A Novel Calcium Sensor and Its Target Kinase in Arabidopsis

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

A Novel Ca2+ Sensor and Its Target Kinase in Arabidopsis A number of stressful factors such as drought, cold, salinity, can severely hamper plant growth and development. To survive these extreme conditions, plants have evolved complex mechanisms to "monitor" the stress conditions and respond by changing their physiological and developmental programs. The long term goal of this project is to understand the signaling pathways that link the environmental stress factors to cellular responses in higher plants. Almost all extracellular signals, including plant hormones, light, stress factors, and pathogenic or symbiotic elicitors, can elicit Ca2+ pulses or transients which serve as "second messengers" in further signaling processes. Because different signals often induce distinct and specific cellular responses, an interesting question is how cells distinguish the Ca2+ messengers produced by different stimuli and respond accordingly. Studies suggest that signaling components that "sense" and "interpret" the Ca2+ parameters hold the key. The laboratory has identified a new family of Ca2+ sensor proteins that play a role in plant response to stress signals. One member of the family, AtCBL1, is strongly responsive to stress factors including drought, cold, and wounding. Another member, AtCBL4 or SOS3, is required for salt tolerance. To understand the molecular mechanism for AtCBL1 function, the laboratory has made a key finding that AtCBL1 specifically associates with a family of novel protein kinases that are found only in higher plants. These AtCBL1-interacting protein kinases (CIPKs) are highly related to each other and their kinase domains are most similar to SNF1 subfamily of protein kinases. However, all CIPKs contain a unique C-terminal non-kinase domain that is responsible for interaction with AtCBL1. Further studies with one of the CIPKs, CIPK1, show that AtCBL1 interaction with CIPK1 requires micromolar levels of Ca2+, suggesting that Ca2+-binding changes the conformation of AtCBL1 and triggers association with CIPK1. Kinase assays using recombinant CIPK1 determined that CIPK1 is a serine/threonine kinase but has unique cofactor preference. For instance, CIPK1 highly prefers Mn2+ for its activity and functions as a Mn2+-binding protein. This project will further explore the functional significance of AtCBL1-CIPK1 complex using a combination of biochemical, cell biology, and molecular genetic approaches. In the first objective, biochemical and cell biology approaches will be used to dissect the native form of CIPK1 holoenzyme and identify the functional substrates for CIPK1. In the second objective, molecular genetic approaches using transgenic plant and "knockout" mutant models will be used to unravel the function of AtCBL1-CIPK1 in plant growth and developmental processes especially under stress conditions. This study will establish a new paradigm for Ca2+ signal transduction, which will have major impact on cell biology in general and on signal transduction research in particular.