SARCOPLASMIC CALCIUM-BINDING PROTEINS IN APLYSIA NERVE AND MUSCLE-CELLS

Muscle (body wall, buccal mass, heart) and neural tissue of the marine mollusc Aplysia californica was analysed for calcium-binding proteins using transblot/Ca-45 overlay, Western blotting and two-dimensional p olyacrylamide gel electrophoresis, and several low molecular weight ca lcium-binding proteins were identified. Our results show that Aplysia muscle contains an abundant protein with a M(r) of approximately 20 00 0 with strong Ca-45(2+)-binding ability and cross-reactivity to antibo dies against the sarcoplasmic calcium-binding protein isoform II (SCP II) from Amphioxus. Immunocytochemical studies revealed that isoforms of SCP are distributed in a tissue-specific manner, SCP II-like protei n is exclusively present in muscle fibres closely associated with the contractile machinery, whereas the isoform I (SCP I-like protein) is e xclusively present in a subset of neurons, suggesting a function in th eir calcium regulation. In addition, a novel Ca-45(2+)-binding protein of M(r) 43 000, pI 4.7, was found in muscle and in neurons. A third p rotein of M(r) 40 000, pI 4.8, cross-reacts with anti-parvalbumin and anti-calbindin D-28K antibodies.