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Identification of a cDNA Clone Coding for a Novel Calcium-Binding Protein from Potato Tuber

Calcium acts as a second messenger in many higher plant cellular activities including cell elongation, cell division, protoplasmic streaming, and enzyme secretion (Marme, 1988). Many of the effects of Ca2+ are mediated by CaBPs that undergo conformational activation upon association with one to four Ca2+ ions. Structural analyses of parvalbumin and other CaBPs (Kretsinger and Nockolds, 1973; Babu et al., 1985; Herzberg and James, 1985) defined a Ca2+-binding "EF-hand" domain consisting of a loop of 12 residues flanked on each side by a 12-residue a-helix. CaBPs are known to possess one or more copies of this conserved EF-hand domain (Kressinger, 1976). Calmodulin is the most ubiquitous and highly conserved CaBP (Anderson et al., 1980; Watterson et al., 1980), whereas others, including troponin C (muscle), calcineurin (brain), and parvalbumin (muscle), transduce Ca2+ signals in a tissueand species-specific manner. Sequence homology between them is largely restricted to the calcium-binding domains. Plants may possess a unique set of CaBPs, considering the distinctness of their biochemistry and physiology. For example, Kajiwara et al. (1990) identified and partially sequenced a rice leaf CaBP with some limited homology to parvalbumin. We report the isolation of a 865-bp cDNA clone from a potato (Solanum tuberosum) tuber cDNA library that codes for a nove1 22.5-kD CaBP designated CAST (Table I, Fig. 1). The predicted coding sequence is 199 amino acid residues and possesses 34% sequence identity with potato calmodulin. Homology also exists with troponin C and various related CaBPs, including caltractin, myosin light chain, CDC31, calcineurin B subunit, and calbindin. The COOH-terminal half of CAST (residues 128-199) exhibits greatest sequence homology to the CaBP family, including a putative Ca*+-binding (EF-hand) domain (Fig. 1). A second EF-hand domain may exist in the region encompassing residues 178 to 188. Secondary structure predictions of CAST, based on the parameters of Novotnq and Auffray (1984) for a-helix, @-sheet, and @-turn propensity, indicate that CAST may possess conformational similarities to calmodulin. The function of CAST is unknown; however, this sequence