A Heteromeric Membrane-Bound Prenyltransferase Complex from Hop Catalyzes Three Sequential Aromatic Prenylations in the Bitter Acid Pathway
作者:Haoxun Li、Zhaonan Ban、Hao Qin、Liya Ma、Andrew J. King、Guodong Wang
DOI:10.1104/pp.114.253682
日期:2015.3
Bitter acids (α and β types) account for more than 30% of the fresh weight of hop (Humulus lupulus) glandular trichomes and are well known for their contribution to the bitter taste of beer. These multiprenylated chemicals also show diverse biological activities, some of which have potential benefits to human health. The bitter acid biosynthetic pathway has been investigated extensively, and the genes for the early steps of bitter acid synthesis have been cloned and functionally characterized. However, little is known about the enzyme(s) that catalyze three sequential prenylation steps in the β-bitter acid pathway. Here, we employed a yeast (Saccharomyces cerevisiae) system for the functional identification of aromatic prenyltransferase (PT) genes. Two PT genes (HlPT1L and HlPT2) obtained from a hop trichome-specific complementary DNA library were functionally characterized using this yeast system. Coexpression of codon-optimized PT1L and PT2 in yeast, together with upstream genes, led to the production of bitter acids, but no bitter acids were detected when either of the PT genes was expressed by itself. Stepwise mutation of the aspartate-rich motifs in PT1L and PT2 further revealed the prenylation sequence of these two enzymes in β-bitter acid biosynthesis: PT1L catalyzed only the first prenylation step, and PT2 catalyzed the two subsequent prenylation steps. A metabolon formed through interactions between PT1L and PT2 was demonstrated using a yeast two-hybrid system, reciprocal coimmunoprecipitation, and in vitro biochemical assays. These results provide direct evidence of the involvement of a functional metabolon of membrane-bound prenyltransferases in bitter acid biosynthesis in hop.
苦酸(α和β型)占啤酒花腺毛重量的30%以上,是啤酒苦味的来源。这些多烯基化合物的生物活性多种多样,其中一些可能对人体健康有益。苦酸的生物合成途径已经过广泛研究,苦酸合成早期步骤的基因也已克隆并确定功能。然而,人们对催化β-苦酸合成途径中三个连续烯基化步骤的酶知之甚少。在此,我们采用酵母(酿酒酵母)系统对芳香烯基转移酶(PT)基因的功能进行了鉴定。利用酵母系统对从啤酒花腺毛特异性互补DNA文库中获得的两个PT基因(HlPT1L和HlPT2)进行了功能鉴定。在酵母中,经过密码子优化的PT1L和PT2与上游基因共表达可产生苦酸,但单独表达任一PT基因时均未检测到苦酸。PT1L和PT2中富含天冬氨酸的基序的逐步突变进一步揭示了β-苦酸生物合成中这两种酶的烯基化序列:PT1L仅催化第一个烯基化步骤,PT2催化随后的两个烯基化步骤。利用酵母双杂交系统、相互共沉淀和体外生化分析,证明了PT1L和PT2之间的相互作用形成的代谢体。这些结果直接证明了膜结合烯基转移酶的功能代谢体参与啤酒花苦酸生物合成。