The fungal toxin fusicoccin (FC) binds to a receptor (FCBP) in the plasma membrane (PM) of virtually all higher plant cells and causes enhanced proton excretion by the plasma membrane proton- (ATPase). The ubiquitous nature of the response suggests that the FCBP, a receptor for a compound a plant normally never would encounter, is a conserved protein with a different but essential function. If so, a loss of FCBPs should be lethal. To test this hypothesis, a search is being made for mutant Arabadopsis thaliana plants which are insensitive to FC and lack FCBPs. To characterize the FCBP, the FCBP gene will first be isolated from an Arabadopsis genomic library. This will require microsequencing the 31 kDa FCBP polypeptide which has been purified, and then constructing an oligonucleotide probe. The manner in which FC-FCBP enhances ATPase activity is uncertain. One possibility is that it alters the pH profile for the ATPase to enchance its activity in the cytoplasmic pH range. It may do so by increasing the content of lysophospholipids (LPL) in the PM. Two questions concerning this hypothesis will be tested. First, are the FCBP and ATPase sufficient for a FC effect or are other PM proteins required? Secondly, does FC enhance the LPL content of the PM, and can increased LPL replace the requirement for FC? Cells move chemicals across their membranes by a system of active transport. The pumps are proteins but they are difficult to separate and purify because they are located across and embedded in membranes. It is now possible to study one of these proteins from plants because it has been purified. This protein is thought to play a role in plant growth hormone action.
