgeranyl transferase (Rab-GGTase), a multi-subunit enzyme that contains a catalytic heterodimer and an accessory component, termed Rab escort protein (REP)-1 [, ]. REP-1 presents newly-synthesised Rab proteins to the catalytic component, and forms a stable complex with the prenylated proteins following the transfer reaction.cDNA cloning of component A of rat Rab geranylgeranyl transferase (REP) confirms its resemblance to Rab3A guanine nucleotide dissociation inhibitor (GDI) and its identity with the human choroideremia gene product []. A genetic defect in REP underlies human choroideremia. Choroideraemia (or tapetochoroidal dystrophy) is a common form of X-linked blindness characterised by progressive dystrophy of the choroid, retinal pigment epithelium and retina [, , ].
Zinc finger (Znf) domains are relatively small protein motifs which contain multiple finger-like protrusions that make tandem contacts with their target molecule. Some of these domains bind zinc, but many do not; instead binding other metals such as iron, or no metal at all. For example, some family members form salt bridges to stabilise the finger-like folds. They were first identified as a DNA-binding motif in transcription factor TFIIIA from Xenopus laevis (African clawed frog), however they are now recognised to bind DNA, RNA, protein and/or lipid substrates [, , , , ]. Their binding propertiesdepend on the amino acid sequence of the finger domains and of the linker between fingers, as well as on the higher-order structures and the number of fingers. Znf domains are often found in clusters, where fingers can have different binding specificities. There are many superfamilies of Znf motifs, varying in both sequence and structure. They display considerable versatility in binding modes, even between members of the same class (e.g. some bind DNA, others protein), suggesting that Znf motifs are stable scaffolds that have evolved specialised functions. For example, Znf-containing proteins function in gene transcription, translation, mRNA trafficking, cytoskeleton organisation, epithelial development, cell adhesion, protein folding, chromatin remodelling and zinc sensing, to name but a few []. Zinc-binding motifs are stable structures, and they rarely undergo conformational changes upon binding their target. This (predicted) zinc finger is found in the bassoon and piccolo proteins, both of which are components of the presynaptic cytoskeletal matrix (PCM) assembled at the active zone of neurotransmitter release, where Piccolo plays a role in the trafficking of synaptic vesicles (SVs) [, , ]. The Piccolo zinc fingers were found to interact with the dual prenylated rab3A and VAMP2/Synaptobrevin II receptor PRA1. There are eight conserved cysteines in Piccolo-type zinc fingers, suggesting that they coordinates two zinc ligands.
