Isolation of human glycogen branching enzyme cDNAs by screening complementation in yeast

Functional complementation of the Saccharomyces cerevisiae glycogen branching enzyme deficiency was screened to isolate human cDNAs that encode this enzyme. Human hepatoma cell line HepG2-derived cDNA libraries using the pAB23BXN yeast expression vector yielded four cDNAs capable of complementing the glc3::TRP1 glycogen branching enzyme mutation. Complementation was recognized by an altered iodine-staining trait. This illustrates that interspecies complementation can be used to isolate rare plasmids from libraries by screening if there is sufficient resolution. The human and yeast glycogen branching enzymes have a 67% identical amino acid sequence over a major portion of their length. The human gene is on chromosome 3.