CA-2+ BINDING BY MYXICOLA NEUROFILAMENT PROTEINS

Titrimetric, 45Ca dialysis, and autoradiographic methods were used to examine how axoplasmic proteins from the giant neuron of the marine annelid Myxicola infundibulum bind caicium. Following the autoradiographic method of Maruyama et al. [1], the 150-160 kD neurofilament subunits were identified as prominent intracellular Ca-binding peptides. Using equilibrium dialysis, extracts of axoplasmic proteins (>50% neurofilament subunits) were examined in 300 mM KCl at different concentrations of free Ca and Mg, and at different pH. Axoplasmic proteins showed a high affinity Ca binding site (K 1 2 3-6 μM, capacity 3-7 μmole g-1 protein) at pH 6.8 or pH 7.5. Changing the Mg concentration from 0 to 5 mM had no effect on the Ca binding. Elevating the dialysis pH from 7.0 to 9.0 reduced the apparent number of binding sites for Ca. Using microelectrodes to record the free Ca, microtitrations of axopiasmic proteins were completed by adding small amounts of CaCl2 to 100 μl volumes of protein solutions. In a medium containing ionic constituents closely resembling those of the Myxicola axon, a Ca binding capacity of 5.0 μmole g-1 protein and a K 1 2 of ∼1 μM were measured. © 1990.