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/45Ca overlay, Western blotting and two-dimensional polyacrylamide gel electrophoresis, and several low molecular weight calcium-binding proteins were identified. Our results that Aplysia muscle contains an abundant protein with a M(r) of approximately 20,000 with strong 45Ca(2+)-binding ability and cross-reactivity to antibodies 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 protein is exclusively present in muscle fibres closely associated with the contractile machinery, whereas the isoform I (SCP I-like protein) is exclusively present in a subset of neurons, suggesting a function in their calcium regulation. In addition, a novel 45Ca(2+)-binding protein of M(r) 43,000, pl 4.7, was found in muscle and in neurons. A third protein of M(r) 40,000, pl 4.8, cross-reacts with anti-parvalbumin and anti-calbindin D-28K antibodies.