ISOLATION OF AN ARABIDOPSIS CDNA SEQUENCE ENCODING A 22-KDA CALCIUM-BINDING PROTEIN (CABP-22) RELATED TO CALMODULIN

Complementary DNA sequences were isolated from a library of cloned Arabidopsis leaf mRNA sequences in lambdagt10 that encoded a 21.7 kDa polypeptide (CaBP-22), which shared 66% amino acid sequence identity with Arabidopsis calmodulin. The putative Ca2+-binding domains of CaBP-22 and calmodulin, however, were more conserved and shared 79% sequence identity. Ca2+ binding by CaBP-22, which was inferred from its amino acid sequence similarity with calmodulin, was demonstrated indirectly by Ca2+-induced mobility shifting of in vitro translated CaBP-22 during SDS-polyacrylamide gel electrophoresis. CaBP-22 is encoded by a ca. 0.9 kb mRNA that was detected by northern blotting of leaf poly(A)+ RNA; this mRNA was slightly larger than the 809 bp CaBP-22 cDNA insert, indicating that the deduced amino acid sequence of CaBP-22 is near full-length. CaBP-22 mRNA was detected in RNA fractions isolated from leaves of both soil-grown and hydroponically grown Arabidopsis, but below the limits of detection in RNA isolated from roots, and developing siliques. Thus, CaBP-22 represents a new member of the EF-hand family of Ca2+,-binding proteins with no known animal homologue and may participate in transducing Ca2+ signals to a specific subset of response elements.