Kinetic analysis of genetic complementation in heterokaryons of propionyl CoA carboxylase-deficient human fibroblasts

The authors studied genetic complementation of propionyl CoA carboxylase (PCC) deficiency in cultures of polyethylene glycol (PEG)-induced heterokaryons, using mutant fibroblast lines assigned to 5 mutant classes, designated bio, pcc A, pcc B, pcc C, and pcc BC. By measuring PCC activity directly in extracts of fused cells or indirectly in intact cells by [1-¹⁴C]propionate utilization, the authors confirmed the nonlinear nature of the PCC deficiency complementation map described by Gravel et al. When they studied the kinetics of complementation, they detected 3 distinct patterns using the [1-¹⁴C]propionate utilization assay. When either pcc A or pcc C lines were fused to bio cells, ¹⁴C-fixation increased to half of the maximally restored values within 4 hr. In pcc A x pcc C crosses or in pcc A x pcc B crosses, however, complementation was much slower. In fusions between pcc B and pcc C cells, a third pattern was elicited; complementation was incomplete, maximum restoration of PCC activity being <20% of that observed in other complementing crosses. From these data and previous biochemical evidence, the authors suggest: (1) that the bio and pcc mutations affect different genes; (2) that complementation between pcc A and either pcc B, pcc C, or pcc BC lines is intergenic and involves subunit exchange and synthesis of new PCC molecules; and (3) that complementation between pcc B and pcc C mutants is interallelic.