2020 Program

July 7, 2020, a worldwide ISPL

ABSTRACTS:

Selected organism-level similarities and differences in lipid signaling and lipid storage.

Kent D. Chapman

In areas of lipid signaling and lipid storage, research efforts in the Chapman laboratory over the last 27 years have revealed several mechanistic similarities and differences between plants and other organisms.  As one example, the endocannabinoid signaling system in mammals shares some remarkable similarities to N-acylethanolamine (NAE) signaling in plants with respect to the types of lipophilic ligands and the enzymes that metabolize these bioactive molecules. In both animals and plants, these acylethanolamide signals are inactivated by a conserved amidase enzyme—the fatty acid amide hydrolase (FAAH), but structural differences in these enzymes from mammals and plants point to evolutionary divergence in substrate specificity resulting in divergent cellular communication outcomes.  In terms of lipid storage, essentially all cells package neutral lipids into cytoplasmic lipid droplets (LDs), which on the surface may look quite different depending upon the cell, tissue or organism type. However, recent research has pointed to more unifying mechanisms at the core of LD biogenesis in animals and plants despite apparent lineage-specific protein families.  The 2020 Galliard lecture will briefly review these two aspects of plant lipid metabolism—lipid signaling and storage—in which the Chapman laboratory and a rich group of talented collaborators have made contributions, with a perspective on the evolutionary conservation evident among organisms that move and those that grow in place.

Understanding processes that limit lipid accumulation

Jantana Keereetaweep

The first committed step in fatty acid synthesis is mediated by Acetyl-CoA carboxylase (ACCase). By using Arabidopsis cell suspension cultures, our recent study showed that ACCase can be regulated by short-term (reversible) and longer-term (irreversible) inhibition by the oversupply of fatty acids (FA) upon feeding with Tween80. Inactive analogs of biotin carboxyl transfer proteins (BCCPs), Biotin-Attachment-Domain-Containing (BADC), can displace BCCP subunits within ACCase complex and downregulate its activity. While the reversible phase of ACCase inhibition was similar for cells derived from badc1badc3 and wild-type, the irreversible phase of inhibition was reduced by 50% in badc1badc3 relative to wild-type. We present the two important homeostatic roles for BADC proteins in the regulation of ACCase activity: firstly, during normal growth and development, and secondly, by contributing to its long-term irreversible feedback inhibition from oversupply of FA.  Another study demonstrated the involvement of a catalytic α-subunit of the SUCROSE-NON-FERMENTING1-RELATED PROTEIN KINASE1 (SnRK1) in the phosphorylation-dependent proteasomal degradation of WRINKLED1 (WRI1). We recently showed that trehalose-6-phosphate (T6P) an inhibitor of SnRK1, directly binds to KIN10, weakening its association with GEMINIVIRUS REP-INTERACTING KINASE1 (GRIK1), required to activate KIN10, thereby stabilizing WRI1. We tested the hypothesis that the phosphate group on T6P can preferentially bind to a site on KIN10 in an area of positively charged residues i.e., lysines or arginines. Upon the inspection of the surface of a KIN10 homology model, several such potential T6P binging sites were identified. Lysines and arginines within these sites were substituted by alanine residues and T6P binding assays were performed. Equilibrium dissociation constants (K_d) obtained from microscale thermophoresis between KIN10 K63A-R65A-R66A and T6P was significantly increased relative to that of WT. KIN10 K63A-R65A-R66A-K69A-L73A also showed weakened association with GRIK1 compared to WT. Therefore, we propose the potential binding site(s) for T6P on KIN10 and rationalize how it weakens the interaction between KIN10 and GRIK1 blocking its activation.