Interaction of caldesmon with endoplasmic reticulum membrane: effects on the mobility of phospholipids in the membrane and on the phosphatidylserine base-exchange reaction.

We have previously demonstrated by tryptophan fluorescence the interaction of caldesmon with anionic phospholipid vesicles [Czurylo, Zborowski and Dabrowska (1993) Biochem. J. 291, 403-408]. In the present work we investigated the interaction of caldesmon with natural-membrane (rat liver endoplasmic reticulum) phospholipids by co-sedimentation assay. The results indicate that 1 mol of caldesmon binds approx. 170 mol of membrane phospholipids with a binding affinity constant of 7.3 x 10(6) M-1. The caldesmon-membrane phospholipid complex dissociates with increasing salt concentration and in the presence of Ca2+/calmodulin. As indicated by EPR measurements of membrane lipids labelled with 5-doxyl stearate and TEMPO-phosphatidylethanolamine, binding of caldesmon results in an increase in mobility of the acyl chains (in the region of carbon 5) and a decrease in polar headgroup mobility of phospholipids. Interaction of caldesmon with phospholipids is accompanied by inhibition of phosphatidylethanolamine synthesis via a phospholipid base-exchange reaction, with phosphatidylserine as substrate. This shows that, of the endoplasmic reticulum membrane phospholipids, the main target of caldesmon is phosphatidylserine.