Acetylcholinesterases of the nematode Steinernema carpocapsae. Characterization of two types of amphiphilic forms differing in their mode of membrane association.

We analyzed the molecular forms of acetylcholinesterase (AChE) in the nematode Steinernema carpocapsae. Two major AChEs are involved in acetylcholine hydrolysis. The first class of AChE is highly sensitive to eserine (IC50 = 0.05 microM). The corresponding molecular forms are: an amphiphilic 14S form converted into a hydrophilic 14.5S form by mild proteolysis and two hydrophilic 12S and 7S forms. Reduction of the amphiphilic 14S form with 10 mM dithiothreitol produces hydrophilic 7S and 4S forms, indicating that it is an oligomer of hydrophilic catalytic subunits linked by disulfide bond(s) to a hydrophobic structural element that confers the amphiphilicity to the complex. Sedimentation coefficients suggest that 4S, 7S, 12S forms correspond to hydrophilic monomer, dimer, tetramer and that the 14S form is also a tetramer linked to one structural element. The second class of AChE is less sensitive to eserine (IC50 = 0.1 mM). Corresponding molecular forms are hydrophilic and amphiphilic 4S forms (monomers) and a major amphiphilic 7S form converted into a hydrophilic dimer by Bacillus thuringiensis phosphatidylinositol-specific phospholipase C. This amphiphilic 7S form thus possesses a glycolipid anchor. It appears that Steinernema (a very primitive invertebrate) presents AChEs with two types of membrane association that closely resemble those described for amphiphilic G2 and G4 forms of AChE in more evolved animals.