アクティブボード・２０１１年　５月
　　　　　・・・・・２０１１年　５月　４日更新・・・・・

研究発表を行った学会；
・　American Society for Cell Biology (ASCB) 50th annual meeting
　　２０１０年１２月１１日〜１５日（Philadelphia, PA, USA）
　

タイトル；Posttranslational regulation of ATP binding cassette transporter G5 and G8 by HRD1.
　
発表者；鈴木　伸悟　氏
　　　（熊本大学　大学院薬学教育部　遺伝子機能応用学分野）

Abstract；
ATP-binding cassette transporters ABCG5 and ABCG8 form heterodimer in endoplasmic reticulum (ER) and are expressed to the plasma membrane, where they work as a lipidic transporter. Mutation in these transporter genes prevents trafficking to the plasma membrane, resulting in sitosterolemia, a rare autosomal recessive disorder of sterol trafficking characterized by sterol accumulation and premature atherosclerosis. Since ABCG5 and ABCG8 are also known to associate with absorption of dietary sterol and billary excretion of cholesterol, the decrease in their expression at the plasma membrane exacerbates a lipid metabolism disorder such as atherosclerosis. In this study, we sought to determine the regulatory mechanism of degradation of monomeric and heterodimeric ABCG5 and/or ABCG8 during their quality control. Here, we showed that monomeric immature h (human) ABCG5 and hABCG8 are degraded in a proteosomal-dependent fashion because protein expression is inhibited by the proteasomal inhibitor MG-132, but not by the lysosomal inhibitor chloroquine. To further determine the molecules involved in the ER-associated proteasomal degradation of hABCG5 and hABCG8, we focused on several E3 ubiquitin ligases that could be potentially involved. Although no particular E3 ligases that directly induce the degradation of hABCG5 and hABCG8 were determined, we identified HRD1 as a modulator of glycosylation state of hABCG8. Overexpression of HRD1 decreased the expression of glycosylated immature hABCG8, and increased the accumulation of non-glycosylated immature hABCG8. Based on mutational analysis, we demonstrated Asn-619 to be the major site of N-glycan addition in hABCG8, and showed that Asn-619 glycosylation is required for the stability of hABCG8 and the maturation of hABCG5/8 heterodimer. Thus, our data suggest HRD1 as a key molecule that selectively reduces the stability of hABCG8 by increasing the non-glycosylated form of hABCG8.