Despite the physiologic importance of vitamin E, in particular its alpha-tocopherol (alpha-T) isoform, the molecular mechanisms involved in the cellular uptake of this antioxidant from plasma lipoproteins have not been well-defined. Recent studies have suggested that selective lipid uptake, rather than endocytosis, is important for alpha-T delivery to cells. Here we show that the scavenger receptor class B type I (SR-BI), which mediates cellular selective cholesteryl ester uptake from lipoproteins, facilitates efficient transfer of alpha-T from HDL to cultured cells. In SR-BI-deficient mutant mice, relative to wild-type control animals, there was a significant increase in plasma alpha-T levels (1.1- to 1.4-fold higher) that was mostly due to the elevated alpha-T content of their abnormally large plasma HDL-like particles. This increase in plasma alpha-T in SR-BI knockout mice was accompanied by a significant decrease (65-80%) in the alpha-T concentrations in bile and several tissues including ovary, testis, lung and brain. SR-BI deficiency did not alter the alpha-T concentrations of the liver, spleen, kidney or white fat. These data show that SR-BI plays an important role in transferring alpha-T from plasma lipoproteins to specific tissues. Also, in the case of the liver as was previously shown for SR-BI-dependent hepatic cholesterol transport, SR-BI-mediated uptake of alpha-T was primarily coupled to biliary excretion rather than to tissue accumulation. Defective tissue uptake of lipoprotein alpha-T in SR-BI-deficient mice may contribute to the reproductive and cardiovascular pathologies exhibited by these animals.

The structure and expression of the potato mitochondrial gene rps10, encoding ribosomal protein S10, has been characterized. The RPS10 polypeptide of 129 amino acids is encoded by two exons of 307 bp and 80 bp respectively, which are separated by a 774-bp class-II intron. Editing of the complete rps10 coding region was studied by sequence analysis of spliced cDNAs. Four C residues are edited into U, resulting in the creation of a putative translational initiation codon, a new stop codon which eliminated ten carboxy-terminal residues, and two additional amino-acid alterations. All these changes increase the similarity between the potato and liverwort polypeptides. One additional C-to-U RNA editing event, observed in the intron sequence of unspliced cDNAs, improves the stability of the secondary structure in stem I (i) of domain I and may thus be required for the splicing reaction. All spliced cDNAs, and most unspliced cDNAs, were completely edited, suggesting that editing is an early step of rps10 mRNA processing and precedes splicing. Earlier work on potato rps10 (Zanlungo et al. 1994) is now known to comprise only a partial analysis of the gene, since the short downstream exon was not identified.

PDZK1 is a multi-PDZ domain-containing adaptor protein that binds to the C terminus of the high density lipoprotein receptor, scavenger receptor, class B, type I (SR-BI), and controls the posttranscriptional, tissue-specific expression of this lipoprotein receptor. In the absence of PDZK1 (PDZK1(؊/؊) mice), murine hepatic SR-BI protein levels are very low (<5% of control). As a consequence, abnormal plasma lipoprotein metabolism (ϳ1.5-1.7-fold increased total plasma cholesterol carried in both normal size and abnormally large high density lipoprotein particles) resembles, but is not as severely defective The HDL 2 receptor SR-BI plays an important role in lipoprotein-mediated lipid transport and metabolism (1). SR-BI mediates HDL binding to cells and subsequently facilitates the net transfer of cholesteryl esters from the particle core but not the protein or most of the lipid components of the outer shell of the lipoprotein, a process called selective lipid uptake (1-4). This receptor also mediates bidirectional movement of unesterified cholesterol between cells and lipoproteins (5-7). SR-BI is most highly expressed in hepatocytes, where it helps control plasma lipoprotein metabolism, and in steroidogenic cells, where it delivers lipoprotein cholesterol for storage and subsequent conversion into steroid hormones (4,8,9). Normal expression and moderate transgene-mediated overexpression of hepatic SR-BI have been shown to protect against atherosclerosis in several murine models (10 -16), and SR-BI transgene expression in the liver can prevent the female infertility seen in otherwise SR-BI null mice (SR-BI (Ϫ/Ϫ)) (12,17,18).SR-BI deficiency in SR-BI (Ϫ/Ϫ) mice causes an ϳ2-fold elevation in plasma cholesterol carried in both normal size and abnormally large HDL particles (12, 19), which exhibit an abnormally high ratio of unesterified cholesterol-to-total (unesterified plus esterified) cholesterol (14, 20) as well as a 30 -50% decrease in biliary cholesterol secretion rates and concentrations (12,21,22). These phenotypes are thought to be consequences of the reduced hepatic uptake of cholesterol from circulating HDL and can be reversed by hepatic SR-BI transgene expression (17,21,23,24).Tissue SR-BI expression can be regulated by both transcriptional and posttranscriptional mechanisms (25). In the liver, but not in steroidogenic tissues, posttranscriptional control of SR-BI protein expression depends on the presence of an adaptor protein, PDZK1 (26 -28). Cytoplasmic adaptor proteins that bind to membrane-associated proteins regulate a variety of biological processes, including signal transduction, adhesion, membrane trafficking, and cellular transport (29). They often comprise combinations of modular protein interaction domains such as Src homology (SH2, SH3), phosphotyrosinebinding (PTB), and PDZ domains that recognize short peptide or phosphopeptide motifs (e.g. PDZ domains usually bind to the C-terminal 3-4 residues of interacting proteins) (30).PDZK1 is a single chain, four-PDZ domain-containing polypeptide tha...

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