CaM/BAG5/Hsc70 signaling complex dynamically regulates leaf senescence

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.

Triphosphate Tunnel Metalloenzyme Function in Senescence Highlights a Biological Diversification of This Protein Superfamily

“…Their precise molecular role(s) during these processes are not clear yet, but their mitochondrial localization is likely crucial for their biological function, since TTM1 DTM, which does not localize to the mitochondria, lost its ability to complement the ttm1 mutant phenotype. The involvement of mitochondria in plant PCD has been suggested but is not understood well (Lam et al, 2001;Qamar et al, 2015;Li et al, 2016). Based on the presence of the C-terminal transmembrane domains ( Fig.…”

Genome-Wide Identification and Expression Analysis of the Ascorbate Oxidase Gene Family in Gossypium hirsutum Reveals the Critical Role of GhAO1A in Delaying Dark-Induced Leaf Senescence

“…Numerous genes have been reported to be differentially expressed during the leaf senescence process [44], and many SAGs have been verified to be involved in leaf senescence [33,34,[45][46][47][48][49]. The detected SAGs of PIFs (PIF3, PIF34, and PIF35), SENs (SEN1 and SEN4), transcription factors (TFs; WRKY6, NAC2, and NAP), and cell death-related genes (PDCD5 and ACD1), showed significant abundant accumulation in the Arabidopsis ao1-1 mutant (Figure 8, Supplementary Figure S1), indicating that the function of AO is closely associated with the expression of these genes.…”

The Life and Death of a Plant Cell

“…This feature is intriguing given the role of the mitochondria in intrinsic apoptosis pathways and that none of the animal BAGs are localized to the mitochondria. Li et al (101) recently suggested that AtBAG5 forms a complex with Hsc70 and CaM that regulates plant senescence. AtBAG6 is the largest member of this family and was originally identified as a CaMbinding protein (88).…”