In 1901 a physician, Archibald Garrod, observed a patient with black urine. He used this simple observation to demonstrate that a single mutant gene can produce a discrete block in a biochemical pathway, which he called an “inborn error of metabolism”. Garrod’s brilliant insight anticipated by 40 years the one gene-one enzyme concept of Beadle and Tatum. In similar fashion the chemist Linus Pauling and the biochemist Vernon Ingram, through study of patients with sickle cell anemia, showed that mutant genes alter the amino acid sequences of proteins. Clearly, many fundamental advances in biology were spawned by perceptive studies of human genetic diseases (1). We began our work in 1972 in an attempt to understand a human genetic disease, familial hypercholesterolemia or FH. In these patients the concentration of cholesterol in blood is elevated many fold above normal and heart attacks occur early in life. We postulated that this dominantly inherited disease results from a failure of end-product repression of cholesterol synthesis. The possibility fascinated us because genetic defects in feedback regulation had not been observed previously in humans or animals, and we hoped that study of this disease might throw light on fundamental regulatory mechanisms. Our approach was to apply the techniques of cell culture to unravel the postulated regulatory defect in FH. These studies led to the discovery of a cell surface receptor for a plasma cholesterol transport protein called low density lipoprotein (LDL) and to the elucidation of the mechanism by which this receptor mediates feedback control of cholesterol synthesis (2,3). FH was shown to be caused by inherited defects in the gene encoding the LDL receptor, which disrupt the normal control of cholesterol metabolism. Study of the LDL receptor in turn led to the understanding of receptor-mediated endocytosis, a genera! process by which cells communicate with each other through internalization of regulatory and nutritional molecules (4). Receptor-mediated endocytosis differs from previously described biochemical pathways because it depends upon the continuous and highly controlled movement of membraneembedded proteins from one cell organelle to another in a process termed

https://science.sciencemag.org/content/sci/232/4746/34.full.pdf

A receptor-mediated pathway for cholesterol homeostasis.

Rapid progress has been made in the understanding of the molecular interactions that result in cell adhesion. Many adhesive proteins present in extracellular matrices and in the blood contain the tripeptide arginine-glycine-aspartic acid (RGD) as their cell recognition site. These proteins include fibronectin, vitronectin, osteopontin, collagens, thrombospondin, fibrinogen, and von Willebrand factor. The RGD sequences of each of the adhesive proteins are recognized by at least one member of a family of structurally related receptors, integrins, which are heterodimeric proteins with two membrane-spanning subunits. Some of these receptors bind to the RGD sequence of a single adhesion protein only, whereas others recognize groups of them. The conformation of the RGD sequence in the individual proteins may be critical to this recognition specificity. On the cytoplasmic side of the plasma membrane, the receptors connect the extracellular matrix to the cytoskeleton. More than ten proved or suspected RGD-containing adhesion-promoting proteins have already been identified, and the integrin family includes at least as many receptors recognizing these proteins. Together, the adhesion proteins and their receptors constitute a versatile recognition system providing cells with anchorage, traction for migration, and signals for polarity, position, differentiation, and possibly growth.

https://science.sciencemag.org/content/sci/238/4826/491.full.pdf

New perspectives in cell adhesion : RGD and integrins

Integrins: a family of cell surface receptors.

Apolipoprotein E is a plasma protein that serves as a ligand for low density lipoprotein receptors and, through its interaction with these receptors, participates in the transport of cholesterol and other lipids among various cells of the body A mutant form of apolipoprotein E that is defective in binding to low density lipoprotein receptors is associated with familial type III hyperlipoproteinemia, a genetic disorder characterized by elevated plasma cholesterol levels and accelerated coronary artery disease Apolipoprotein E is synthesized in various organs, including liver, brain, spleen, and kidney, and is present in high concentrations in interstitial fluid, where it appears to participate in cholesterol redistribution from cells with excess cholesterol to those requiring cholesterol Apolipo-protein E also appears to be involved in the repair response to tissue injury; for example, markedly increased amounts of apolipoprotein E are found at sites of peripheral nerve injury and regeneration Other functions of apolipoprotein E, unrelated to lipid transport, are becoming known, including immunoregulation and modulation of cell growth and differentiation

https://science.sciencemag.org/content/sci/240/4852/622.full.pdf

Apolipoprotein E: cholesterol transport protein with expanding role in cell biology.

Restriction isotyping of human apolipoprotein E by gene amplification and cleavage with HhaI.

First recognized as a major determinant in lipoprotein metabolism and cardiovascular disease, apolipoprotein (apo) E has emerged as an important molecule in several biological processes not directly related to its lipid transport function, including Alzheimer's disease and cognitive function, immunoregulation, and possibly even infectious diseases. ApoE is a polymorphic protein arising from three alleles at a single gene locus. The three major isoforms, apoE4, apoE3, and apoE2, differ from one another only by single amino acid substitutions, yet these changes have profound functional consequences at both the cellular and molecular levels. ApoE3 seems to be the normal isoform in all known functions, while apoE4 and apoE2 can each be dysfunctional. Isoform (allele)-specific effects include the association of apoE2 with the genetic disorder type III hyperlipoproteinemia and with both increased and decreased risk for atherosclerosis and the association of apoE4 with increased risk for both atherosclerosis and Alzheimer's disease, impaired cognitive function, and reduced neurite outgrowth; isoform-specific differences in cellular signaling events may also exist. Functional differences in the apoE isoforms that affect (or did affect) survival before the reproductive years probably account, at least in part, for the allele frequencies of the present day.

Apolipoprotein E: far more than a lipid transport protein.

Plasma lipoproteins: apolipoprotein structure and function.

Human E apoprotein heterogeneity. Cysteine-arginine interchanges in the amino acid sequence of the apo-E isoforms.

Human apolipoprotein E. The complete amino acid sequence.

Epidemiological, pathological and genetic studies show a strong positive correlation between elevated plasma concentrations of low-density lipoprotein (LDL) cholesterol and the risk of premature coronary heart disease. Apolipoprotein (apo) B-100 is the sole protein component of LDL and is the ligand responsible for the receptor-mediated uptake and clearance of LDL from the circulation. Apo B-100 is made by the liver and is essential for the assembly of triglyceride-rich very low-density lipoproteins (VLDL) in the cisternae of the endoplasmic reticulum and for their secretion into the plasma. VLDL transports triglyceride to peripheral muscle and adipose tissue, where the triglyceride is hydrolysed by lipoprotein lipase. The resultant particle, relatively enriched in cholesteryl ester, constitutes LDL. LDL delivers cholesterol to peripheral tissues where it is used for membrane and steroid hormone biosynthesis and to the liver, the only organ which can catabolize and excrete cholesterol. Plasma LDL levels are therefore determined by the balance between their rate of production from VLDL and clearance by the hepatic LDL (apo B/E) receptor pathway. Here we report the complete 4,563-amino-acid sequence of apo B-100 precursor (relative molecular mass (Mr) 514,000 (514K] determined from complementary DNA clones. Numerous lipid-binding structures are distributed throughout the extraordinary length of apo B-100 and must underlie its special functions as a nucleus for lipoprotein assembly and maintenance of plasma lipoprotein integrity. A domain enriched in basic amino-acid residues has been identified as important for the cellular uptake of cholesterol by the LDL receptor pathway.

Stanley C. Rall

Papers

Apolipoprotein E: far more than a lipid transport protein.

Plasma lipoproteins: apolipoprotein structure and function.

Human E apoprotein heterogeneity. Cysteine-arginine interchanges in the amino acid sequence of the apo-E isoforms.

Human apolipoprotein E. The complete amino acid sequence.

Complete protein sequence and identification of structural domains of human apolipoprotein B