CaM/BAG5/Hsc70 signaling complex dynamically regulates leaf senescence.

Luhua Li,Xue Yang,Qi Li,Xuejie Wang,Shang Guo,Dong Chang,Shuzhen Men,Shasha Fang,Yuequan Shen,Boyang Cui,Yangfei Xing

doi:10.1038/srep31889

Abstract

Calcium signaling plays an essential role in plant cell physiology, and chaperone-mediated protein folding directly regulates plant programmed cell death. The Arabidopsis thaliana protein AtBAG5 (Bcl-2-associated athanogene 5) is unique in that it contains both a BAG domain capable of binding Hsc70 (Heat shock cognate protein 70) and a characteristic IQ motif that is specific for Ca2+-free CaM (Calmodulin) binding and hence acts as a hub linking calcium signaling and the chaperone system. Here, we determined crystal structures of AtBAG5 alone and in complex with Ca2+-free CaM. Structural and biochemical studies revealed that Ca2+-free CaM and Hsc70 bind AtBAG5 independently, whereas Ca2+-saturated CaM and Hsc70 bind AtBAG5 with negative cooperativity. Further in vivo studies confirmed that AtBAG5 localizes to mitochondria and that its overexpression leads to leaf senescence symptoms including decreased chlorophyll retention and massive ROS production in dark-induced plants. Mutants interfering the CaM/AtBAG5/Hsc70 complex formation leads to different phenotype of leaf senescence. Collectively, we propose that the CaM/AtBAG5/Hsc70 signaling complex plays an important role in regulating plant senescence.

Highlights

Senescence occurs at the final stage of leaf development in plants and is a critical process for the recycling of nutrients from senescent leaves to other parts of the plant
In addition to the short α-helix (Ha) that lies between helices H1 and H2 (Fig. 1B), the BAG domain generally retains the highly conserved helix bundle structure shared by other Arabidopsis thaliana BAG (AtBAG) family members[17]
The presence of the canonical BAG domain suggests the recognition of AtBAG5 by the molecular chaperones Hsp70/Hsc[70]

Summary

Introduction

Senescence occurs at the final stage of leaf development in plants and is a critical process for the recycling of nutrients from senescent leaves to other parts of the plant. The specific regulation mechanism of the mitochondrial calcium signaling pathway in the process of plant senescence has not been well characterized. In Arabidopsis, there are seven homologs of the BAG protein family, which regulate apoptosis-like processes ranging from pathogen attack and development to abiotic stresses. Based on their characteristic structural components, the Arabidopsis thaliana BAG (AtBAG) protein family can be divided into two groups. The IQ motif is a plant-specific CaM-binding motif with the consensus sequence IQXXXRGXXXR, which preferentially binds to apo-CaM19 This novel feature implicates the involvement of BAG proteins in the Ca2+/CaM signaling pathway in plants. We identified a signaling complex CaM/AtBAG5/Hsc[70] which plays important role in plant senescence

Methods

Results

Conclusion