EVIDENCE THAT BINDING OF CTP - PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE TOMEMBRANES IN RAT HEPATOCYTES IS MODULATED BY THE RATIO OF BILAYER-FORMING TO NON-BILAYER-FORMING LIPIDS
H. Jamil et al., EVIDENCE THAT BINDING OF CTP - PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE TOMEMBRANES IN RAT HEPATOCYTES IS MODULATED BY THE RATIO OF BILAYER-FORMING TO NON-BILAYER-FORMING LIPIDS, Biochemical journal, 291, 1993, pp. 419-427
The mechanism by which phospholipase C (PLC) digestion of cultured cel
ls mediates binding of CTP:phosphocholine cytidylyltransferase to cell
ular membranes was investigated. Incubation of choline-depleted rat he
patocyes with PLC caused a translocation of enzyme from cytosol to mem
branes concomitant with a decrease in the concentration of phosphatidy
lcholine with no effect on the concentration of other phospholipids. R
emoval of PLC and supplementation with choline restored the amount of
phosphatidylcholine in the cells and translocated cytidylyltransferase
to the cytosol. However, when phosphatidylcholine levels were decreas
ed by incubation with phospholipase A2 (PLA2), there was no significan
t redistribution of cytidylyltransferase activity. With PLA2 the conce
ntration of phosphatidylethanolamine, as well as of phosphatidylcholin
e, was significantly decreased. Since PLC, but not phospholipase A2, r
aised the cellular concentration of diacylglycerol, possibly diacylgly
cerol mediated the binding of cytidylyltransferase to membranes. This
possibility was examined, but is unlikely, since addition of lysophosp
hatidylcholine to PLC-treated cells restored the concentration of phos
phatidylcholine and released cytidylyltransferase into the cytosol, bu
t did not lower diacylglycerol levels to normal values. Studies in vit
ro, incubations of cells with choline analogues and a survey of the li
terature suggested that the over-riding common factor in regulation of
cytidylyltransferase binding to membranes may be the ratio of bilayer
to non-bilayer lipids in that membrane.