APO-B, E Receptor Binding of LDL: Evidence for Species-specific Segments of APO-B100 in the Binding Site Domain

Abstract

The low-density lipoprotein (LDL) is a complex macromolecule, about half of whose weight is represented by cholesterol, in both free and esterified forms. This quasi-spherical, pseudomicellar particle accounts for the transport of about twothirds of the cholesterol in human plasma, assuring its delivery to diverse tissues and organs for eventual utilisation in cellular membrane synthesis. The specific interaction of LDL with cells occurs by way of a highly-specialised membrane receptor, the LDL (apo-B, E) receptor [1, 2]. Indeed, this receptor mediates a major portion of the catabolism of LDL, an essential process in the regulation of circulating LDL levels and of cholesterol homeostasis in the intact animal in vivo [1, 2]. Substantial progress has recently been made in our knowledge of the structure and biosynthesis of the LDL receptor protein, as well as in its gene structure [3,4]. By contrast, we remain largely ignorant of the precise nature of the interaction of LDL with the receptor, although the specificity for this interaction is embodied in the major protein component of the lipoprotein particle, apolipoprotein B100 [5, 6]. Moreover, certain of these residues are located in the trypsin-accessible regions of apo-B100, their removal (or cleavage) diminishing the ability of LDL to bind to its receptor [7–9]. To further define the receptor binding site region on LDL/B100, and to evaluate the extent to which this domain may be species-specific, we have determined i) the characteristics of the high affinity binding of homologous (porcine) and heterologous (human) LDL to apo-B, E receptors of porcine adrenocor-tical membranes, and ii) the effect of limited proteolysis (using trypsin) of the heterologous (human) LDL on its interaction with the porcine receptor.