Tubb Matthew R, Silva R A Gangani D, Pearson Kevin J, Tso Patrick, Liu Min, Davidson W Sean
Department of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio 45237.
Laboratory of Experimental Gerontology, NIA, National Institutes of Health, Baltimore, Maryland 21224.
J Biol Chem. 2007 Sep 28;282(39):28385-28394. doi: 10.1074/jbc.M704070200. Epub 2007 Aug 8.
Apolipoprotein A-IV (apoA-IV) is a 376-amino acid exchangeable apolipoprotein made in the small intestine of humans. Although it has many proposed roles in vascular disease, satiety, and chylomicron metabolism, there is no known structural basis for these functions. The ability to associate with lipids may be a key step in apoA-IV functionality. We recently identified a single amino acid, Phe(334), which seems to inhibit the lipid binding capability of apoA-IV. We also found that an intact N terminus was necessary for increased lipid binding of Phe(334) mutants. Here, we identify Trp(12) and Phe(15) as the N-terminal amino acids required for the fast lipid binding seen with the F334A mutant. Furthermore, we found that individual disruption of putative amphipathic alpha-helices 3-11 had little effect on lipid binding, suggesting that the N terminus of apoA-IV may be the operational site for initial lipid binding. We also provide three independent pieces of experimental evidence supporting a direct intramolecular interaction between sequences near amino acids 12/15 and 334. This interaction could represent a unique "switch" mechanism by which apoA-IV changes lipid avidity in vivo.
载脂蛋白A-IV（apoA-IV）是一种由人类小肠产生的含376个氨基酸的可交换载脂蛋白。尽管它在血管疾病、饱腹感和乳糜微粒代谢中具有多种假定作用，但这些功能尚无已知的结构基础。与脂质结合的能力可能是apoA-IV发挥功能的关键步骤。我们最近鉴定出一个单一氨基酸苯丙氨酸（Phe334），它似乎会抑制apoA-IV的脂质结合能力。我们还发现完整的N端对于增加Phe334突变体的脂质结合是必需的。在此，我们确定色氨酸（Trp12）和苯丙氨酸（Phe15）是F334A突变体快速脂质结合所需的N端氨基酸。此外，我们发现假定的两亲性α螺旋3-11的个别破坏对脂质结合影响不大，这表明apoA-IV的N端可能是初始脂质结合的作用位点。我们还提供了三条独立的实验证据，支持氨基酸12/15附近序列与334之间存在直接分子内相互作用。这种相互作用可能代表一种独特的“开关”机制，通过该机制apoA-IV在体内改变脂质亲和力。
