The 28-kDa vitamin D-dependent calcium-binding protein has a six-domain structure.

Vitamin D-dependent 28-kDa calcium-binding protein (CaBP28) cDNA clones were isolated from a chicken intestinal library. The nucleotide sequence analysis of the CaBP28 cDNA shows an open reading frame of 786 nucleotides, coding for a 262-amino acid 30.167-kDa protein. Interestingly, the protein contains six repeats of a domain with the feature of a calcium-binding site. In two of the six domains, oxygen-containing amino acids important for the positioning of calcium are absent, suggesting that these two sites have lost their calcium-binding capability and might have adopted a new function in evolution. In the chicken intestine, three different sized species of CaBP28 mRNA (2.0, 2.8, and 3.1 kilobases) are detected. Primer extension and S1 nuclease mapping show that the three CaBP28 mRNA species share a common 5' end but differ in the length of their 3' noncoding sequence. A similar triplet of CaBP28 mRNAs is identified in the rat kidney by the chicken probe, showing an interspecies conservation of the CaBP28. In the rat intestine, however, no CaBP28 mRNA could be detected. Instead, a vitamin D-dependent 9-kDa CaBP (CaBP9) is expressed, with an mRNA size of approximately equal to 0.7 kilobase that does not cross-hybridize with the CaBP28 probe. This indicates that the CaBP28 and CaBP9 are the product of two independent genes.