Residues in Torpedo californica acetylcholinesterase necessary for processing to a glycosyl phosphatidylinositol-anchored form

Acetylcholinesterase from Torpedo californica (TcAChE) can be found as a glycosyl phosphatidylinositol (GPI)-anchored, membrane associated form. The C-terminal amino-acid sequence of the precursor protein resembles the signal peptide sequence found in proteins and enzymes destined for GPI-modification. Characteristics of such a signal peptide are a relatively polar stretch of amino acids, separating a cleavage- and modification-site (omega-site) residue from a hydrophobic C-terminus. We have introduced mutations, both at putative omega-sites and in the hydrophobic region, and analysed their effects on GPI-anchoring of TcAChE. Our results show that substitution of all three Ser residues in the region Ser(542)-Ser(544) prevents GPI-modification and membrane anchoring. Individual substitution of each of these residues resulted in no or only a minor effect on the modification. We therefore conclude that more than one residue within this sequence can be utilised as the omega-site. Our analyses of double substitutions indicate that Ser-543 and Ser-544 are the preferred residues for GPI-modification. Moreover, the hydrophobic region is shown to be essential for GPI-anchoring of TcAChE.