The presence of sn-1-palmitoyl lysophosphatidylinositol monophosphate correlates positively with the fusion-permissive state of the plasma membrane of fusogenic carrot cells grown in suspension culture

Abstract

Two analogues of lysophosphatidylinositol monophosphate (LPIP) occur in fusogenic carrot cells and protoplasts and are distinguished from each other on thin-layer plates by differing R t values. The lower R f LPIP comigrates with sn -1-palmitoyl LPIP (16:0 LPIP) synthesized from sn -1-palmitoyl lysophosphatidylinositol. The upper R f LPIP is presumed to be sn -2-linoleoyl LPIP (18:2 LPIP) based on fatty acid analysis of phosphatidylinositol monophosphate (PIP) and comigration with the upper R f product resulting from the base-catalyzed hydrolysis of PIP. The 18:2 analogue is only a minor component of the LPIP recovered from the fusogenic cells and protoplasts, but it is the predominant form of LPIP in nonfusogenic cells and protoplasts. LPIP is found primarily in the plasma membranes of these cells representing from 10 to 20% of the total [ 3 H]inositol-labeled lipid recovered from isolated plasma membrane. Even though the fusogenic cells lose some of the LPIP as a result of cell wall digestion, if 2% or more of the total protoplast inositol lipid is LPIP, the protoplasts fuse spontaneously with a fusion frequency of greater than 60%. As the fusogenic protoplasts lose their fusion potential, they also lose the relative amount of 16:0 LPIP. Inhibiting the in vivo metabolism of phosphatidylinositol bisphosphate (PIP 2 ) and PIP in cells and protoplasts with the aminoglycoside, neomycin, does not inhibit fusion. These data indicate that the presence and metabolism of 16:0 LPIP but not the polyphosphoinositides, PIP and PIP 2 , correlate positively with protoplast fusion.