Generation by Limited Proteolysis of a Catalytically Active 39‐kDa Protein from the 115‐kDa form of Phosphatidylinositol‐glycan‐specific Phospholipase D from Bovine Serum

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Abstract
It has been suggested previously that small amounts of the mature 115-kDa form of phosphatidylinositol (PtdIns)-glycan-specific phospholipase D from bovine serum may exist as a 47-kDa form which can also be generated in vitro by treatment with proteases. In this study, we investigated the possible proteolytic processing by trypsin of partially purified PtdIns-glycan- specific phospholipase D from bovine serum and found that tryptic digestion caused an apparent activation of the enzyme when assayed in the presence of 0.1% (mass/vol.) Triton X-100. Trypsin cleaved the 115-kDa form of PtdIns-glycan-specific phospholipase D into three major polypeptides with molecular masses of 33, 39, and 47 kDa. Under non-denaturing conditions, the polypeptides remained tightly but noncovalently associated with each other. However, in the presence of 6 M urea, the polypeptides could be separated by anion-exchange chromatography. After renaturation, PtdIns-glycan-specific phospholipase D activity was found to be associated with a 39-kDa fragment. Based on its size and its amino acid sequence, the active-site-containing fragment consisted of approximately 275 residues of the N-terminal region of PtdIns-glycan-specific phospholipase D. The active 39-kDa fragment hydrolyzed the PtdIns-glycan-anchors of solubilized acetylcholinesterase from bovine erythrocytes and variant surface glycoprotein from blood stream trypanosomes. However, this fragment was inactive on membrane-associated acetylcholinesterase and PtdIns.