It was previously found (Białkowska, K., Zembroń, A. and Sikorski, A. F. (1994) Biochim. Biophys. Acta 1191, 21-26.) that isolated erythrocyte ankyrin inhibited interaction of spectrin with phospholipid liposomes, mainly those prepared from lipid mixtures containing aminophospholipids. In this communication we report on the effect of isolated ankyrin on red blood cell spectrin interaction with phospholipids in monolayers. Our data indicate that spectrin interaction with monolayers containing PE and, to a smaller extent, PS is sensitive to inhibition by ankyrin while interaction with monolayers containing only PC is not. Tetrameric spectrin affects monolayer surface pressure similarly to the heterodimer. However, an interaction of tetrameric spectrin with phospholipids was much more effectively inhibited by purified ankyrin indicating that one site per spectrin tetramer engaged in this interaction. When interactions of purified individual spectrin subunits (α or β) with phospholipid monolayers was studied, the β-subunit caused a strong, saturable effect on the surface pressure of the PE/PC monolayer, in contrast to the α-chain which induced much smaller and monotonic changes on the surface pressure of the same monolayer. Also interactions of the β-subunit with amino-phospholipids/PC monolayers were more sensitive to inhibition by ankyrin than those with native αβ-heterodimer of spectrin, e.g. threefold lower concentration of ankyrin was necessary to induce the same effect, while interaction of the α-subunit with phospholipid monolayers was entirely insensitive to ankyrin. Phosphorylation of spectrin in vitro with either cAMP-dependent protein kinase, or metabolically in intact cells, induced a decrease in the effect of either dimer or tetramer on the surface pressure of phospholipid monolayers. The sensitivity of this interaction to ankyrin was also greatly diminished. When isolated ankyrin was phosphorylated by the same cAMP-dependent protein kinase its ability to compete with phospholipid for spectrin was also diminished. The described effects may indicate a physiological significance of spectrin’s interaction with phospholipids, particularly in situations when there is not enough functional ankyrin to accommodate all spectrin molecules in the membrane.
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