Acetylcholinesterase of mammalian neuromuscular junctions: presence of tailed asymmetric acetylcholinesterase in synaptic basal lamina and sarcolemma.

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Abstract
A sarcolemma-rich fraction can be isolated after subcellular fractionation of mouse intercostal muscles by sedimentation on a discontinuous sucrose gradient. The quantitative recovery of the acetylcholine receptor in this fraction is .apprx. 50%, which indicates the presence of a high proportion of postsynaptic membranes. Acetylcholinesterase (AcChoEase; EC 3.1.1.7) is found mainly in 3 different layers: the top layer, which contains soluble AcChoEase, the intermediate layer (fraction A), and the last, AcChoR-rich, layer (fraction C). The relative proportions of the molecular forms of AcChoEase are different in the 3 layers. The 16S AcChoEase is in a higher proportion in both types of membrane fractions (A and C) compared to soluble AcChoEase. Both total AcChoEase and 16S AcChoEase are enriched in the A and C fractions. In the C fraction, the sequential use of homogenizations in the presence of detergent and high ionic strength allows the solubilization of distinct AcChoEase pools. One is detergent-soluble and mainly composed of slow-sedimenting forms; the other one is detergent-insoluble, high-ionic strength-soluble, and composed mainly of collagen-like, tailed, asymmetric (16S) AcChoEase. Most of the asymmetric AcChoEase is specifically localized in the synaptic extracellular matrix of the mammalian muscle fiber. In the A fraction, most of the 16S AcChoEase found is solubilized by detergent alone, suggesting an association with microsomal membranes. At least some of the basal lamina-embedded 16S AsChoEase is preassembled intracellularly in the sarcoplasmic reticulum.