Polypeptide composition of acetylcholine receptor purified from teleost and elasmobranch electroplax membranes

The presynaptic calcium channel is part of a transmembrane complex linking a synaptic laminin (alpha4beta2gamma1) with non-erythroid spectrin

Nerve regeneration studies at the neuromuscular junction (NMJ) suggest that synaptic basal lamina components tell the returning axon where to locate neurotransmitter release machinery, including synaptic vesicle clusters and active zones. Good candidates for these components are the synaptic laminins (LNs) containing alpha4, alpha5, or beta2 chains. Results from a beta2 laminin knockout mouse have suggested a linkage of this extracellular laminin to cytosolic synaptic vesicle clusters. Here we report such a transmembrane link at the electric organ synapse, which is homologous to the NMJ. We immunopurified electric organ synaptosomes and found on their surface two laminins of 740 and 900 kDa. The 740 kDa laminin has a composition of alpha4beta2gamma1 (laminin-9). Immunostaining reveals that as in the NMJ, alpha4 and beta2 chains are concentrated at the electric organ synapse. Using detergent-solubilized synaptosomes, we immunoprecipitated a complex containing alpha4beta2gamma1 laminin, the voltage-gated calcium channel, and the cytoskeletal protein spectrin. Other presynaptic proteins such as 900 kDa laminin are not found in this complex. We hypothesize that alpha4beta2gamma1 laminin in the synaptic basal lamina attaches to calcium channel, which in turn is attached to cytosolic spectrin. Spectrin could then organize synaptic vesicle clusters by binding vesicle-associated proteins.

Functional stability of Torpedo acetylcholine receptor. Effects of protease treatment

The effect of tryptic degradation on structural and functional properties of the membrane-bound acetylcholine receptor from Torpedo californica has been investigated. Under conditions of proteolysis which resulted in extensive degradation of receptor subunits, the membrane preparations retained their full capability of mediating agonist-induced cation flux as measured in rapid kinetic experiments. Low concentrations on trypsin also cleaved receptor dimers to monomers, and this effect was paralleled by degradation of the Mr 65 000 subunits which are known to contain sulfhydryl group(s) involved in receptor dimerization through an interchain disulfide bond(s). This conversion to monomers occurred at lower trypsin concentrations when the enzyme was added to the outside of the vesicles compared with the effects observed when the enzyme was present inside the vesicles. Similarly Mr 43 000 protein consistently found in preparations of the membrane-bound acetylcholine receptor, which can readily be removed without apparent effect on receptor function, displayed greater susceptibility to proteolysis when trypsin was added to the exterior medium rather than inside the vesicles. The results emphasize the full functionality of the monomeric form of the acetylcholine receptor comprised of four polypeptides.

Immunochemical properties of junctional and extrajunctional acetylcholine receptor

Immunological assays were performed to compare two distinct forms of the nicotinic acetylcholine receptor (AChR): junctional (JR) and extrajunctional receptor (EJR). Antibodies from myasthenia gravis patients' sera inhibited the binding of [125I]alpha-bungarotoxin (BGT), to EJR more effectively than binding to JR. Immunological differences between JR and EJR were confirmed by other assay methods. In all cases, EJR appeared to have antigenic determinants not found on JR. It was established that enzymatic removal of carbohydrates from EJR caused it to more closely resemble JR. Thus differences between JR and EJR may be due, in part, to carbohydrate residues found on EJR that are absent on JR. The extent of antibody binding to EJR was examined by gel filtration methods. Immunochemical studies of bands from SDS gels showed that antibodies are present in myasthenic serum which react with the 3 subunits (42, 53, 64 kdaltons) of AChR to varying degrees.

Lipids of synaptic vesicles: relevance to the mechanism of membrane fusion

Synaptic vesicles from the electric organ of the marine ray Narcine brasiliensis, purified to at least 90% homogeneity, were analyzed for the lipid and fatty acid content of their membranes. The major lipids (mol %) were phosphatidylcholine (32.3%), phosphatidylethanolamine (20.5%), phosphatidylserine (6.1%), sphingomyelin (3.0%), and cholesterol (33.3%), a composition which did not differ greatly from that of the parent electric organ. While the number of double bonds per fatty acid molecule was similar for both synaptic vesicle and whole electric organ phospholipids, the vesicles were highly enriched in docosahexenoic acid (22:6). Reaction with the amine labeling reagents isethionylacetimidate and trinitrobenzenesulfonic acid indicated that 40% of the phosphatidylserine and 60% of the phosphatidylethanolamine are present on the external (cytoplasmic) surface of the synaptic vesicle. These data on a natural fusing membrane have relevance to models of membrane fusion, which have been based largely on studies of in vitro fusion using synthetic membranes.