Galactose-Specific Lectins Protect Isolated Thylakoids against Freeze-Thaw Damage

We have measured freeze-thaw damage to isolated spinach (Spinacia oleracea L.) chloroplast thylakoid membranes in the presence of different galactose-specific seed lectins to determine whether the binding of proteins to the membrane surface can lead to cryoprotection. Of the seven lectins investigated, five were protective to different degrees and two showed no measurable effect. Protection was afforded by a reduction of the solute permeability of the membranes. This reduced the solute influx during freezing and thereby osmotic rupture of the thylakoid vesicles during thawing. Using model membranes and fluorescently labeled lectins, we could show that the proteins bound exclusively to the digalactosyl lipids in the membranes. Binding was a prerequisite for the protective effect, because the presence of up to 5 mM galactose in the samples completely inhibited both binding of the lectins to thylakoid and model membranes and cryoprotection. The degree of binding was, in contrast, not related to the cryoprotective efficiency of different lectins; cryoprotection was a function of the hydrophobicity of the proteins.