An essential enzyme in glutathione (L-gamma-glutamyl-L-cysteinylglycine, GSH) biosynthesis, GSH is essential for growth and differentiation to prespore stage (PubMed:15993406). Catalyzes the condensation of glutamate to cysteine (PubMed:15993406). ATP + L-cysteine + L-glutamate = ADP + gamma-L-glutamyl-L-cysteine + H(+) + phosphate Sulfur metabolism; glutathione biosynthesis; glutathione from L-cysteine and L-glutamate: step 1/2. Monomer. Expressed throughout development, expression peaking at the aggregation stage. Expression is localized in the prespore region and tip region of the tipped aggregate. At slug stage, expression is predominantly in the posterior prespore region. In the fruiting body expression is detected in basal disk and spore region containing upper cup and lower cup. Cells show defects in prespore differentiation. Belongs to the glutamate--cysteine ligase type 3 family.