Catalyzes the reduction of malonyl-CoA to malonate semialdehyde, a key step in the 3-hydroxypropanoate and the 3-hydroxypropanoate/4-hydroxybutyrate cycles. Can also use succinyl-CoA and succinate semialdehyde as substrates but at a lower rate than malonyl-CoA. 3-oxopropanoate + CoA + NADP(+) = H(+) + malonyl-CoA + NADPH Divalent metal cations such as Mg(2+) and Mn(2+) ions at 5 mM. Activated by dithioerythritol (5 mM) and inhibited by the thiol-blocking agent iodoacetamide (0.1 mM). Optimum pH is 7.2, with half-maximal activities at pH 6 and 8. Optimum temperature is 85 degrees Celsius. Homodimer and possibly a tetramer. This enzyme contains bound RNA, but the physiological role is not known. Belongs to the aspartate-semialdehyde dehydrogenase family.