Electron microscopic studies on stretched and globular acetylcholinesterase molecules of the electric eel (Electrophorus electricus)

Abstract

Electron micrographs of the various molecular forms of acetylcholinesterase (as previously characterized) are presented. The “globular” forms G and G' appear as tetramers, and dimers, respectively, whereas the “native” forms, A, C, and D show a structure “en grappe” where a cluster of globules (respectively 4, 6 to 8, and at least 10) is associated with a semi‐rigid “tail”. The globules seem identical in all forms and probably represent the sub‐units of the enzyme (Mw 60000—80000). The structures are 50 nm long in their greatest dimension; the tail is 2—3 nm thick and its visible part is longest in A and shortest in D; presumably part of it is masked by the head. These results explain our previous physico‐chemical observations which had led us to propose that the native forms were asymmetric in structure. We discuss a possible model of the sub‐unit structure for the “native” forms: A, C and D are proposed to consist of identical tails associated with respectively one, two, and three tetramers. This hypothesis provides a rationale for the occurrence of three well‐defined “native” acetylcholinesterases, as well as for the derived “globular” forms, the dimer and the tetramer. We further propose that the interactions inside the structures are not equivalent for the two kinds of subunits: for example, one of them might be linked directly to the tail, the other only through the tetramer interactions.