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). A number of isoforms are produced. According to EST sequences. Present in developing tissues from roots, leaves, flowers, siliques and seeds (at protein level).