High impact information on OSBP

• There have been recent advances in the study of lipid transfer proteins, such as ceramide transfer protein (CERT) and homologues of oxysterol binding protein (OSBP) [1].
• Here, evidence that OSBP and ORPs are bona fide sterol carriers is discussed [2].
• The crystal structure of a homolog of oxysterol binding protein solved by Jim Hurley and coworkers (Im et al., 2005) shows that this large family acts as cholesterol transporters, taking not only the lipid but also large numbers of water molecules into an internal pocket [3].
• Identification of Kes1p as a negative effector for Golgi function provides the first direct insight into the biological role of any member of the OSBP family, and describes a novel pathway for the regulation of Golgi-derived transport vesicle biogenesis [4].
• Kes1p shares homology with human oxysterol binding protein and participates in a novel regulatory pathway for yeast Golgi-derived transport vesicle biogenesis [4].

Biological context of OSBP

• Both OSBP genes contain 14 exons and have similar exon sizes and splice sites suggesting they may have arisen from a gene duplication event [5].
• Prompted by the multitude of OSBP-related genes in the yeast genome, we carried out a search for human expressed sequence tags (ESTs) displaying homology to the sterol-binding domain of OSBP [6].
• The human cDNA was used to determine the chromosomal localization of the OSBP gene by Southern blot hybridization to panels of somatic cell hybrid clones containing subsets of human or mouse chromosomes and by RFLP analysis of recombinant inbred mouse strains [7].
• The OSBP locus mapped to the long arm of human chromosome 11 and the proximal end of mouse chromosome 19 [7].
• We have used bioinformatics and data mining as a starting point for identifying new family members in humans based on the presence of the OSBP signature EQVSHHPP [8].

Anatomical context of OSBP

• In the yeast Saccharomyces cerevisiae an OSBP homologue is involved in membrane trafficking through the Golgi complex [6].
• Thus, cholesterol functions through its interaction with OSBP outside of membranes to regulate the assembly of an oligomeric phosphatase that controls a key signaling pathway in the cell [9].
• Novel members of the human oxysterol-binding protein family bind phospholipids and regulate vesicle transport [10].
• In human tissues there was specific isoform distribution, with most tissues expressing varied levels of isoforms with the complete OSBP domain; while only whole brain, kidney, spleen, thymus, and thyroid expressed high levels of the isoforms associated with the truncated OSBP domain [11].
• We previously demonstrated that this gene is highly expressed in CD34(+) hematopoietic progenitor cells, and deduced that the "full-length" ORP3 gene comprises 23 exons and encodes a predicted protein of 887 amino acids with a C-terminal OSBP domain and an N-terminal pleckstrin homology domain [11].

Associations of OSBP with chemical compounds

• Together with OSBP, the identified gene products constitute a novel human protein family that may provide a link between organellar sterol status and membrane dynamics [6].
• Our study implicates this new yeast gene family in ergosterol synthesis and provides comparative evidence of a role for human OSBP in cholesterol synthesis [12].
• Cytosolic OSBP formed a approximately 440-kilodalton oligomer with a member of the PTPPBS family of tyrosine phosphatases, the serine/threonine phosphatase PP2A, and cholesterol [9].
• Upon addition of 25-hydroxycholesterol, most of the OSBP became concentrated in large perinuclear structures that stained with lentil lectin, a protein that stains the Golgi apparatus [13].
• The structures that contained OSBP were disrupted by brefeldin A, confirming their identification as Golgi [13].

Physical interactions of OSBP

• The ORP have two major structural features: a highly conserved OSBP-type sterol-binding domain in the C-terminal half and a pleckstrin homology domain present in the N-terminal region of most family members [14].
• Oxysterol binding protein (OSBP) is a cytosolic protein that undergoes ligand-induced binding to the Golgi apparatus and has been implicated in the regulation of cellular cholesterol metabolism [6].
• The oxysterol-binding protein homologue ORP1L interacts with Rab7 and alters functional properties of late endocytic compartments [15].
• Oxysterol-binding-protein (OSBP)-related protein 4 binds 25-hydroxycholesterol and interacts with vimentin intermediate filaments [16].
• By using a combination of yeast two-hybrid, glutathione S-transferase pull-down and immunoprecipitation experiments, the VAP-A-binding region in OSBP was localized to amino acids 351-442 [17].

Regulatory relationships of OSBP

• OSBP is a cholesterol-regulated scaffolding protein in control of ERK 1/2 activation [9].
• Analysis of a chimeric ORP3:OSBP protein suggests that ligand binding by the C-terminal domain of OSBP induces allosteric changes that activate the N-terminal targeting modules of ORP3 [18].
• We found that decreasing OSBP expression by approximately 90% did not affect 25-HC-induced inhibition of transcription of 3-hydoxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and squalene epoxidase to any extent [19].

Other interactions of OSBP

• We have investigated the structure, oxysterol-binding activity, cellular localization and function of ORP4 (also designated OSBP2 or HLM), a homologue that shares the highest degree of similarity with OSBP [16].
• Here, OSBP was found to function as a cholesterol-binding scaffolding protein coordinating the activity of two phosphatases to control the extracellular signal-regulated kinase (ERK) signaling pathway [9].
• ORP2 [OSBP (oxysterol-binding protein)-related protein 2] belongs to the 12-member mammalian ORP gene/protein family [20].
• The intracellular targeting determinants of oxysterol binding protein (OSBP)-related protein 3 (ORP3) were studied using a series of truncated and point mutated constructs [18].
• The previously noted potential leucine zipper sequence in OSBP was not required for Golgi localization, nor was it essential for homodimer formation [13].

Analytical, diagnostic and therapeutic context of OSBP

• Cladistic analysis divides the human OSBP genes into five groups, whose members share similarities in sequence and gene structure; RT-PCR analysis indicates that expression patterns among group members vary widely [8].
• In the transfected cells, some of the oxysterol binding protein (OSBP) was distributed diffusely in the cytoplasm, and some was bound to small vesicles near the nucleus, as revealed by indirect immunofluorescence [13].
• Western blot analysis using peptide antibodies specific to OSBP1 and OSBP2 detected the proteins in different subcellular fractions in the retinal monkey tissue [5].
• Northern blot analyses indicate that OSBP1 is broadly expressed in human and monkey tissues [5].
• In vivo phosphopeptide mapping and mutagenesis of OSBP was used to identify the cholesterol-sensitive phosphorylation sites at serines 381, 384, and 387 that were responsible for the altered mobility on SDS-PAGE [21].

References