cDNA for a 14-kilodalton polypeptide from Madagascar periwinkle (Catharanthus roseus). Eight conserved cysteines in six related proteins from different plants suggest common functional elements.

A change from MX growth medium to a solution of 8% SUC induces the expression of a large number of proteins in cell cultures of Catharanthus roseus (Vetter et al., 1992; our unpublished results). In a differential screen for induced mRNAs, we identified a cDNA for a basic 14-kD polypeptide with four putative membrane-associated helices (Fig. 1, Table I). It also contains a Pro-Lys-Pro motif that is present in different variations in many Glyor Pro-rich cell wall proteins and has been identified as a motif in extensin-like proteins (Zhou et al., 1992). A search of the data libraries identified three proteins of similar size with more than 50% identity: (a) from Zea mays (John et al., 1992), (b) from Daucus carota (Aleith and Richter, 1990), and (c) from Arabidopsis thaliana (gene EXTA, accession No. X67421). Figure 1 shows the comparison, with a few small gaps to optimize the alignment. It appears that the four proteins share nine Cys's in conserved positions. The C. roseus polypeptide also possesses significant similarity with the C-terminal 100 amino acids of two larger proteins: a 37.4-kD protein preferentially expressed in tomato fruits (TPRP-Fl; 48% identity) (Salts et al., 1992) and an auxin-repressed 22.5-kD polypeptide from soybean (ADR11; 45% identity) (Datta et al., 1993). It is interesting that the alignment indicates that eight Cys's conserved in the smaller polypeptides are also present in the larger proteins (Fig. 1). The precise functions of the six proteins are not understood, but the sequence similarity and, in particular, the conservation of the Cys's suggest common functional elements. It is also of interest that the expression of at least three of the proteins appears to be regulated by auxin: the 14-kD polypeptide from C. roseus was induced by transfer of the cells from MX growth medium to an auxin-free SUC solution, the D. carota protein was induced after remova1 of auxin from the cell cultures (Aleith and Richter, 1990), and the soybean protein was identified as an auxin down-regulated cDNA clone (Datta et al., 1993).