title: Antibodies to junctional sarcoplasmic reticulum proteins: probes for the Ca2+-release channel.
abstract: The junctional face membrane plays a key role in excitation-contraction coupling in skeletal muscle. A protein of 350 kDa, tentatively identified as a component of the junctional feet, connects transverse tubules to terminal cisternae of sarcoplasmic reticulum [Kawamoto, Brunschwig, Kim & Caswell (1986) J. Cell Biol. 103, 1405-1414]. The membrane topology and protein composition of sarcoplasmic reticulum Ca2+-release channels of rabbit skeletal muscle were investigated using an immunological approach, with anti-(junctional face membrane) and anti-(350 kDa protein) polyclonal antibodies. Upon preincubation of the terminal cisternae with anti-(junctional face membrane) antibodies, Ca2+-ATPase and Ca2+-loading activities were not affected, whereas anti-(350 kDa protein) antibodies stimulated Ca2+-ATPase activity by 25% and inhibited Ca2+-loading activity by 50% (at an antibody/terminal cisternae protein ratio of 1:1). Specific photolabelling of terminal cisternae proteins with [14C]doxorubicin was prevented by both anti-(junctional face membrane) and anti-(350 kDa protein) antibodies. Stimulation of Ca2+ release by doxorubicin was prevented by both anti-(junctional face membrane) and anti-(350 kDa protein) antibodies. Half-maximal inhibition was obtained at an antibody/terminal cisternae protein ratio of 1:1. Kinetic measurements of Ca2+ release indicated that anti-(350 kDa protein) antibodies prevented Ca2+-induced Ca2+ release, whereas the ATP-stimulation and the inhibition by Mg2+ were not affected. These results suggest that: (i) Ca2+- and doxorubicin-induced Ca2+ release is mediated by Ca2+ channels which are selectively localized in the junctional face membrane; (ii) the 350 kDa protein is a component of the Ca2+-release channel in native terminal cisternae vesicles; and (iii) the Ca2+-activating site of the channel is separate from other allosteric sites.
authors.affiliation: Centro di Studio per la Biologia e la Fisiopatologia Muscolare del Consiglio Nazionale delle Ricerche, Universita di Padova, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Zorzato
authors.forename: A
authors.initials: A
authors.lastname: Chu
authors.forename: P
authors.initials: P
authors.lastname: Volpe
mesh_terms: D000818:Animals; D000906:Antibodies; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004317:Doxorubicin; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000276:immunology* / Q000378:metabolism
chemical_list: D000906:Antibodies; D015220:Calcium Channels; D004317:Doxorubicin; D000252:Calcium-Transporting ATPases; D002118:Calcium