Advances in our understanding of the roles of phosphatidylinositol phosphates in controlling cellular functions such as endocytosis, exocytosis and the actin cytoskeleton have included new insights into the phosphatases that are responsible for the interconversion of these lipids. One of these is an entirely novel class of phosphatase domain found in a number of well characterized proteins. Proteins containing this Sac phosphatase domain include the yeast Saccharomyces cerevisiae proteins Sac1p and Fig4p. The Sac phosphatase domain is also found within the mammalian phosphoinositide 5-phosphatase synaptojanin and the yeast synaptojanin homologues Inp51p, Inp52p and Inp53p. These proteins therefore contain both Sac phosphatase and 5-phosphatase domains. This review describes the Sac phosphatase domain-containing proteins and their actions, with particular reference to the genetic and biochemical insights provided by study of the yeast Saccharomyces cerevisiae.
The factors regulating the activity of cellular phospholipase D (PLD) have been well characterized; however, the cellular distribution of specific PLD isoforms and the factors defining localization are less clear. Two specific PLD1 isoforms, PLD1a and PLD1b, are shown in the present study to be localized in endosomal compartments with early endosomal autoantigen 1, internalizing epidermal growth factor receptor (ErbB1) and lysobisphosphatidic acid. Novel C-terminal splice variants of PLD1, PLD1a2 and PLD1b2, do not exhibit this endosomal localization. Studies using catalytically inactive and C-terminal deletion mutants of the four PLD1 isoforms led to the conclusion that the C-terminus plays an important part in the catalytic activity of PLD1, but that the endosomal localization of PLD1a and PLD1b is defined by the C-terminus and not catalytic activity.
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