Protein interactors of acyl-CoA-binding protein ACBP2 mediate cadmium tolerance in Arabidopsis

In our recent paper in the Plant Journal, we reported that Arabidopsis thaliana lysophospholipase 2 (lysoPL2) binds acyl-CoA-binding protein 2 (ACBP2) to mediate cadmium [Cd(II)] tolerance in transgenic Arabidopsis. ACBP2 contains ankyrin repeats that have been previously shown to mediate protein-protein interactions with an ethylene-responsive element binding protein (AtEBP) and a farnesylated protein 6 (AtFP6). Transgenic Arabidopsis ACBP2-overexpressors, lysoPL2-overexpressors and AtFP6-overexpressors all display enhanced Cd(II) tolerance, in comparison to wild type, suggesting that ACBP2 and its protein partners work together to mediate Cd(II) tolerance. Given that recombinant ACBP2 and AtFP6 can independently bind Cd(II) in vitro, they may be able to participate in Cd(II) translocation. The binding of recombinant ACBP2 to [¹⁴C]linoleoyl-CoA and [¹⁴C]linolenoyl-CoA implies its role in phospholipid repair. In conclusion, ACBP2 can mediate tolerance to Cd(II)-induced oxidative stress by interacting with two protein partners, AtFP6 and lysoPL2. Observations that ACBP2 also binds lysophosphatidylcholine (lysoPC) in vitro and that recombinant lysoPL2 degrades lysoPC, further confirm an interactive role for ACBP2 and lysoPL2 in overcoming Cd(II)-induced stress.Key words: acyl-CoA-binding protein, cadmium, hydrogen peroxide, lysophospholipase, oxidative stressAcyl-CoA-binding proteins (ACBP1 to ACBP6) are encoded by a multigene family in Arabidopsis thaliana.¹ These ACBP proteins are well studied in Arabidopsis in comparison to other organisms,^1–4 and are located in various subcellular compartments.¹ Plasma membranelocalized ACBP1 and ACBP2 contain ankyrin repeats that have been shown to function in protein-protein interactions.^5,6 ACBP1 and ACBP2 which share 76.9% amino acid identity also confer tolerance in transgenic Arabidopsis to lead [Pb(II)] and Cd(II), respectively.^1,5,7 Since recombinant ACBP1 and ACBP2 bind linolenoyl-CoA and linoleoyl-CoA in vitro, they may possibly be involved in phospholipid repair in response to heavy metal stress at the plasma membrane.^5,7 In contrast, ACBP3 is an extracellularly-localized protein⁸ while ACBP4, ACBP5 and ACBP6 are localized to cytosol.^9,10 ACBP1 and ACBP6 have recently been shown to be involved in freezing stress.^9,11 ACBP4 and ACBP5 bind oleoyl-CoA ester and their mRNA expressions are lightregulated.^12,13 Besides acyl-CoA esters, some ACBPs also bind phospholipids.^9,11,13 To investigate the biological function of ACBP2, we have proceeded to establish its interactors at the ankyrin repeats, including AtFP6,⁵ AtEBP⁶ and now lysoPL2 in the Plant Journal paper. While the significance in the interaction of ACBP2 with AtEBP awaits further investigations, some parallels can be drawn between those of ACBP2 with AtFP6 and with lysoPL2.