A novel Ca^(2+)-binding protein with Mr of 23 K (designated p23) has beenidentified in avian erythrocytes and thrombocytes. p23 localizes to the marginal bands(MBs), centrosomes and discrete sites around the nuclear membrane in mature avianerythrocytes. p23 appears to bind Ca^(2+) directly and its interaction with subcellularorganelles seems to be modulated by intracellular [Ca^(2+)]. However, its unique proteinsequence lacks any known Ca^(2+)-binding motif. Developmental analysis reveals that p23association to its target structures occurs only at very late stages of bone marrowdefinitive erythropoeisis. In primitive erythroid cells, p23 distributes diffusely in thecytoplasm and lacks any distinct localization. It is postulated that p23 association tosubcellular structures may be induced in part by decreased intracellular [Ca^(2+)]. In vitroand in vivo experiments indicate that p23 does not appear to act as a classicalmicrotubule-associated protein (MAP) but p23 homologues appear to be expressed inMB-containing cells of a variety of species from different vertebrate classes. It has beenhypothesized that p23 may play a regulatory role in MB stabilization in a Ca^(2+)-dependentmanner.
Binucleated (bnbn) turkey erythrocytes were found to express a truncated p23variant (designated p21) with identical subcellular localization as p23 exceptimmunostaining reveals the presence of multi-centrosomes in bnbn cells. The p21sequence has a 62 amino acid deletion at the C-terminus and must therefore have anadditional ~40 amino acids at the N-terminus. In addition, p21 seems to have lost theability to bind Ca^(2+) and its supramolecular interactions are not modulated by intracellular[Ca^(2+)]. These apparent differences between p23 and p21 raised the possibility that the p23/p21 allelism could be the Bn/bn genotype. However, genetic analysis suggested thatp23/p21 allelism had no absolute correlation with the Bn/bn genotype.
