Genetic Interactions Between BOB1 And Multiple 26S Proteasome Subunits Suggest A Role For Proteostasis In Regulating Arabidopsis Development

Protein folding and degradation are both required for protein quality control, an essential cellular activity that underlies normal growth and development. We investigated how BOB1, an Arabidopsis thaliana small heat shock protein, maintains normal plant development. bob1 mutants exhibit organ polarity defects and have expanded domains of KNOX gene expression. Some of these phenotypes are ecotype specific suggesting that other genes function to modify them. Using a genetic approach we identified an interaction between BOB1 and FIL, a gene required for abaxial organ identity. We also performed an EMS enhancer screen using the bob1-3 allele to identify pathways that are sensitized by a loss of BOB1 function. This screen identified genetic, but not physical, interactions between BOB1 and the proteasome subunit RPT2a. Two other proteasome subunits, RPN1a and RPN8a, also interact genetically with BOB1. Both BOB1 and the BOB1-interacting proteasome subunits had previously been shown to interact genetically with the transcriptional enhancers AS1 and AS2, genes known to regulate both organ polarity and KNOX gene expression. Our results suggest a model in which BOB1 mediated protein folding and proteasome mediated protein degradation form a functional proteostasis module required for ensuring normal plant development

A genetic interaction between chaperone and proteasome activity in Arabidopsis

Organisms contain cellular mechanisms that help maintain the correct structure of proteins in order to ensure for their proper function. These include the activity of several families of protein chaperones called heat shock proteins (HSP) that aid in protein folding. If proteins are unable to be correctly folded they are degraded in an ubiquitin dependent manner by proteolytic protein complexes such as the 26S proteasome. BOBBER1 (BOB1) is the only known Arabidopsis small heat shock protein (sHSP) that is required for thermotolerance as well as for normal development. A partial loss of function allele of BOB1, bob1-3, exhibits pleiotropic developmental phenotypes including serrated leaves, delayed growth, and defects in thermotolerance. A strong allelic combination, bob1-3/bob1-1 (a null allele), results in much more severe phenotypes including abaxial leaf spurs, polarity defects in floral organs, and fasciated meristems. In order to identify developmental pathways that BOB1 functions in we performed an EMS modifier screen in a bob1-3 background. bom has bob1-3 dependent phenotypes and displays phenotypes similar to those observed in strong bob1-3/bob1-1 allelic combinations. We used next generation sequencing to simultaneously map and clone bom. BOM encodes the RPT2a AAA-ATPase subunit of 26S proteasome. Phenotypic analysis of rosette size, total above ground growth, and roots, of single and double mutant plants suggests that there is a complex genetic interaction between BOB1 and BOM. These data demonstrate a genetic link between protein folding and protein degradation and suggest that plants with reduced chaperone activity are sensitized to decreases in proteasome activity