ANALYSIS OF THE LIPOPROTEIN BINDING-SITE OF RAT-LIVER MEMBRANES

Intermediate density lipoproteins (IDL) were shown to bind to high- an d low-affinity binding sites an rat liver membranes. The low-affinity sites were named lipoprotein binding sites (LBS), since they bind all classes of lipoproteins. This study was undertaken to further characte rize the interaction of I-125-labelled IDL with the LBS of rat liver m embranes to determine the chemical nature of the LBS. We found that th e binding of IDL to the LBS is insensitive to EDTA and sensitive to he parin and that it is present on plasma membranes. Also, membranes were pretreated with various enzymes that have an effect on the membrane c onstituents, and the activity of the LBS on these treated membranes wa s determined. Our results reveal that the LBS of rat liver membranes i s insensitive to heparinase I, chondroitinase ABC, and phospholipase C , while it is partially sensitive to phospholipase A, and sensitive to proteases and heat. Rat liver membrane proteins were solubilized with Triton X-100, reconstituted in liposomes, and analyzed for their abil ity to bind lipoproteins. I-125-labelled IDL were shown to bind to hig h- and low-affinity sites that are similar, in affinity and specificit y, to the ones observed with intact rat liver membranes, indicating th at a LBS activity is detectable on these liposomes. We found that the binding capacity of low-affinity sites in liposomes containing either no protein or containing proteins solubilized from Escherichia coli me mbranes is five times weaker than low-affinity sites in liposomes cont aining liver membrane proteins. Thus, a protein solubilized from rat l iver membranes has LBS activity when reconstituted in liposomes. Taken altogether our results provide new information on the binding of IDL to the LBS and indicate that the LBS activity is in part mediated by a protein. Thus, the LBS appears to be a bona fide receptor.