ABSTRACT This paper describes the first physiological response at the translational level towards heterologous protein production in Saccharomyces cerevisiae . In yeast, the phosphorylation of eukaryotic initiation factor 2α (eIF-2α) by Gcn2p protein kinase mediates derepression of GCN4 mRNA translation. Gcn4p is a transcription factor initially found to be required for transcriptional induction of genes responsible for amino acid or purine biosynthesis. Using various GCN4-lacZ fusions, knockout yeast strains, and anti-eIF-2α-P/anti-eIF-2α antibodies, we observed that heterologous expression of the membrane-bound α 1 β 1 Na,K-ATPase from pig kidney, the rat pituitary adenylate cyclase seven-transmembrane-domain receptor, or a 401-residue soluble part of the Na,K-ATPase α 1 subunit derepressed GCN4 mRNA translation up to 70-fold. GCN4 translation was very sensitive to the presence of heterologous protein, as a density of 1‰ of heterologous membrane protein derepressed translation maximally. Translational derepression of GCN4 was not triggered by misfolding in the endoplasmic reticulum, as expression of the wild type or temperature-sensitive folding mutants of the Na,K-ATPase increased GCN4 translation to the same extent. In situ activity of the heterologously expressed protein was not required for derepression of GCN4 mRNA translation, as illustrated by the expression of an enzymatically inactive Na,K-ATPase. Two- to threefold overexpression of the highly abundant and plasma membrane-located endogenous H-ATPase also induced GCN4 translation. Derepression of GCN4 translation required phosphorylation of eIF-2α, the tRNA binding domain of Gcn2p, and the ribosome-associated proteins Gcn1p and Gcn20p. The increase in Gcn4p density in response to heterologous expression did not induce transcription from the HIS4 promoter, a traditional Gcn4p target.