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 Ca-45 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.