MOLECULAR-CLONING AND EVOLUTIONARY ANALYSIS OF THE CALCIUM-MODULATED CONTRACTILE PROTEIN, CENTRIN, IN GREEN-ALGAE AND LAND PLANTS

Centrin (= caltractin) is a ubiquitous, cytoskeletal protein which is a member of the EF-hand superfamily of calcium-binding proteins. A cen trin-coding cDNA was isolated and characterized from the prasinophyte green alga Scherffelia dubia. Centrin PCR amplification primers were u sed to isolate partial, homologous cDNA sequences from the green algae Tetraselmis striata and Spermatozopsis similis. Annealing analyses su ggested that centrin is a single-copy-coding region in T. striata and S. similis and other green algae studied. Centrin-coding regions from S. dubia, S. similis and T. striata encode four colinear EF-hand domai ns which putatively bind calcium. Phylogenetic analyses, including hom ologous sequences from Chlamydomonas reinhardtii and the land plant At riplex nummularia, demonstrate that the domains of centrins are congru ent and arose from the two-fold duplication of an ancestral EF hand wi th Domains 1 + 3 and Domains 2 + 4 clustering. The domains of centrins are also congruent with those of calmodulins demonstrating that, like calmodulin, centrin is an ancient protein which arose within the ance stor of all eukaryotes via gene duplication. Phylogenetic relationship s inferred from centrin-coding region comparisons mirror results of sm all subunit ribosomal RNA sequence analyses suggesting that centrin-co ding regions are useful evolutionary markers within the green algae.