Identification and Functional Characterization of the BAG Protein Family in Arabidopsis thaliana

Abstract

The genes that control mammalian programmed cell death are conserved across wide evolutionary distances. Although plant cells can undergo apoptosis-like cell death, plant homologs of mammalian regulators of apoptosis have, in general, not been found. This is in part due to the lack of primary sequence conservation between animal and putative plant regulators of apoptosis. Thus, alternative approaches beyond sequence similarities are required to find functional plant homologs of apoptosis regulators. Here, we present the results of using advanced bioinformatic tools to uncover the Arabidopsis family of BAG proteins. The mammalian BAG (Bcl-2-associated athanogene) proteins are a family of chaperone regulators that modulate a number of diverse processes ranging from proliferation to growth arrest and cell death. Such proteins are distinguished by a conserved BAG domain that directly interacts with Hsp70 and Hsc70 proteins to regulate their activity. Our searches of the Arabidopsis thaliana genome sequence revealed seven homologs of the BAG protein family. We further show that plant BAG family members are also multifunctional and remarkably similar to their animal counterparts, as they regulate apoptosis-like processes ranging from pathogen attack to abiotic stress and development. Programmed cell death (PCD)2 plays an indispensable role in development and physiology, but it is unclear whether the mechanisms governing PCD in both plants and animals are similar. The genes that control programmed cell death are conserved across wide evolutionary distances from Caenorhabditis elegans to humans (1), although whether such conservation extends to plants is unknown. PCD plays a normal physiological role in many plant processes, and although the biochemical mechanisms responsible for cell suicide in plants are largely unknown, a number of reports suggest similarities to animal PCD (reviewed in Ref. 2). Moreover, ectopic expression of certain animal anti-apoptotic genes in transgenic plants is known to confer protection from pathogens and other stresses by death suppression (3-5).The identification of homologs of animal cell death regulators is of considerable interest. However, to date, few endogenous plant genes that show significant sequence similarity to animal apoptotic genes have been identified. Examination of the completed genome sequence of Arabidopsis thaliana, as well as other partially or nearly complete plant genomes by tools such as BLAST and FASTA, has not revealed any apparent homologs to the core apoptosis regulators. This may be explained by high sequence divergence of functional plant homologs of animal apoptotic proteins. Therefore, one approach to identify candidate PCD modulators in plants is through advanced tools of bioinformatics. We describe the results of such efforts, viz. the identification and characterization of the BAG protein family of Arabidopsis by profilesequence (Pfam) and profile-profile (FFAS) algorithms.The BAG proteins are an evolutionarily conserved family of multifunctional proteins...

Aspartyl Protease-Mediated Cleavage of BAG6 Is Necessary for Autophagy and Fungal Resistance in Plants

“…While plant and human BAG proteins have relatively low sequence identity that initially obscured detection by BLAST searches, these proteins were found to be remarkably similar in several key structural regions. Importantly, these observations extend to functional similarity as well (Doukhanina et al, 2006). In particular, human and plant BAG proteins show high degrees of structural conservation in the functional portions of the protein (e.g., helical structure and hydrophobicity; Doukhanina et al, 2006) that when taken together account for functional similarity.…”

“…T-DNA knockout lines (bag6) showed enhanced susceptibility to the necrotrophic fungus Botrytis cinerea (Doukhanina et al, 2006). B. cinerea challenge to wild-type Arabidopsis (Col-0) typically resulted in an initial short growth phase emanating from the point of inoculation, transitioning to a rapid delimited cell death phenotype.…”

“…To cope with both biotic and abiotic environmental pressures, plants have developed a number of defense strategies (Jones and Dangl, 2006). Recent studies have established that control of programmed cell death (PCD) pathways can be an important component that controls the outcome of a given stress response in plants (Dickman et al, 2001;Doukhanina et al, 2006;Lord and Gunawardena, 2012;Kabbage et al, 2013). Apoptosis and autophagy are two principal forms of PCD broadly studied in eukaryotes.…”

“…In particular, human and plant BAG proteins show high degrees of structural conservation in the functional portions of the protein (e.g., helical structure and hydrophobicity; Doukhanina et al, 2006) that when taken together account for functional similarity. In accordance, our studies have indicated that plant BAG family members are also multifunctional and similar to their animal counterparts, as they block several biotic and abiotic cell death-mediated processes while conferring cytoprotection in situations ranging from pathogen attack to drought stress to plant development (Doukhanina et al, 2006;Williams et al, 2010).…”

“…As an approach to address whether PCD-related genes have structural and, thus, possibly functional similarities to their mammalian counterparts, we used computational approaches searching for plant genes that structurally resembled apoptotic PCD genes, and as a result we uncovered the Bcl-2-associated athanogene (BAG) protein family in Arabidopsis thaliana (Doukhanina et al, 2006;Kabbage and Dickman, 2008). While plant and human BAG proteins have relatively low sequence identity that initially obscured detection by BLAST searches, these proteins were found to be remarkably similar in several key structural regions.…”