This protein is a component of the acetyl coenzyme A carboxylase complex; first, biotin carboxylase catalyzes the carboxylation of the carrier protein and then the transcarboxylase transfers the carboxyl group to form malonyl-CoA. Lipid metabolism; fatty acid biosynthesis. Acetyl-CoA carboxylase is a heterohexamer composed of biotin carboxyl carrier protein, biotin carboxylase and 2 subunits each of ACCase subunit alpha and ACCase plastid-coded subunit beta (accD). Primarily expressed in 7 to 10 days after flowering seeds at levels approximately 2-fold less abundant than BCCP1.