Alpha subunit of the biotin-independent and biotin-dependent malonate decarboxylase multienzyme complex (EC 4.1.1.88 and EC 7.2.4.4, respectively). Acts as an acyl-carrier protein (ACP) transferase component. This first step in malonate decarboxylation involves the exchange of an acetyl thioester residue bound to the activated ACP subunit for a malonyl thioester residue. Has a weak activity with acetyl-CoA as substrate. acetyl-[ACP] + malonate = acetate + malonyl-[ACP] Optimum pH is 5.5.