Some biochemical properties of the upregulation of the squid nerve Na+/Ca2+ exchanger by MgATP and phosphoarginine.

In squid nerve MgATP upregulation of Na+/Ca2+ exchange requires a soluble cytosolic regulatory protein (SCRP) of about 13 kDa; phosphoarginine (PA) stimulation does not. MgATP-gamma-S mimics MgATP. When a 30-10-kDa cytosolic fraction is exposed to 0.5 mM [32P]ATP in the same solution used for transport assays, and in the presence of native membrane vesicles, a 13-kDa and a 25-kDa band become phosphorylated. Membrane vesicles alone do not show these phosphorylated bands and heat denaturation of the cytosolic fraction prevents phosphorylation. Moreover, staurosporine, a general inhibitor of kinases, does not affect MgATP + SCRP stimulation of the exchanger or the phosphorylation of the 13 kDa but prevents phosphorylation of the 25-kDa cytosolic band. The 30-10-kDa fraction phosphorylated in the presence of staurosporine stimulates Na+/Ca2+ exchange in vesicles in the absence of ATP but with Mg2+ in the medium. The 30-10-kDa fraction is not phosphorylated by PA. In membrane vesicles two protein bands, at about 60 kDa and 70 kDa identified as the low molecular weight neurofilament (NF), are phosphorylated by PA, but not by MgATP. This phosphorylation is specific for PA, insensitive to staurosporine (similar to the PA-stimulated fluxes), and labile. In addition, co-immunoprecipitation was observed between the NF and the exchanger protein. Under the conditions of these experiments no phosphorylation of the exchanger is detected, either with MgATP or PA.