title: Targeting of Nir2 to lipid droplets is regulated by a specific threonine residue within its PI-transfer domain.
abstract: Nir2, like its Drosophila homolog retinal degeneration B (RdgB), contains an N-terminal phosphatidylinositol-transfer protein (PI-TP)-like domain. Previous studies have suggested that RdgB plays an important role in the fly phototransduction cascade and that its PI-transfer domain is critical for this function. In this domain, a specific mutation, T59E, induces a dominant retinal degeneration phenotype. Here we show that a similar mutation, T59E in the human Nir2 protein, targets Nir2 to spherical cytosolic structures identified as lipid droplets by the lipophilic dye Nile red. A truncated Nir2T59E mutant consisting of only the PI-transfer domain was also targeted to lipid droplets, whereas neither the wild-type Nir2 nor the Nir2T59A mutant was associated with lipid droplets under regular growth conditions. However, oleic-acid treatment caused translocation of wild-type Nir2, but not translocation of the T59A mutant, to lipid droplets. This treatment also induced partial targeting of endogenous Nir2, which is mainly associated with the Golgi apparatus, to lipid droplets. Targeting of Nir2 to lipid droplets was attributed to its enhanced threonine phosphorylation. These results suggest that a specific threonine within the PI-transfer domain of Nir2 provides a regulatory site for targeting to lipid droplets. In conjunction with the role of PI-TPs in lipid transport, this targeting may affect intracellular lipid trafficking and distribution and may provide the molecular basis underlying the dominant effect of the RdgB-T59E mutant on retinal degeneration.
authors.affiliation: Department of Neurobiology, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Litvak
authors.forename: Yoav D
authors.initials: YD
authors.lastname: Shaul
authors.forename: Mark
authors.initials: M
authors.lastname: Shulewitz
authors.forename: Roy
authors.initials: R
authors.lastname: Amarilio
authors.forename: Shari
authors.initials: S
authors.lastname: Carmon
authors.forename: Sima
authors.initials: S
authors.lastname: Lev
mesh_terms: D019943:Amino Acid Substitution; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002458:Cell Fractionation; D002462:Cell Membrane / Q000378:metabolism; D002499:Centrifugation, Density Gradient; D003600:Cytosol / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005136:Eye Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D005799:Genes, Dominant; D006056:Golgi Apparatus / Q000378:metabolism; D006367:HeLa Cells / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D050356:Lipid Metabolism; D008565:Membrane Proteins; D009363:Neoplasm Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D019301:Oleic Acid / Q000494:pharmacology; D010766:Phosphorylation; D010769:Phosphothreonine / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000187:drug effects; D012162:Retinal Degeneration / Q000235:genetics / Q000378:metabolism*; D013912:Threonine / Q000502:physiology*
chemical_list: D002135:Calcium-Binding Proteins; D005136:Eye Proteins; D008565:Membrane Proteins; D009363:Neoplasm Proteins; C113505:PITPNM1 protein, human; D010769:Phosphothreonine; D019301:Oleic Acid; D013912:Threonine; D017346:Protein Serine-Threonine Kinases