A genetic screen for ethanolamine auxotrophs has identified a novel mutant allele of the morphogenesis checkpoint dependent (MCD)-4 gene, designated mcd4-P301L. In the presence of a null allele for the phosphatidylserine (PtdSer) decarboxylase 1 gene (psd1 Delta), the mcd4-P301L mutation confers temperature sensitivity for growth on minimal medium. This growth defect is reversed by either ethanolamine or choline supplementation. Incubation of mutant cells with [(3)H]serine followed by analysis of the aminoglycerophospholipids demonstrated a 60% decrease in phosphatidylethanolamine (PtdEtn) formation compared to parental cells. Chemical analysis of phospholipid content after culture under non-permissive conditions also demonstrated a 60% decrease in the PtdEtn pool compared to the parental strain. Although the morphogenesis checkpoint dependent (MCD)-4 gene and its homologues have been shown to play a role in glycosylphosphatidylinositol (GPI) anchor synthesis, the mcd4-P301L strain displayed normal incorporation of [(3)H]inositol into both proteins and lipids. Thus, a defect in GPI anchor synthesis does not explain either the ethanolamine auxotrophy or biochemical phenotype of this mutant. We also examined the growth characteristics and PtdSer metabolism of a previously described mcd4-174 mutant strain, with defects in GPI anchor synthesis, protein modification and cell wall maintenance. The mcd4-174, psd1 Delta strain is a temperature sensitive ethanolamine auxotroph that requires osmotic support for growth, and displays normal PtdEtn formation compared to parental cells. These results reveal important genetic interactions between PSD1 and MCD4 genes, and provide evidence that Mcd4p can modulate aminoglycerophospholipid metabolism, in a way independent of its role in GPI anchor synthesis.