Biosymthetic Regulation and Intracellular Transport of Phosphatidylserine

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1.Biosynthetic regulation of phosphatidylserine (PS)The PS biosynthesis in Chinese hamster ovary cells is catalyzed by at least two different PS synthases named PSS I and PSS II. In this study, we showed that (1) PSS II in CHO-K1 cells is inhibited by exogenous PS, (2) overproduction of PSS II leads to the loss of normal control of PSS II activity by exogenous PS, (3) the activity of over produced PSS II in CHO Kl cells is depressed for maintenance of the normal PS biosynthetic rate, probably through molecular mechanisms different from those for the exogenous PS-mediated inhibition, and (4) the Arg97 of PSS II is a critical residue for both the exogenous PS mediated inhibition of PSS II and the depression of overproduced PSS II activity.2.Intracellular transport of PSPhosphatidylethanolamine synthesis through the phosphatidylserine (PS) decarboxylation pathway requires PS transport from the endoplasmic reticulum or mitochondria-associated membrane to the mitochondrial inner membrane in mammalian cells. The transport-dependent PS decarboxylation in permeabilized Chinese hamster ovary (CHO) cells was enhanced by cytosolic factors from bovine brain. A cytosolic protein factor exhibiting this enhancing activity was purified, and its amino acid sequence was partially determined. The sequence determined was identical with part of the amino acid sequence of an EF-hand type calcium-binding protein, S100B. A His_<6>-tagged recombinant CHO S100B protein was able to remarkably enhance the transport-dependent PS decarboxylation in permeabilized CHO cells. Under the standard assay conditions for PS decarboxylase, the recombinant S100B protein did not stimulate PS decarboxylase activity and exhibited no PS decarboxylase activity. These results implicated the S100B protein in the transport of PS to the mitochondrial inner membrane.