L'acétylcholinestérase des organes électriques de Poissons (torpille et gymnote); complexes membranaires

Molecular weights for the series of six Electrophorus acetylcholinesterase forms have been determined either by the sedimentation‐diffusion equilibrium method or, particularly in the case of the very scarce G′ and G″ forms, from their Stokes radius and sedimentation coefficient values. Both methods are in excellent agreement. The results provide good evidence for the model previously proposed, G″, G′ and G containing one, two and four subunits, whereas A, C and D possess, in addition to respectively one, two and three tetrameric sets of such subunits, a structural element, the tail. Although the amino acid composition of ‘tailed’ and globular forms did not reveal any significant feature of this element, its mass, about 100000 daltons, could be deduced from a comparison of molecular weights for the two classes of acetylcholinesterase forms. This value is in close agreement with electron microscopic data. The tail is thought to consist of three 30000‐dalton strands.

supporting

confidence: 56%

mentioning

confidence: 99%

Molecular Forms of Electrophorus Acetylcholinesterase

Bon¹,

Huet

Lemonnier³

et al. 1976

“…The early studies of Massoulie and Rieger [42] provided evidence that Torpedo electric organ also contains asymmetric forms of AChE which are similar to those observed in Electrophorus. Subsequent physicochemical studies revealed the presence of A12, A8 and A4 forms which were homologous to the corresponding forms in Electrophorus and could be similarly extracted at high ionic strength in the absence of detergent [32,51,52].…”

Section: Molecular Structurementioning

confidence: 91%

Modes of attachment of acetylcholinesterase to the surface membrane

Silman

Futerman

1987

173

Acetylcholinesterase (AChE) occurs in multiple molecular forms differing in their quaternary structure and mode of anchoring to the surface membrane. Attachment is achieved by post‐translational modification of the catalytic subunits. Two such mechanisms are described. One involves attachment to catalytic subunit tetramers, via disulfide bridges, of a collagen‐like fibrous tail. This, in turn, interacts, primarily via ionic forces, with a heparin‐like proteoglycan in the extracellular matrix. A second such modification involve the covalent attachment of a single phosphatidylinositol molecule at the carboxyl‐terminus of each catalytic subunit polypeptide; the diacylglycerol moiety of the phospholipid serves to anchor the modified enzyme hydrophobically to the lipid bilayer of the plasma membrane. The detailed molecular structure of these two classes of acetylcholinesterase are discussed, as well as their biosynthesis and mode of anchoring.

“…It was thus found that the aggregated fraction contained the asymmetric molecules A (9.2 S), C (14.2 S) and D (18.5 S), while the nonaggregated fraction contained only the globular form G (tetramer, 11.8 S) and G' (dimer 7.4 S). In an early publication [3] we had described the aggregation of the heavy D and C molecules, but since a slow-sedinienting peak remained unaggregated in low-ionicstrength conditions, we had assumed that the A form was not aggregating. However, the existence of dimers G', sedimenting in the same range, was not recognized at the time.…”

Section: Formation and Sedimentation Properties Of Acttylcholinestermentioning

confidence: 99%

The Dependence of Acetylcholinesterase Aggregation at Low Ionic Strength upon a Polyanionic Component

Bon¹,

Cartaud²,

Massoulié³

1978

111

The low ionic strength aggregation of Electrophoixs asymmetric forms of acetylcholinesterase has been found to depend upon a distinct aggregating agent, present in large excess in the electric organ extracts. The aggregates appeared, in electron micrographs, as bundles of a few molecules, the tails of which were linked in a side-to-side association, leaving the head groups at both ends. The proportion of aggregated enzyme, but not the sedimentation properties of the aggregates, depended in a complex manner upon ionic conditions and aggregating agent levels. The aggregates were not dissociated by dilution even at very low concentrations. Properties of the aggregating agent suggested that it might be a non-proteic polymer, interacting with positively charged groups of the enzyme tail.Various polyanions were found to promote aggregation of acetylcholinesterase, but induced aggregates of different sedimentation coefficients. Polycarboxylates were effective only at high concentrations while sulfate and phosphate polyanions were very efficient. Aggregates induced by these latter polyanions possessed the same morphological features as those produced by the endogenous preparation.Analysis of the Electrophorus aggregating preparation demonstrated that its chondroitin sulfate content fully accounts for its aggregating capacity, and this was in agreement with its sensitivity to specific mucopolysaccharidases.Aggregates were also obtained in Electrophorusl Torpedo mixed systems but their characteristics were found to depend both on the nature of the enzyme and of the aggregating agent. Aggregation had no effect on catalytic efficiency, and we discuss its relevance to the synaptic localization of the enzyme, through interactions between the tails of acetylcholinesterase and chondroitin-sulfatetype glycosaminoglycan components.It has been known for a long time that certain varieties of acetylcholinesterase from electric fish (Electrophorus and Torpedo) aggregate in low-salt media [l-51. Studies of the pH and ionic strength dependence of aggregate formation [ 1 -31 indicate that this phenomenon is controlled by ionic interactions. We have recently described the structure of the aggregates, as observed by high-resolution electron microscopy. These structures consist of a few, of the order of 6-10, asymmetric molecules, the tails of which are linked side by side. This bundle of rod-like tails constitutes the core of the aggregate, leaving two groups of heads at both extremities [6].In the present paper we examine the nature of the interactions involved in acetylcholinesterase aggregation and demonstrate that it is characteristic of the tailed forms and depends upon the presence of an agAhbipviation. Nbsz, 5,5'-dithio-bis(2 nitrobenzoic acid).gregating polyanionic agent distinct from the acetylcholinesterase molecule. We compare the electron microscopic appearance of aggregates inducted by various polyanions and present some evidence regarding the chemical nature of the endogenous aggregating agent. Finally we discuss the relevan...