Subfamily III of mammalian oxysterol-binding protein (OSBP) homologues: the expression and intracellular localization of ORP3, ORP6, and ORP7.
The human OSBP related protein (ORP) family consists of 12 members, which can be divided into six subfamilies based on the genomic organization and amino acid homology. Here we performed basic characterization of subfamily III, which consists of three members: ORP3, ORP6, and ORP7. According to cDNA hybridization, the three genes are expressed in a tissue-specific manner. While ORP3 mRNA is most abundant in kidney, lymph nodes, and thymus, ORP6 shows highest expression in brain and skeletal muscle, and ORP7 in the gastrointestinal tract. Using monospecific peptide antibodies, we confirmed the presence of the three proteins in human and mouse tissues. ORP6 gene expression was induced upon differentiation of F9 embryonic carcinoma cells into parietal endoderm, while ORP3 and ORP7 mRNA levels were unchanged. In the F9 cells, endogenous ORP6 associated predominantly with the nuclear envelope. When expressed from the cDNA in cultured cells, the three proteins were distributed between the cytosol and endoplasmic reticulum (ER) membranes, with a minor portion found at the plasma membrane. Experiments with truncated constructs showed that the N-terminal portion of the proteins, containing a pleckstrin homology (PH) domain, has markedly strong plasma membrane targeting specificity, while the C-terminal half remains largely cytosolic. The expression data demonstrates that ORP3, -6, and -7 are not merely redundant gene products but show marked quantitative differences in tissue expression, suggesting tissue-specific aspects in their function. The dual targeting of the proteins indicates a putative role in communication between the ER and the plasma membrane.
Department of Molecular Medicine, National Public Health Institute, Biomedicum, PO Box 104, 00251 Helsinki, Finland.
Markku
M
Lehto
Jukka
J
Tienari
Sanna
S
Lehtonen
Eero
E
Lehtonen
Vesa M
VM
Olkkonen
D000818:Animals; D015153:Blotting, Western; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D018076:DNA, Complementary; D005786:Gene Expression Regulation; D005822:Genetic Vectors; D006801:Humans; D007150:Immunohistochemistry; D017404:In Situ Hybridization, Fluorescence; D051379:Mice; D008969:Molecular Sequence Data; D009928:Organ Specificity; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger; D011987:Receptors, Steroid / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014170:Transformation, Genetic; D000098825:Oxysterol Binding Proteins
D002352:Carrier Proteins; D018076:DNA, Complementary; D020033:Protein Isoforms; D012333:RNA, Messenger; D011987:Receptors, Steroid; D000098825:Oxysterol Binding Proteins