Evolutionary insights into the role of the essential centromere protein CAL1 in Drosophila

Centromeres are essential cis-elements on chromosomes that are crucial for the stable transmission of genetic information during mitotic and meiotic cell divisions. Different species employ a variety of centromere configurations, from small genetically defined centromeres in budding yeast to holocentric centromeres that occupy entire chromosomes in Caenorhabditis, yet the incorporation of nucleosomes containing the essential centromere-specific histone H3 variant CENP-A is a common feature of centromeres in all eukaryotes. In vertebrates and fungi, CENP-A is specifically deposited at centromeres by a conserved chaperone, called HJURP or Scm3, respectively. Surprisingly, homologs of these proteins have not been identified in Drosophila, Caenorhabditis, or plants. How CENP-A is targeted to centromeres in these organisms is not known. The Drosophila centromeric protein CAL1, found only in the Diptera genus, is essential for CENP-A localization, is recruited to centromeres at a similar time as CENP-A, and interacts with CENP-A in both chromatin and pre-nucleosomal complexes, making it a strong candidate for a CENP-A chaperone in this lineage. Here, we discuss the conservation and evolution of this essential centromere factor and report the identification of a "Scm3-domain"-like region with similarity to the corresponding region of fungal Scm3 as well as a shared predicted alpha-helical structure. Given the lack of common ancestry between Scm3 and CAL1, we propose that an optimal CENP-A binding region was independently acquired by CAL1, which caused the loss of an ancestral Scm3 protein from the Diptera lineage.