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

We studied genetic complementation of propionyl CoA carboxylase (PCC) deficiency in cultures of polyethylene glycol (PEG)-induced heterokaryons, using mutant fibroblast lines assigned to five 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, we confirmed the nonlinear nature of the PCC deficiency complementation map described by Gravel et al. [1]. When we studied the kinetics of complementation, we detected three 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 hrs. 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 begin less than 20% of that observed in other complementing crosses. From these data and previous biochemical evidence, we 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.