Protein 4.1 is involved in a structural thermotropic transition of the red blood cell membrane detected by a spin-labeled stearic acid

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Abstract
Proteins involved in a structural transition in red blood cell membranes detected at 8 .+-. 1.5.degree. C by a stearic acid spin-label have been investigated. Calcium loading of red blood cells with ionophore A23187 caused the disappearance of the 8.degree. C transition. Protein 4.1 appears to be the most susceptible protein to Ca2+ treatment. Antibodies specific for spectrin, band 3 (43K cytoplasmic domain), and protein 4.1 have been utilized as specific probes to modify membrane thermotropic properties. The 8.degree. C transition was eliminated by anti-4.1 protein antibodies but was not modified by the other antibodies. To further characterize the protein(s) involved in the transition, ghosts were subjected to sequential extraction of skeletal proteins. The extraction of band 6, spectrin, and actin did not modify the 8.degree. C transition. In contrast, high-salt extraction (1 M KCl) of spectrin-actin-depleted vesicles, a procedure that extracts proteins 2.1 and 4.1, was able to eliminate the 8.degree. C transition. Rebinding of purified 4.1 to the high salt extracted vesicles restored the 8.degree. C transition. These results indicate the involvement of protein 4.1 in the transition and suggest a functional membrane association of this protein. The binding of protein 4.1 to the membrane seems to contribute significantly to the thermotropic properties of red blood cells.

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