Structural And Functional Studies Of BAG Proteins In Plant

The recently identified plant Bcl-2-associated athanogene (BAG) family plays an extensive role in plant programmed cell death (PCD) processes ranging from growth and development to stress responses and even cell death. There are seven members in the Arabidopsis thaliana BAG (AtBAG) protein family, four of which (AtBAG1-4) have a domain organization similar to that of mammalian BAG1. Here, crystal structures of the BAG domains of AtBAG1-4have been determined; they have high homology and adopt a structure comprising three short parallel a-helices, similar to some mammalian BAG proteins. The crystal structure of a complex of the AtBAG1ubiquitin-like domain and BAG domain (UBD) with the Hsc70nucleotide-binding domain (NBD) was also determined. The binding of the AtBAG1BD to the Hsc70NBD induces conformational change of the Hsc70NBD to the open state and reduces the affinity of the NBD for ADP. The crystal structure also helps to explain the relationship between the ubiquitin-like domain and BAG domain in the BAG proteins.The other three members of AtBAG protein family (AtBAG5-7) all contain a conserved IQ motif located immediately before the BD. Here we reported the crystal structure of AtBAG5BD, which shows a divergently evolved structural and functional feature. We also provided the first evidence of the interaction between IQ motif-containing protein and calmodulin (CaM) in plant. The IQ motif adjacent to the BAG domain enables AtBAG5to act as a hub for simultaneous binding of apo-CaM and Hsc70. Our results reveal unique features of the plant BAG protein family, including their functions as co-chaperones. These results provide important information that aids in the understanding of the chaperone system in plants.