A triple yeast mutant was constructed which lacks BST1, the gene for sphingosine lyase, besides the phosphatidylserine decarboxylases PSD1 and PSD2. In this yeast mutant, which can only be grown in the presence of exogenous ethanolamine, phosphatidylethanolamine can be depleted to very low levels. Under those conditions, respiration as well as glucose and 3-O-methylglucose uptake proceed unaffected. Plasma membrane ATPase is as active in these cells as that of control cells grown in the presence of ethanolamine. Drastically decreased, however, are H+/amino acid symporters. The activities of arginine (Can1p), proline (Put4p) and general amino acid permease (Gap1p) are decreased more than 20-fold. Amino acid transport in yeast is dependent on coupling to the proton motive force. It can be envisaged that phosphatidylethanolamine might play a role in this process or in the early steps of the secretion pathway common for all amino acid permeases or, eventually, it could affect the transport proteins directly at the plasma membrane Transformation of the triple mutant with a CEN plasmid harbouring BST1 wild-type gene totally reversed its phenotype to that observed in the double mutant.
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