Novel protein-disulfide isomerases from the early-diverging protist Giardia lamblia

Protein-disulfide isomerase is essential for formation and reshuffling of d isulfide bonds during nascent protein folding in the endoplasmic reticulum, The two thioredoxin-like active sites catalyze a variety of thiol-disulfid e exchange reactions. We have characterized three novel protein-disulfide i somerases from the primitive eukaryote Giardia lamblia, Unlike other protei n-disulfide isomerases, the giardial enzymes have only one active site. The active-site sequence motif in the giardial proteins (CGHC) is characterist ic of eukaryotic protein-disulfide isomerases, and not other members of the thioredoxin superfamily that have one active site, such as thioredoxin and Dsb proteins from Gram-negative bacteria, The three giardial proteins have very different amino acid sequences and molecular masses (26, 50, and 13 k Da). All three enzymes were capable of rearranging disulfide bonds, and gia rdial protein-disulfide isomerase-2 also displayed oxidant and reductant ac tivities. Surprisingly, the three giardial proteins also hard Ca2+-dependen t transglutaminase activity. This is the first report of protein-disulfide isomerases with a single! active site that have diverse roles in protein cr oss-linking. This study may provide clues to the evolution of key functions of the endoplasmic reticulum in eukaryotic: cells, protein disulfide forma tion, and isomerization.