Department of Biosciences, Minnesota State University-Moorhead, USA.
Department of Biology, University of San Diego, San Diego, CA, USA.
J Exp Bot. 2018 Feb 23;69(5):1171-1181. doi: 10.1093/jxb/erx471.
In C4 plants, the pyruvate phosphate dikinase regulatory protein (PDRP) regulates the C4 pathway enzyme pyruvate phosphate dikinase (PPDK) in response to changes in incident light intensity. In maize (Zea mays) leaves, two distinct isoforms of PDRP are expressed, ZmPDRP1 and ZmPDRP2. The properties and C4 function of the ZmPDRP1 isoform are well understood. However, the PDRP2 isoform has only recently been identified and its properties and function(s) in maize leaves are unknown. We therefore initiated an investigation into the maize PDRP2 isoform by performing a side by side comparison of its enzyme properties and cell-specific distribution with PDRP1. In terms of enzyme functionality, PDRP2 was found to possess the same protein kinase-specific activity as PDRP1. However, the PDRP2 isoform was found to lack the phosphotransferase activity of the bifunctional PDRP1 isoform except when PDRP2 in the assays is elevated 5- to 10-fold. A primarily immuno-based approach was used to show that PDRP1 is strictly expressed in mesophyll cells and PDRP2 is strictly expressed in bundle sheath strand cells (BSCs). Additionally, using in situ immunolocalization, we establish a regulatory target for PDRP2 by showing a significant presence of C4 PPDK in BSC chloroplasts. However, a metabolic role for PPDK in this compartment is obscure, assuming PPDK accumulating in this compartment would be irreversibly inactivated each dark cycle by a monofunctional PDRP2.
在 C4 植物中，丙酮酸磷酸二激酶调节蛋白（PDRP）通过响应入射光强的变化来调节 C4 途径酶丙酮酸磷酸二激酶（PPDK）。在玉米（Zea mays）叶片中，表达了两种不同的 PDRP 同工型，ZmPDRP1 和 ZmPDRP2。ZmPDRP1 同工型的性质和 C4 功能已经得到很好的理解。然而，PDRP2 同工型最近才被鉴定出来，其在玉米叶片中的性质和功能尚不清楚。因此，我们通过对 PDRP2 同工型的酶性质和细胞特异性分布与 PDRP1 进行并列比较，开始对其进行研究。就酶功能而言，发现 PDRP2 具有与 PDRP1 相同的蛋白激酶特异性活性。然而，发现 PDRP2 同工型除了在测定中 PDRP2 升高 5-10 倍之外，缺乏双功能 PDRP1 同工型的磷酸转移酶活性。主要基于免疫的方法表明，PDRP1 严格表达在叶肉细胞中，PDRP2 严格表达在束鞘链细胞（BSC）中。此外，通过原位免疫定位，我们通过显示 BSC 叶绿体中存在大量 C4 PPDK 来确定 PDRP2 的调节靶标。然而，假设在这个隔室中积累的 PPDK 会在每个暗周期被单功能 PDRP2 不可逆地失活，因此在这个隔室中 PPDK 可能具有代谢作用是不清楚的。
