Phosphatidylserine Decarboxylases Research Articles

Cloning of a gene (PSD1) encoding phosphatidylserine decarboxylase from Saccharomyces cerevisiae by complementation of an Escherichia coli mutant.

A gene (PSD1) encoding a phosphatidylserine decarboxylase of Saccharomyces cerevisiae was cloned by complementation of a conditional lethal mutation in the homologous gene in Escherichia coli strain EH150. Expression of the cDNA clone in EH150 corrected growth, phospholipid, and phosphatidylserine decarboxylase activity defects. Expression of the genomic clone in wild type yeast resulted in 20-fold amplification of phosphatidylserine decarboxylase activity. A 1500-base pair open reading frame encodes a 56,558-Da protein with a potential mitochondrial targeting sequence. Upstream regulatory elements found in other enzymes of the phospholipid biosynthetic pathway are present in PSD1. The derived amino acid sequence shows 44 and 35% identity with the phosphatidylserine decarboxylases from Chinese hamster ovary cells and E. coli, respectively. Near the carboxyl terminus is an LGST sequence which, in E. coli, is the site of proteolytic cleavage of the proenzyme into the alpha and beta subunits and formation of the pyruvate prosthetic group (Dowhan, W., and Li, Q.-X. (1992) Methods Enzymol. 209, 348-359). Disruption of the PSD1 gene in a haploid strain of yeast resulted in loss of detectable decarboxylase activity but little alteration of the growth properties or phospholipid composition. These results suggest that yeast has a second phosphatidylserine decarboxylation activity.

Phosphatidylserine decarboxylase: Analysis of its action towards unsaturated and saturated phosphatidylserine and the effect of triton X-100 on activity

The membrane-bound enzyme phosphatidylserine decarboxylase ( Tetrahymena pyriformis) was found to have activity both in a crude, particulate form and when it is in a soluble form in the presence of the nonionic surfactant Triton X-100. This surfactant has routinely been included in the assay of phosphatidylserine decarboxylases from all sources; its effect on the activity of the Tetrahymena enzyme has now been characterized and a detailed consideration of the functioning of this surfactant in the assay of this membrane-bound enzyme is presented. The activity of the enzyme towards natural phosphatidylserine is found to be greater than towards saturated phosphatidylserine, both with and without Triton present; this finding is considered in terms of the effect of the thermotropic phase transition of the saturated material on the physical state of the phospholipid, rather than simply in terms of the specificity of the enzyme for phosphatidylserine containing unsaturated fatty acid groups. At high molar ratios of Triton to phospholipid, the activity of the enzyme is dramatically decreased. The decreased activity of the enzyme toward unsaturated Phosphatidylserine is considered in terms of a surface dilution model and the greatly diminished activity towards the saturated analogue is suggested to be the result of lipid phase separation.