gamma-Glutamyltransferase activity has been measured in yeast (Saccharomyces cerevisiae) and shown to be 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 (i) control and dicyclohexylcarbodi-imide-treated cells and in (ii) the wild-type and a gap mutant, a treatment and a mutation that it has been shown prevents uptake of the inhibitors. Thus a major portion of the gamma-glutamyltransferase activity appears to exist in a membrane-bound form that is orientated with its gamma-glutamyl-binding site facing the outside. Yeast cells in which gamma-glutamyltransferase has been 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.

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