PII interactions with BADC and BCCP proteins co-regulate lipid and nitrogen metabolism in Arabidopsis

In plants the initiation of fatty acid synthesis is catalyzed by acetyl-CoA carboxylase (ACCase) which produces malonyl-CoA. The heteromeric form of ACCase (htACCase) is a holoenzyme consisting of biotin carboxylase and carboxyltransferase sub-complexes, both of which are subject to extensive regulation. Biotin carboxylase activity is controlled in part by the presence of the catalytic biotin carboxyl carrier proteins (BCCP1/2) and/or the non-catalytic, non-biotinylated, biotin/lipoyl attachment domain-containing proteins (BADC1/2/3) that associate with backbone biotin carboxylase (BC) protein. However, the mechanisms regulating BADC and BCCP interaction with BC and thus ACCase activity in planta are not clear. Here we demonstrate the Arabidopsis thaliana regulatory protein PII modulates htACCase activity through independent interactions with BADC and BCCP proteins in a selective manner. Analysis of badc1/2 and badc1/3 mutant lines and the respective pii triple mutants reveal that changes in seed oil and protein accumulation of badc double mutants are PII/nitrogen dependent. Absolute quantification of htACCase subunits and PII in developing seeds suggests that Arabidopsis exerts tight regulation over individual protein stoichiometry to balance oil and protein accumulation. The effects on vegetative and seed development indicate PII and BADC proteins have distinct but overlapping roles in the regulation of plant metabolism.