Reconstitution of the hepatic asialoglycoprotein receptor with phospholipid vesicles

Surfactants in membrane solubilisation

An understanding of the action of many drugs requires a knowledge of how the drug reaches the site of action in a cell. A detailed knowledge of the structure and function of cell membranes is often required to understand the transport of drugs across the plasma membrane. To obtain this information proteins must be isolated. The isolation and characterisation of cell membrane proteins usually requires the solubilisation of the membrane and a method of separation of the various membrane proteins and glycoproteins. The starting point for such an investigation is the choice of a suitable surfactant (detergent) to solubilise the membrane. This review considers the range of surfactants that are available for membrane solubilisation, how surfactants interact with membranes, the part they play in the separation of integral membrane proteins and in the reconstitution of membrane proteins for functional studies. The solubilisation of specific membrane proteins and glycoproteins including the human erythrocyte anion transporter, mitochondrial porin, sarcoplasmic reticulum Ca(2+)-ATPase, the ATPase-active multidrug transporter P-glycoprotein, bacteriorhodopsin and rhodopsin are also discussed.

A reassessment of the assay for the asialoglycoprotein receptor and its use in the quantification of receptor distribution in hepatocytes

The assay for the fucose-binding protein described by Lehrman & Hill [(1983) Methods Enzymol. 98, 309-319] was adapted for the measurement of the asialoglycoprotein receptor in rat liver. The amount of ligand bound to the plasma-membrane-associated or affinity-purified receptor was acutely sensitive to the concentrations of Triton X-100 and NaCl in the assay: 0.02% (v/v) Triton X-100 increased ligand binding to the two preparations by 100% and 40% respectively. Higher concentrations of detergent progressively decreased binding, and in 0.32% Triton X-100 it was about 30% of the value obtained in detergent-free buffer. The addition of increasing concentrations of NaCl to the assay progressively inhibited ligand binding to the membrane-associated receptor, whereas there was a 60% increase in binding to the pure receptor in the presence of 0.1-0.2 M-NaCl. These effects could not be identified in the original assay procedure described by Hudgin, Pricer, Ashwell, Stockert & Morell [(1974) J. Biol. Chem. 249, 5536-5543]. Using optimal assay conditions the binding of 125I-beta-D-galactosyl-bovine serum albumin to both the membrane-associated and purified receptor was inhibited by 50% by 1 nM-beta-D-galactosyl-bovine serum albumin and -asialoorosomucoid and by approx. 100 microM-beta-L-fucosyl-bovine serum albumin, whereas beta-D-galactose, lactose and beta-L-fucose had no effect on ligand binding up to concentrations of 1 mM, 500 microM and 5 mM respectively. KD values of 0.94 and 1.25 nM and Bmax. values of 40 and 1660 pmol of D-galactosyl-bovine serum albumin bound/mg of receptor were obtained for the membrane-bound and purified receptor respectively. Hill-plot analysis of the same data gave slopes of 0.96 and 1.01. Scatchard analysis of saturation-binding studies with other subcellular fractions indicated that the receptor was distributed in the proportions 72:23:2.5:2.5 between total microsomal fractions, plasma membrane, Golgi and canalicular membrane respectively. The receptor was about 1% of the total protein in each compartment and was estimated to be about 0.3% of the total liver protein.

Role of membrane lipids in peptide hormone function: binding of enkephalins to micelles

In the course of their biological function, peptide hormones must be transferred from an aqueous phase to the lipid-rich environment of their membrane-bound receptor proteins. We have investigated the possible influence of phospholipids in this process, using 360-MHz 1H and 90-MHz 13C NMR spectroscopy to examine the association of the opioid peptides [Met]- and [Leu]enkephalins (Tyr-Gly-Gly-Phe-Met/Leu) with phospholipid micelles. Binding of peptides to lipid was monitored in NMR spectra by selective chemical shift movements (e.g., the Phe aromatic ring protons) and residue-specific line broadening (e.g., of Met/Leu carbonyl- and alpha-carbon resonances). Results established that the zwitterionic hormones associate hydrophobically both with a neutral lipid (lysophosphatidylcholine) and (also electrostatically) with a negative lipid (lysophosphatidylglycerol). An association constant of Ka = 3.7 X 10(1) M-1 was calculated for the hydrophobic binding of enkephalin to lysophosphatidylcholine. NMR data suggested that enkephalin binds to the lipid with Met/Leu, Phe, and likely Tyr side-chain substituents associated with nonpolar interior regions of the micelle, whereas the COOH-terminal carboxylate moiety of the peptide is located in the surface of the lipid particle. An "attraction-interaction" model is proposed for hormone-lipid association wherein negative lipids attract the hormone electrostatically, while site-specific hydrophobic contacts facilitate its entry, concentration, and orientation into the lipid phase.

The hepatic asialoglycoprotein receptor

Asialo- (i.e., galactose-terminal) glycoproteins are specifically and avidly recognized by a mammalian hepatic parenchymal cell receptor. This receptor, itself a glycoprotein, binds ligand molecules and directs their delivery to lysosomes for catabolism. The receptor is reutilized during this process of receptor-mediated endocytosis. Ligand specificity is conferred by galactose or N-acetyl-galactosamine at the nonreducing termini of the oligosaccharide chains. The receptor appears to be a transmembrane protein and is localized both to the cell surface as well as to several membranous intracellular compartments.

Voltage-dependent orientation of membrane proteins

In order to study the influence of electrostatic forces on the disposition of proteins in membranes, we have examined the interaction of a receptor protein and of a membrane-active peptide with black lipid membranes. In the first study we show that the hepatic asialoglycoprotein receptor can insert spontaneously into lipid bilayers from the aqueous medium. Under the influence of a trans-positive membrane potential, the receptor, a negatively charged protein, appears to change its disposition with respect to the membrane. In the second study we consider melittin, an amphipathic peptide containing a generally hydrophobic stretch of 19 amino acids followed by a cluster of four positively charged residues at the carboxy terminus. The hydrophobic region contains two positively charged residues. In response to trans-negative electrical potential, melittin appears to assume a transbilayer position. These findings indicate that electrostatic forces can influence the disposition, and perhaps the orientation, of membrane proteins. Given the inside-negative potential of most or all cells, we would expect transmembrane proteins to have clusters of positively charged residues adjacent to the cytoplasmic ends of their hydrophobic transmembrane segments, and clusters of negatively charged residues just to the extracytoplasmic side. This expectation has been borne out by examination of the few transmembrane proteins for which there is sufficient information on both sequence and orientation. Surface and dipole potentials may similarly affect the orientation of membrane proteins.

Thermodynamics and membrane processes

This review represents a personal view of membrane thermodynamics. I do not intend to deal at all with the irreversible thermodynamics of membrane mass transfer processes. This aspect has been covered far more competently and completely by other people (Bittar, 1970; Paterson, 1970; Rottenberg, Caplan & Essig, 1970; Mitchell, 1970; Rothschildet al.1980; Oster, Perelson & Katchalsky, 1973; Kedem & Katchalsky, 1958; Schwartz, 1971). The recent review on osmosis by Hill (1979) is a particularly succinct appraisal of a facet of irreversible membrane thermodynamics. Arata & Nishimura (1980) have considered the coupling of electron transfer to vectorial processes in biological membranes.

Voltage-dependent translocation of the asialoglycoprotein receptor across lipid membranes

A membrane receptor protein for asialoglycoproteins induces voltage-dependent increases in ion conductance across a lipid bilayer, probably reflecting penetration of the protein into the bilayer towards an electrically positive pole. In the presence of specific ligand for the receptor, this penetration leads to a 'translocation' of the receptor from one side of the bilayer to the other. These observations suggest a mechanism by which biological membranes might regulate the disposition of their proteins, and a way in which membrane receptors involved in endocytosis might be spared lysosomal destruction in order to be recycled to the plasma membrane.