Ptdlns synthase (PIS) catalyses the reaction between inositol and CDP-DAG-producing Ptdlns and CMP. A PIS gene was identified in Arabidopsis by sequence homology and the cDNA expressed in E. coli (Collin et al. 1999) which accumulated Ptdlns, a phospholipid it normally lacks. The cDNA encoding Arabidopsis PIS was also able to complement the yeast PIS deletion mutant (Xue et al. 2000). The presence of a di-lysine retention signal at the C-terminus of the protein suggests PIS is located in the ER but this is yet to be substantiated. The enzyme was shown to require CDP-DAG and myoinositol as substrates and either Mn2+ or Mg2+ as cofactor (Collin et al. 1999; Xue et al. 2000). In vitro evidence has been presented that the Arabidopsis PIS may also catalyse exchange of the inositol head group of Ptdlns with myoinositol in the presence of low concentrations of CMP (Justin et al. 2002). The acyl chain composition of PtdIns in plants is highly saturated compared to other phospholipids but it is not known how this is maintained. Biochemical analysis of the overexpressed PIS protein suggested that the acyl composition of PtdIns is likely to be determined as much by the localisation of the PIS within subdomains of the ER and the CDP-DAG pool surrounding it as by the substrate selectivity of the enzyme itself (Justin et al. 2003). A second Arabidopsis PIS gene has been identified by sequence homology but not yet characterised (Beisson et al. 2003).
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