α-葡聚糖磷酸化酶（α-glucan phosphorylase，Pho）是一种重要的葡萄糖基转移酶，其主要功能是催化淀粉的磷酸解。淀粉合成起始是淀粉生物合成的一个基本问题。质体型Pho（Pho-1）在淀粉合成起始过程中发挥重要作用，其分子机理还不清楚。Pho-1是蛋白激酶的重要靶点。磷酸化的Pho-1能够和质体型D-酶（DPE1）、淀粉分支酶（BEs）组装成磷酸化依赖的Pho-1/DPE1和Pho-1/BE1/BEIIb复合物。目前，Pho-1的磷酸化位点及其生物学功能还未见报道。申请者前期工作鉴定到Pho-1的多个磷酸化位点。本项目拟在此基础上对Pho-1磷酸化位点的生物学功能、磷酸化依赖的Pho-1多酶复合体的生物发生以及Pho-1磷酸化对淀粉合成起始过程的调控进行研究和探讨。本研究对于阐明Pho-1磷酸化调控淀粉生物合成起始过程的分子机制，完善淀粉生物合成的调控网络具有重大意义。

Plastidial α-1,4-glucan phosphorylase (Pho-1) is an important α-1,4-D-glucosyltransferase that catalyzes the phosphorolysis of the terminal residue from the nonreducing ends of a-1,4-linked glucan chains, liberating Glc-1-P. Although this reaction is reversible, it has been argued that the relatively low levels of Glc-1-P and high levels of inorganic phosphate in plant cells favors the glucan degradation. Recently, Pho-1 has been found to play a pivotal role in the initiation process of rice seed starch biosynthesis, however, the mechanism remains unkown. Protein phosphorylation plays an important role in regulating starch biosynthesis. It has been showed that Pho-1 was phosphorylated during storage starch synthesis and this modification promoted the formation of two complexes: Pho-1/DPE1 and Pho-1/BE1/BEIIb. To date, no other information was available about the phosphorylation sites of Pho-1 and the functions mediated by these modifications. Previously, we have isolated natural Pho-1 from developing rice seeds, and identified six amino acid residues as the phosphorylation sites of Pho-1. In this study, we will focus on exploring the biological functions of the phosphorylation sites of Pho-1 and illustrating the roles of pho-1 phosphorylation in regulating the initiation process of starch biosynthesis. Our research will contribute to revealing the molecular mechanism of Pho-1 in rice starch biosynthesis and to providing a basis for the genetic improvement of rice starch quality.