The mechanism of membrane response to chilling. Effect of temperature on phospholipid deacylation and reacylation reactions in the cell surface membrane.

The ciliary membrane of Tetrahymena pyriformis is physically and metabolically remote from the main centers of lipid metabolism. Nevertheless, it possesses an independent capacity to modify its phospholipid molecular species composition rapidly under stress. The role of ciliary phospholipid deacylating and reacylating enzymes in this phenomenon has been evaluated. Isolated cilia showed substantial phospholipase A (combined A1 and A2), acyl-CoA synthetase and acyltransferase activities. Activities of all the three enzymes of cilia from 39 degrees C-grown cells were greatly reduced when the cilia were incubated at 15 degrees C. In contrast, the phospholipase A and acyltransferase activities in cilia from 15 degrees C-grown cells were surprisingly high at 15 degrees C and twice as high at 37 degrees C as were the equivalent activities in preparations from 39 degrees C-grown cells. While the in vivo substrate specificity of phospholipase A could not be meaningfully assessed, the acyltransferases exhibited a temperature-dependent substrate specificity in vivo. Growth temperature also affected the positional distribution of fatty acids incorporated into ciliary phospholipids in vivo. The ability of acyltransferases to utilize added [14C] acyl-CoA could be markedly stimulated, and their lipid class specificity could be significantly altered in vitro by supplementing the incubation mixture with exogenous lysophospholipid acceptors. These findings suggest that the rate-limiting factor in acyl chain turnover is not the activity of acyltransferases per se but rather the availability of suitable substrates and acceptors. Therefore, we postulate that temperature alters the rate and specificity of ciliary membrane phospholipid retailoring primarily by controlling the in situ phospholipase A activity.