Molecular basis for genetic complementation in propionyl CoA carboxylase deficiency

The extent of genetic complementation in polyethylene glycol-induced heterokaryons of propionyl CoA carboxylase deficient fibroblast lines was determined by comparing enzyme activity changes over time in pairwise fusions of the three major complementation groups, bio, pcc A and pcc C, with the activity changes in similarly mixed but unfused cultures. Maximum complementation between bio and pcc. A or pcc C lines was attained within 24 h after fusion and was not inhibited by cycloheximide. In contrast, the complementation between pcc A and pcc C lines only attained 50% of the maximum restored carboxylase activity by 24-36 h and the increase was 93% inhibited by cycloheximide. Maximum restoration of activity was not achieved until 72-96 h after fusion. Removal of cycloheximide at 24 h permitted complementation to take place. Our studies suggest that intergenic complementation between the bio and pcc A or pcc C lines is due to the contribution within the heterokaryon of normal enzymes from each of the respective lines, resulting in almost immediate, protein synthesis-independent, partial restoration of carboxylase activity. Complementation between pcc A and pcc C lines also appears to be intergenic but probably results from the de novo synthesis of normal subunits and protomers which assemble into normal or stabilized propionyl CoA carboxylase molecules with restored enzyme activity.