CPK激酶调控植物生长发育的分子机理及其研究进展

The functional roles of calcium-dependent protein kinases in plant growth and stress response

钙离子依赖性蛋白激酶(calcium-dependent protein kinase,CDPK或CPK)是细胞信号转导中发挥重要调控功能的一类丝氨酸/苏氨酸蛋白激酶.植物基因组编码大量的CPK激酶.当外界钙离子浓度发生变化时,CPK激酶能感知这些变化并与钙离子结合,导致蛋白构象改变以及释放自抑制结构域对激酶活性的抑制,从而激活CPK激酶.激活的CPK激酶通过磷酸化底物蛋白把钙信号向下游传递并引发胞内信号通路的级联反应,最终调控植物生长发育或对外界环境信号的响应.本文概述了CPK激酶的基本结构和分子特征,以及CPK激酶在调控植物生长发育、激素信号机理、生物和非生物胁迫应答过程中的最新研究进展,并对当前研究中存在的问题和未来的研究方向进行探讨与展望.

Calcium-dependent protein kinases (CPKs/CDPKs) are a multi-gene protein kinase family that play crucial roles inregulating plant growth and stress response. Calcium (Ca2+) is one of the most important secondary messengers, variousenvironmental and developmental cues can stimulate Ca2+ signals, which are displayed in the form of transient changes incalcium ion concentrations. These changes can be recognized and sensed by specific calcium sensors, thereby translatingthe chemical signals into transcriptional or metabolic processes. In recent years, four classes of calcium sensors have beenidentified in eukaryotic cells, including calmodulins (CaMs), calmodulin-like proteins (CMLs), calcineurin B-like proteins(CBLs) and CPKs. Among these calcium sensors, CaMs are highly conserved in all eukaryotic cells, whereas the others areonly identified in plants and some protists. Particularly, CPKs are commonly expressed in plants, but not in animal cells.These calcium sensors, together with other key components in Ca2+ signaling pathways, can recognize specific calciumsignatures and trigger downstream signaling events, such as reprogramming of transcriptional processes, activation ofreceptor like kinase cascades, and accumulation of ROS. In addition, these calcium sensors display distinct proteinstructures and activation mechanisms. CaMs, CMLs and CBLs solely have a Ca2+ binding domain and function as sensorsby directly binding to Ca2+ ions. While CPKs have four characterized domains, including a variable N-terminal domain, aSer/Thr kinase catalytic domain, an autoregulatory/autoinhibitory domain and a calmodulin-like domain. Further, CPKsare the only calcium sensors that have a protein kinase activating domain. The cytosolic Ca2+ signals can be sensed byCPKs and followed by calcium-dependent conformation changes. In this regard, CPKs function as direct “sensor decoders”for translating Ca2+ signals. In plants, CPKs convey calcium signals into cellular responses by phosphorylating varioussubstrates, including membrane transporters, kinases, transcription factors, metabolic enzymes, etc. Interestingly, recentstudies revealed that CPKs can autophosphorylate on Ser and Thr amino acids both in vivo and in vitro, whereas very fewCPKs autophosphorylate on Tyr residues. This huge diversity of phosphorylation targets confers key functions of CPKs incell division, pollen tube growth, stomatal development, phytohormone signaling, transcriptional regulation, stressdefense, etc. Importantly, specificities in CPK signaling are determined by differential expression patterns, calcium bindingselectivity, subcellular localizations and substrates. In the past years, various CPK families have been discovered in modelplant systems and crops. For instance, the Arabidopsis (Arabidopsis thaliana) genome encodes 34 CPKs that arecharacterized by their isoform specificities and calcium-dependent kinase activities. Despite extensive studies have beenperformed to elucidate CPKs’ functions and Ca2+-CPKs signaling networks, the molecular mechanisms of most CPK genesremain poorly understood.In this review, we will describe the structures, subcellular localizations and molecular characteristics of CPKs, we focuson the functional roles of CPKs in regulating plant growth, phytohormone signaling and stress tolerance. Finally, we willdiscuss future directions in the studies of how CPKs determine crop quality and agronomic traits.