In methanogens, the acetyl-CoA decarbonylase synthase (ACDS) complex, which has five different subunits, catalyzes synthesis and cleavage of acetyl-CoA according to the reaction: CO2 + 2H+ + 2e- + CH3-H4SPt + CoA <--> acetyl-CoA + H4SPt + H2O, where H4SPt and CH3-H4SPt are tetrahydrosarcinapterin and N5-methyl-tetrahydrosarcinapterin, respectively. We have dissociated the ACDS complex into three protein components by limited proteolytic digestion. Catalysis of acetyl-CoA synthesis was lost in parallel with the loss of the intact beta subunit; however, no decrease in activity was detected in any of three partial reactions found to be catalyzed by distinct protein components of the proteolyzed ACDS complex: (a) CO dehydrogenase, catalyzed by the alpha epsilon component, (b) CH3-H4pteridine:cob(I)amide-protein methyltransferase, catalyzed by the intact gamma subunit and fragments of the delta subunit, and (c) acetyltransferase, catalyzed by a truncated form of the beta subunit. The results indicated that the beta subunit is responsible for binding CoA and acetyl-CoA and suggested that acetyl-enzyme formation occurs on the beta subunit. A value of 5.5 x [H+]-1 M-1 was determined for the equilibrium constant of the following reaction at pH 7.5 and 25 degrees C: CH3-H4SPt + cob(I)amide-protein + H+ <--> H4SPt + CH3-cob(III)amide-protein.