Alternative titles; symbolsZINC FINGER FYVE DOMAIN-CONTAINING PROTEIN 20; ZFYVE20RABENOSYN 5HGNC Approved Gene Symbol: RBSNCytogenetic location: 3p25.1 Genom...
Alternative titles; symbols
HGNC Approved Gene Symbol: RBSN
Cytogenetic location: 3p25.1 Genomic coordinates (GRCh38): 3:15,070,068-15,099,162 (from NCBI)
▼ Cloning and Expression
Using a gel overlay assay, Nielsen et al. (2000) identified ZFYVE20, which they called rabenosyn-5, as a protein that directly interacts with RAB5 (179512). By peptide analysis, EST database analysis, and PCR of a HeLa cell cDNA library, they cloned rabenosyn-5 cDNA. The deduced 784-amino acid protein contains an N-terminal C2H2-type zinc finger domain, a central FYVE finger domain, and 5 copies of the amino acid motif NPF at its C terminus. SDS-PAGE showed rabenosyn-5 with an apparent molecular mass of 110 kD.
▼ Gene Function
Nielsen et al. (2000) determined that rabenosyn-5 colocalized with EEA1 (605070) on RAB5-positive endosomes in HeLa cells. Recruitment of rabenosyn-5 and EEA1 was dependent upon phosphatidylinositol 3-kinase (see 601232). Mutation analysis indicated that the FYVE domain of rabenosyn-5 was required for endosome targeting. Rabenosyn-5 was complexed with the Sec1-like protein VPS45 (VPS45A; 610035), and rabenosyn-5 appeared to serve as a molecular link between VPS45 and RAB5. Both EEA1 and rabenosyn-5 were required for early endosomal fusion, but only rabenosyn-5 overexpression inhibited processing of procathepsin D (116840) to its 47-kD intermediate, suggesting that the 2 proteins play distinct roles in endosomal trafficking.
By immunofluorescence microscopy of HeLa cells, Naslavsky et al. (2004) detected rabenosyn-5 partly colocalized with EHD1 (605888) in vesicular and tubular structures. Depletion of either EHD1 or rabenosyn-5 in HeLa cells by RNA interference (RNAi) delayed recycling of transferrin (190000) and its receptor (TFRC; 190010), as well as major histocompatibility complex class I (see HLA-A; 142800), to the plasma membrane. Depletion of EHD1 caused accumulation of internalized cargo in a compact juxtanuclear compartment, but depletion of rabenosyn-5 caused retention of cargo within a dispersed peripheral compartment. Simultaneous depletion of both rabenosyn-5 and EHD1 resulted in a phenotype similar to that observed with rabenosyn-5 RNAi alone, suggesting that rabenosyn-5 acts before EHD1 in the regulation of endocytic recycling. Naslavsky et al. (2004) concluded that rabenosyn-5 and EHD1 act sequentially in the transport of proteins from early endosomes to the endosomal recycling compartment and back to the plasma membrane.
▼ Biochemical Features
Using a structural proteomic approach, Eathiraj et al. (2005) determined the specificity and structural basis underlying the interaction of the multivalent effector rabenosyn-5 with the Rab family. The results demonstrated that even the structurally similar effector domains in rabenosyn-5 can achieve highly selective recognition of distinct subsets of Rab GTPases exclusively through interactions with the switch and interswitch regions. The observed specificity is determined at a family-wide level by structural diversity in the active conformation, which governs the spatial disposition of critical conserved recognition determinants, and by a small number of both positive and negative sequence determinants that allow further discrimination between Rab GTPases with similar switch conformations.
The International Radiation Hybrid Mapping Consortium mapped the ZFYVE20 gene to chromosome 3 (STS-N59228).
▼ Molecular Genetics
In a 6-year-old girl with seizures, developmental delay, craniofacial dysmorphism, osteopenia, macrocytosis, and megaloblastoid erythropoiesis, Stockler et al. (2014) identified homozygosity for a missense mutation in the RBSN gene (609511.0001).
▼ ALLELIC VARIANTS ( 1 Selected Example):
.0001 VARIANT OF UNKNOWN SIGNIFICANCE
This variant is classified as a variant of unknown significance because its contribution to a pleiotropic phenotype with seizures and evidence of defective endocytic trafficking has not been confirmed.
By whole-exome sequencing in a 6.5-year-old girl with developmental delay, epileptic encephalopathy, and a pleiotropic clinical and biochemical phenotype, Stockler et al. (2014) identified homozygosity for a c.1273G-A transition in exon 14 of the RBSN gene, resulting in a gly425-to-arg (G425R) mutation. The mutation segregated with disease in the family. The girl was the second of 3 sisters born to nonconsanguineous parents of European descent after an uneventful pregnancy. She developed pharmacoresistant infantile spasms at age 5 months, which improved upon ketogenic diet started at age 14 months. At age 40 months, she underwent a brain MRI that showed moderate enlargement of the third ventricle and normal myelination and cortical architecture. Severe hypertriglyceridemia necessitated modification of the ketogenic diet. At age 6.5 years, she was seizure-free on the modified ketogenic diet and 4 anticonvulsive agents. She had microcephaly, short stature, and hypotonia, and was able to sit with support but unable to stand or walk. She had a happy, friendly demeanor, and was nonverbal, unable to feed herself, and not toilet trained. She had thin, wispy hair and dysmorphic facial features characterized by hypertelorism, midface hypoplasia, deep-set eyes with a hooded appearance, broad nasal bridge, short nose, wide mouth, small teeth, and tongue protrusion. At 6 years of age, moderate osteopenia was observed involving the pelvis and long bones as well as coned epiphyses, short digits, and delayed bone age. Bone marrow aspirates showed megaloblastoid erythropoiesis and occasional intercytoplasmic bridging. Prior to the ketogenic diet, the patient had normal biochemical studies except for high urine methylmalonic acid (MMA) and mildly elevated plasma homocysteine as well as low cobalamin (vitamin B12) levels. Treatment resulted in normalized B12 levels; homocysteine levels were normal, but urine and serum MMA levels remained mildly elevated. The patient also had microalbuminuria and high urinary calcium excretion, with normal plasma 25-hydroxy vitamin D, calcium, phosphorus, and alkaline phosphatase levels. She had persistent red blood cell macrocytosis and transient neutropenia. Peripheral blood showed increased target cells and stomatocytes. Studies in fibroblasts revealed expression and localization of the mutated protein in a wildtype manner, but there was a 50% decrease in transferrin (190000) accumulation, which was corrected by wildtype allele transfection. Furthermore, the patient's fibroblasts displayed an impaired proliferation rate as well as cytoskeletal and lysosomal abnormalities. Western blot analysis showed increased precursor cathepsin D (CTSD; 116840) but diminished mature cathepsin D compared to wildtype, indicating compromised processing. The G425R mutation is in the central residue of an NG(D/E) motif that is an island of conservation within an otherwise variable segment and is conserved down to Drosophila. The mutation had a frequency of 0.003987 in the 1000 Genomes Project database.