a CDP-1,2-diacyl-sn-glycerol + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + CMP + H(+) Multimeric. Interacts with ACP, YbgC and PlsB, forming altogether a complex at the inner membrane. Belongs to the CDP-alcohol phosphatidyltransferase class-II family.