Structure of Acetylcholinesterase: Its Relationship to the Postsynaptic Membrane
Acetylcholinesterase (A Ch E), when extracted with high ionic strength solutions from Torpedo and Electrophorus electric organs and from mammalian skeletal muscle, exists in several molecular forms (6,12,19,25,27). Massoulie and Rieger initially demonstrated in Electrophorus three species with sedimentation constants of 8S, 14S and 18S (19). In more recent studies Massoulie and his colleagues have observed that the predominant 18S species consists of three sets of tetrameric head units, each of which is attached to a filament that joins to a tail of 50 × 2 nm in dimensions (5). Each subunit of the head is 80,000 in molecular weight so that the assembled 18S species has a molecular weight greater than 1 million. Although thought not to be of high molecular weight, the tail unit markedly affects the hydrodynamic properties of the enzyme forms. Treatment of the various AChE species with trypsin or other proteolytic enzymes gives rise to a common species with a sedimentation constant of 11S and a molecular weight near 330,000 (8, 20). Electron microscopy reveals that the US species is devoid of a tail unit and in contrast to the other forms, it will not aggregate in solutions of low ionic strength (6, 25). The US form is probably analogous to the enzyme purified in early years when A Ch E was dissociated from the membrane by extensive autolysis (8).
KeywordsSodium Dodecyl Sulfate Postsynaptic Membrane Electric Organ Sedimentation Constant High Molecular Weight Species
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