Introduction

Copper is an essential metal, utilized as a cofactor by numerous enzymes regulating vital cellular functions, including oxidative phosphorylation, neurotransmitter biosynthesis, radical detoxification, iron uptake, and many others (for review, see refs. 1 and 2). The importance of copper for normal cell metabolism is best illustrated by the existence of severe genetic disorders, in which the normal distribution of copper is disrupted (3-5). Menkes disease (MNK) is an inborn copper deficiency associated with severe developmental delays, mental retardation, poor temperature control, and connective tissue abnormalities. All of these symptoms can be ascribed to the malfunction of various enzymes, which require copper as a cofactor. Such enzymes include cytochrome-c oxidase, tyrosinase, lysyl oxidase, peptidyl-6-amidase, and many others. Recent identification of the Menkes disease gene (ATP7A) revealed that it encoded a copper-transporting ATPase or the Menkes disease protein (6-8), which has a dual function: to transport copper from the cytosol to copper-dependent enzymes located within the secretory pathway and to export excess copper out of the cell.

Although the Menkes disease protein is indispensable for copper distribution from the intestine to various tissues, by itself it is insufficient for normal copper homeostasis. The product of another gene, ATP7B, plays a key role in removing excess copper from human body by transporting copper from the liver to the bile (9-11). Mutations in ATP7B lead to vast accumulation of copper in the liver, brain, and kidneys, causing a set of pathological symptoms, known as Wilson's disease (WND). Severe liver lesions, neurological problems, and a wide spectrum of psychiatric abnormalities are common symptoms of WND (12). The Wilson's disease gene, ATP7B, was isolated and fully characterized in 1993-1994; these studies revealed that it encodes a copper-transporting ATPase with over 50% sequence homology to the Menkes disease protein (9-11,13,14).

Although the Menkes disease protein (MNKP) and Wilson's disease protein (WNDP) have significant structural identity and also function similarly in the in vitro systems (15-17), their distinct tissue distribution and alterations in their expression during development (18) suggest that the relative abundance and activity of these proteins are controlled by a specific set of environmental cues. Understanding the regulatory mechanisms acting on these proteins represents one of the most unexplored and exciting area of copper homeostasis.

The detailed biochemical characterization of the Menkes and Wilson's disease proteins is the first step toward elucidation of their specific physiological roles in a cell. Such studies can be aided

From: Handbook of Copper Pharmacology and Toxicology Edited by: E. J. Massaro © Humana Press Inc., Totowa, NJ

Fig. 1. Schematic representation of the transmembrane organization of WNDP. The blocks in the N-terminal portion indicate the position of copper-binding sites with a conserved sequence motif GMTCxxC. The single arrows indicate the beginning and the end of the ATP-binding domain; the double arrow indicates the end of the N-terminal domain. Open circles mark the location of some of the Wilson's disease mutations and TGE, DKTG, TGDN, and GDGxxD are the sequence motifs conserved in all P-type ATPases; W in a block shows the position of a single tryptophan residue in the ATP-binding domain. CPC is a sequence motif that is specific for copper-transporting ATPases.

N-terminal cop ¡je r-binding domain ATP-landing domain

Fig. 1. Schematic representation of the transmembrane organization of WNDP. The blocks in the N-terminal portion indicate the position of copper-binding sites with a conserved sequence motif GMTCxxC. The single arrows indicate the beginning and the end of the ATP-binding domain; the double arrow indicates the end of the N-terminal domain. Open circles mark the location of some of the Wilson's disease mutations and TGE, DKTG, TGDN, and GDGxxD are the sequence motifs conserved in all P-type ATPases; W in a block shows the position of a single tryptophan residue in the ATP-binding domain. CPC is a sequence motif that is specific for copper-transporting ATPases.

considerably by comparative analysis of WNDP or MNKP with much better characterized cation-transporting ATPases, such as Ca2+-ATPase or Na+,K+-ATPase, which belong to the same protein family as WNDP and MNKP.

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