Sarcoplasmic calcium-binding proteins (SCPs) are members of the EF-hand calcium-binding protein family which are characterized by the presence of helix-loop-helix motifs in their amino acid sequence. SCPs have an M(r) of approximately 20,000, a pI of approximately 5 and interact with two to three calcium ions (Ca2+) with a KD of 10(-7) to 10(-8) M. Mg2+ ions antagonize Ca2+ ion binding in a complex manner so that these proteins are exquisitely fine-tuned to interfere with the Ca2+ signal. SCPs apparently fulfil no specific activatory function. They exhibit strong polymorphism, show a marked homology to coelenterate photoproteins (aequorin, luciferin) and have been found only in invertebrates, predominantly in muscle and neurons. In mollusks, SCPs are distributed in a tissue-specific manner, with immunoreactivity to SCP I-like isoforms localized in electrically silent neurons colocalized with serotonin, and immunoreactivity to SCP II-like isoforms exclusively present in muscle.