Novel avian thymic parvalbumin displays high degree of sequence homology to oncomodulin

CPV3 is a novel avian parvalbumin. It displays an isoelectric point of 4.6, intermediate between that of avian thymic hormone (pI = 4.3) and the muscle parvalbumin isoform (pI = 5.2). Expression of CPV3, like that of avian thymic hormone (ATH), is restricted to the thymic stroma. However, the CPV3 content of chicken thymus tissue (120 micrograms/g tissue) is 4 times lower than that of ATH (500 micrograms/g tissue). The polymerase chain reaction (PCR) was used to gain access to the nucleotide sequence of CPV3. A 147-base pair fragment of the coding sequence, corresponding to residues 48-97, was amplified from total chicken cDNA using degenerate PCR primers. A RACE-PCR strategy was then used to extend the known sequence in both the 5' and 3' directions. The cDNA sequence thus obtained includes 671 base pairs. Primer extension analysis suggests that the cloned cDNA corresponds to a full-length transcript. Northern analysis of chicken mRNA indicates that the average CPV3 transcript is approximately 800 nucleotides in length, significantly smaller than the ATH message (approximately 1000 nucleotides). Southern analysis suggests the presence of a single CPV3 gene in the chicken genome. The translated nucleotide sequence, displaying 108 residues between the initiator and termination codons, is that of a beta-parvalbumin. The CPV3 sequence exhibits 58% identity with ATH and 52% identity with the chicken muscle isoform. Interestingly, CPV3 and the mammalian oncodevelopmental parvalbumin called oncomodulin are identical at 73 of 108 residues (68% identity). Correspondingly, flow-dialysis measurements with 45Ca2+ indicate that the Ca(2+)-binding domains are inequivalent, as in oncomodulin. The apparent dissociation constants, at pH 7.4 in 150 mM NaCl, are approximately 10 nM and 80 nM.