Guo Woei-Jiun, Ho Tuan-Hua
Institute of Plant and Microbial Biology, Academia Sinica, Taipei 115, Taiwan.
Plant Physiol. 2008 Aug;147(4):1710-22. doi: 10.1104/pp.108.120238. Epub 2008 Jun 26.
Plant HVA22 is a unique abscisic acid (ABA)/stress-induced protein first isolated from barley (Hordeum vulgare) aleurone cells. Its yeast homolog, Yop1p, functions in vesicular trafficking and in the endoplasmic reticulum (ER) network in vivo. To examine the roles of plant HVA22, barley HVA22 was ectopically expressed in barley aleurone cells. Overexpression of HVA22 proteins inhibited gibberellin (GA)-induced formation of large digestive vacuoles, which is an important aspect of GA-induced programmed cell death in aleurone cells. The effect of HVA22 was specific, because overexpression of green fluorescent protein or another ABA-induced protein, HVA1, did not lead to the same effect. HVA22 acts downstream of the transcription factor GAMyb, which activates programmed cell death and other GA-mediated processes. Moreover, expression of HVA22:green fluorescent protein fusion proteins showed network and punctate fluorescence patterns, which were colocalized with an ER marker, BiP:RFP, and a Golgi marker, ST:mRFP, respectively. In particular, the transmembrane domain 2 was critical for protein localization and stability. Ectopic expression of the most phylogenetically similar Arabidopsis (Arabidopsis thaliana) homolog, AtHVA22D, also resulted in the inhibition of vacuolation to a similar level as HVA22, indicating function conservation between barley HVA22 and some Arabidopsis homologs. Taken together, we show that HVA22 is an ER- and Golgi-localized protein capable of negatively regulating GA-mediated vacuolation/programmed cell death in barley aleurone cells. We propose that ABA induces the accumulation of HVA22 proteins to inhibit vesicular trafficking involved in nutrient mobilization to delay coalescence of protein storage vacuoles as part of its role in regulating seed germination and seedling growth.
植物HVA22是一种独特的脱落酸（ABA）/胁迫诱导蛋白，最初从大麦（Hordeum vulgare）糊粉层细胞中分离得到。其酵母同源物Yop1p在体内的囊泡运输和内质网（ER）网络中发挥作用。为了研究植物HVA22的作用，将大麦HVA22在大麦糊粉层细胞中异位表达。HVA22蛋白的过表达抑制了赤霉素（GA）诱导的大型消化液泡的形成，这是GA诱导糊粉层细胞程序性细胞死亡的一个重要方面。HVA22的作用具有特异性，因为绿色荧光蛋白或另一种ABA诱导蛋白HVA1的过表达不会产生相同的效果。HVA22在转录因子GAMyb的下游起作用，GAMyb激活程序性细胞死亡和其他GA介导的过程。此外，HVA22:绿色荧光蛋白融合蛋白的表达显示出网络状和点状荧光模式，分别与ER标记BiP:RFP和高尔基体标记ST:mRFP共定位。特别是，跨膜结构域2对蛋白质定位和稳定性至关重要。系统发育上最相似的拟南芥（Arabidopsis thaliana）同源物AtHVA22D的异位表达也导致液泡化受到抑制，其程度与HVA22相似，这表明大麦HVA22与一些拟南芥同源物之间功能保守。综上所述，我们表明HVA22是一种定位于内质网和高尔基体的蛋白，能够负调控大麦糊粉层细胞中GA介导的液泡化/程序性细胞死亡。我们提出，ABA诱导HVA22蛋白的积累，以抑制参与营养物质动员的囊泡运输，从而延迟蛋白质储存液泡的融合，这是其在调节种子萌发和幼苗生长中的作用的一部分。
