AbstractMuscle (body wall, buccal mass, heart) and neural tissue of the marine molluscAplysia californicawas analysed for calcium‐binding proteins using transblot45Ca overlay, Western blotting and two‐dimensional polyacrylamide gel electrophoresis, and several low molecular weight calcium‐binding proteins were identified. Our results show thatAplysiamuscle contains an abundant protein with aMrof ∼20 000 with strong45Ca2+‐binding ability and cross‐reactivity to antibodies against the sarcoplasmic calcium‐binding protein isoform II (SCP II) fromAmphioxus.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 novel45Ca2+‐binding protein ofMr43 000, pl 4.7, was found in muscle and in neurons. A third protein ofMr40 000, pl 4.8, cross‐reacts with anti‐parvalbumin and anti
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