A novel 13 kDa cytoplasmic soluble protein is required for the nucleotide (MgATP) modulation of the Na/Ca exchange in squid nerve fibers.
The Na/Ca exchange is a highly regulated transport mechanism in which MgATP, a powerful modulatory intracellular substrate, has important implications for its function. As occurs with some preparations, in squid axons, nucleotide regulation is lost after membrane vesicle isolation. This has been a significant obstacle in the biochemical characterization of the MgATP effect. An important clue in solving this long-standing puzzle is presented in this work by showing that prolonged intracellular dialysis of squid axons produces a complete run down of the MgATP effect. Here we report that a soluble cytoplasmic factor isolated from fresh squid axoplasm and brain reconstitutes the MgATP stimulation of the Na-gradient-dependent 45Ca uptake in squid optic nerve membrane vesicles. Partial purification of this factor uncovers the presence of a novel 13 kDa soluble cytoplasmic protein (SCPr) which, when microinjected in ATP de-regulated dialyzed squid axons, completely restores the MgATP stimulation of Na(o)-dependent Ca efflux. We propose that in the squid preparation this SCPr constitutes the link between the nucleotide and target effector: the Na/Ca exchanger itself, or other plasma membrane structures which may secondarily interact with the exchanger.
DiPolo
R
R
Center de Biofisica y Bioquímica, IVIC, Caracas, Venezuela.
Berberián
G
G
Delgado
D
D
Rojas
H
H
Beaugé
L
L
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism*; D049832:Decapodiformes; D009412:Nerve Fibers / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D011506:Proteins; D019831:Sodium-Calcium Exchanger; D000255:Adenosine Triphosphate; D012964:Sodium; D002118:Calcium