γ-Glutamyltransferase is not involved in the bulk uptake of amino acids, peptides or γ-glutamyl-amino acids in yeast (Saccharomyces cerevisiae)

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Abstract
.gamma.-Glutamyltransferase activity was measured in yeast (S. cerevisiae). It is associated mainly with the membrane fraction. A similar level of activity is found in a wild-type strain and in gap and gpp strains, the latter mutants being defective in the general amino acid and peptide permeases, respectively. The activity is inhibited in whole cells by 6-diazo-5-oxo-L-norleucine (N2O-Nle), azaserine and serine-borate complex; this inactivation seemingly acts from without for it is similar in control and dicyclohexylcarbodiimide-treated cells, and in the wild-type and a gap mutant, a treatment and a mutation that prevents uptake of the inhibitors. Thus, a major portion of the .gamma.-glutamyltransferase activity apparently exists in a membrane-bound form that is orientated with its .gamma.-glutamyl-binding site facing the outside. Yeast cells in which .gamma.-glutamyltransferase was inactivated by N2O-Nle show no significant change in their rates of uptake of a variety of amino acids, dipeptides and .gamma.-glutamyl-amino acids. The results preclude a major, direct role for .gamma.-glutamyltransferase in the transport of these substrates.