Residues in the C-Terminus of Torpedo californica Acetylcholinesterase Important for Modification into a Glycophospholipid Anchored Form

Abstract

Acetylcholinesterase (AChE) is an enzyme that exists in several structurally distinct forms. Two major forms of AChE molecules are found in the electric organ of Torpedo californica. A hydrophilic form that is attached by a collagen-like tail to the basal lamina in the synaptic cleft, and a hydrophobic dimeric form (G2-AChE) that is attached to the cell membrane via a glycosyl-phosphatidyl inositol (GPI) anchor. These two different forms arise due to alternative splicing of two exons, exon 5 and exon 6. Exon 5 encodes the last 31 carboxy-terminal amino acids found in the hydrophobic form of AChE. This C-terminal peptide (GPIsp) contains the signal for GPI modification. The GPI-moiety is attached to a specific amino acid residue, encoded by exon 5, at the cleavage/modification site or co-site. This residue is found about 20 amino acids upstream of the C-terminus. Studies of other GPI-modified proteins have not revealed a definitive consensus amino acid sequence in the C-terminal region, but some characteristics are found. The first 2 amino acids downstream of the co-site, positions ω+1 and ω+2, are believed to interact with the active site of a putative transamidase, catalyzing the GPI-modification reaction. The ω+2 position in the GPIsp is the most conserved residue. In natural proteins only a few amino acids (Gly, Ala, Ser and Thr) are found in this position (Kodukula et al., 1993). These residues are followed by a stretch of 5 to 10 small and relatively polar amino acids, the “spacer region”. The “spacer region” is followed by a stretch of 10–15 hydrophobic amino acids.