Amino acid sequences of four isoforms of amphioxus sarcoplasmic calcium-binding proteins.

The protochordate amphioxus (Branchiostoma lanceolatum) contains different isoforms of sarcoplasmic, high-affinity Ca2(+)-binding proteins (SCP). The amino acid sequences of the two major isoforms SCP I and II, reported previously [Takagi, T., Konishi, K. & Cox, J.A. (1986) Biochemistry 25, 3585-3592], have been corrected and differ from each other by seven amino acid substitutions in a 17-residue-long segment (positions 20-36). We also report on the isolation and amino acid sequence determination of two minor isoforms, i.e. amphioxus SCP III and IV. Although they behave very differently from the major forms with respect to net charge, they differ from SCP I by only one amino acid: SCP III has Met at position 20 (Tyr in SCP I) and SCP IV has Asn at position 23 (Asp in SCP I). Together the sequence data on amphioxus SCP suggest that, in contrast with SCP of other invertebrate phyla, the isoforms are generated by alternative splicing of the primary RNA transcript with a mutually exclusive pattern.