PtdSer is present at low concentrations in plants. There is biochemical evidence that it can be made both by PtdSer synthase (PSS) which uses CDP-DAG and serine and by head-group exchange of serine with Cho or Etn from PtdCho or PtdEtn respectively depending on the plant species and tissue (Kinney 1993). A cDNA encoding a protein that showed strong PSS activity was cloned from wheat in a screen to identify genes that enhance resistance to aluminium toxicity (Delhaize et al. 1999). However, a recent BLAST search reveals that no homologs of the wheat PSS are identified in any other plant species and that the closest homolog to the wheat PSS is a gene from a cereal fungal pathogen (Gibberella). It now seems likely that the "wheat" clone was isolated from fungus infected plant material. On the other hand, a clear homolog of the mouse PSS is present in the Arabidopsis genome (At1g15110), although the mouse protein syn-thesises PtdSer by a base-exchange mechanism using PtdEtn or PtdCho rather than transfer of serine to CDP-DAG (Stone et al. 1998). It was later shown that the PSS from mouse is located in the mitochondria-associated membrane, an ER-type membrane that purifies with mitochondria rather than the bulk ER (Stone and Vance 2000). There is as yet no definitive information on the location of the phospholipid base-exchange enzyme in plants. A plant gene encoding a CDP-DAG:serine phosphatidyltrans-ferase has yet to be identified in plants. The plasma membrane of plants is relatively rich in PtdSer and there is evidence that this lipid may be supplied as small, PtdSer-enriched vesicles that are released from the ER in an adenosine triphosphate (ATP)-dependent process (Sturbois-Balcerzak et al. 1999; Vincent et al. 2001). However, the plasma membrane itself has been shown to have high phospholipid head-group serine exchange activity suggesting that this membrane obtains its PtdSer from two sources (Vincent et al. 1999).
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