Acetylcholinesterase: Key Enzyme in Neurotransmission Moonlights with Unexpected Additional Roles?
Joel L. Sussman

At cholinergic synapses, the entire course of signal transmission, viz release of acetylcholine (ACh), its diffusion across the synaptic cleft, reversible interaction with the nicotinic ACh receptor and, finally, hydrolysis by acetylcholinesterase (AChE), occurs within a few milliseconds. In accordance with its physiological role, AChE has an unusually high turnover number, especially for a serine hydrolase, operating at a rate close to diffusion control. As a consequence of its key physiological role, AChE is the target of a repertoire of natural toxins and man-made poisons; these include alkaloids, such as galanthamine and (-)-huperzine A[1], the 3-fingered polypeptide toxin, fasciculin, and synthetic organophosphate nerve agents and insecticides[2]. AChE is also the target of drugs designed to alleviate the cholinergic deficiency associated with Alzheimer’s disease[3]. Solution of the 3D structure of TcAChE[4] enhanced understanding of the structural elements underlying its specificity and catalytic power. Subsequent elucidation of the human and Drosophila AChE structures permitted a structure-based approach to design of antiChE drugs and insecticides.
Studies during the last decade have identified a family of neural cell adhesion proteins, which are single-pass transmembrane proteins, with substantial sequence similarity to cholinesterases (ChEs). The regions of sequence similarity correspond to only part of their complete sequences, thus establishing the ChE domain as a modular domain incorporated into different proteins, i.e. cholinesterase-like adhesion molecules (CLAMs)[5,6]. CLAMs, however, are devoid of catalytic activity, since they lack residues crucial for catalysis. They appear to play a key role in the earliest stages of the development of the CNS and mutations, in the ChE domain of one of them, i.e. neuroligin, has been associated with autism[7].
The cytoplasmic domains of CLAMs bear no sequence homology to any known protein, and physicochemical studies show that they are ‘intrinsically unfolded’ when expressed in E. coli[6]. We developed a web-based tool, FoldIndex© (see http://bioportal.weizmann.ac.il/fldbin/findex)[8], which has proven to be very useful in predicting regions of a new protein sequence that are likely to be disordered and have applied it to examine the CLAMs family as well as cholinesterase molecules. FoldIndex© is also being used in a routine way in the ISPC[9] (see http://www.weizmann.ac.il/ISPC) to aid in crystallization of proteins by first predicting which regions of a protein sequence are likely to be intrinsically disordered and then not including these regions in the construct that is cloned.
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This work was supported by The Israel Ministry of Science and Technology (MOST) for the ISPC, by The EC Vth Framework ‘SPINE’ Project (Structural Proteomics in Europe) Grant number: QLG2-CT-2002-00988, the Divadol Foundation and the Minerva Foundation.
References:
1. Xu, Shen, Luo, Silman, Sussman, Chen & Jiang. J Am Chem Soc 125, 11340-11349 (2003).
2. Silman & Sussman in Cholinesterases & Cholinesterase Inhibitors (ed. Giacobini) 9-25 (M. Dunitz, London, 2000).
3. Greenblatt, Dvir, Silman & Sussman. J Mol Neurosci 20, 369-384 (2003).
4. Sussman, Harel, Frolow, Oefner, Goldman, Toker & Silman. Science 253, 872-879 (1991).
5. Botti, Felder, Sussman & Silman. Protein Eng 11, 415-420 (1998).
6. Zeev-Ben-Mordehai, Rydberg, Solomon, Toker, Botti, Auld, Silman & Sussman. Proteins 53, 758-767 (2003).
7. Jamain et al. & Bourgeron. Nature Genet 34, 27-29 (2003).
8. Prilusky, Felder, Zeev-Ben-Mordehai, Rydberg, Man, Beckmann, Silman & Sussman. Bioinformatics 21, 3435-3438 (2005).
9. Albeck et al. & Sussman. Acta Cryst D61, 1364-1372 (2005).