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The genetical and biochemical approach is used as a powerful tool for the elucidation of the metabolic regulation as well as the biological significance of various constituents in living cells. This technique has been successfully applied to the membrane phospholipids in mammalian cells, actually to phosphatidylserine, sphingomyelin, and cardiolipin, all of which are rather minor constituents in membrane phospholipids. Isolation and characterization of the mutant cells defective in biosyntheses and biosynthetic regulations of these phospholipids led to the new findings of the in vivo biosynthetic pathways and also of the biological significance of these phospholipids. By analyzing the five different kinds of mutant cells unusual in metabolisms of phosphatidylserine in CHO-K1 cells, we found a series of the following evidence. Phosphatidylserine is synthesized by the two kinds of base-exchange enzymes, namely serine-exchange enzyme I and II, through the chain reactions in participation of phosphatidylserine decarboxylase; phosphatidylcholine--&gt;phosphatidylserine--&gt;phosphatidylethanolamine--&gt; phosphatidylserine. The substrates of serine-exchange enzyme I are phosphatidylcholine, and either choline, serine, or ethanolamine, while those of serine-exchange enzyme II are phosphatidylethanolamine, and serine or ethanolamine but not choline. The last phosphatidylserine produced in consequence of the chain reaction again becomes a substrate of phosphatidylserine decarboxylase, and so this metabolic flow of the biosynthesis of phosphatidylserine is finally connected to a cyclic reaction between phosphatidylethanolamine and phosphatidylserine catalyzed by serine-exchange enzyme II and phosphatidylserine decarboxylase. Localization of serine-exchange enzyme I is on the cytoplasmic side of endoplasmic reticulum, while that of enzyme II is on its lumen side. There is a lot of evidence including our immunological observation that phosphatidylserine decarboxylase is exclusively located in the inner membrane of mitochondria. Thus, during the process of the above-mentioned reactions, phosphatidylserine have to move from endoplasmic reticulum to mitochondria, and phosphatidylethanolamine does from mitochondria to endoplasmic reticulum. The biological significance of these movements is now under investigation. Enveloped virus infection and its maturation are closely associated with the membrane functions of host mammalian cells. In order to get further information concerning the biological functions of phosphatidylserine and/or phosphatidylethanolamine in mammalian cell membranes, Sindbis virus infection system was examined. Deficiency of phosphatidylserine and/or phosphatidylethanolamine was found to give a profound effect on the maturation of the virus, mainly due to low efficiency of membrane fusion between the viral envelope and endosomal membranes of the host cells. All these results indicate that phosphatidylserine and/or phosphatidylethanolamine are essential to membrane fusion at least in enveloped viral infection.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetical and Biochemical Studies on the Metabolisms and Biological Roles of Membrane Phospholipids in Mammalian Cells