Sequence and Regulation of a Late Embryogenesis Abundant Group 3 Protein of Maize

The group 3 class of Lea proteins is expressed both in seeds and in drought-stressed tissues, where they are thought to play a role as desiccation protectants (reviewed by Mundy and Skriver, 1990). In wheat, there is experimental evidence that expression of Lea 3 is correlated with resistance to drought stress (Ried and Walker-Simmons, 1993). Lea group 3 proteins are distinguished by a tandemly repeated 11-amino acid motif (Dure et al., 1989). Comparing these proteins from a variety of plants, Dure (1993) has proposed a model for the structure of the tandemly arrayed units and for the possible function of these proteins in desiccation protection. In an earlier paper (Thomann et al., 1992), we described MLG3, an ABA-responsive embryo maturation protein that w a s ant igen ica l l y re la ted to the L e a group 3 p r o t e i n from wheat. Using a cDNA of wheat Lea group 3, we isolated a full-length 1.1-kb cDNA (pMlg3) from maize (Zea mays L.). The clone has one open reading frame of 666 bases. pMlg3 detected a single hybridizing band in genomic Southern blots, consistent with our genetic evidence for a single locus (Thomann et al., 1992). The predicted translation product of Mlg3 contains a tandemly repeated 11-amino acid motif having apolar residues (chiefly Ala and Thr) at positions 1, 2, 5, and 9; positively charged amino acids (chiefly Lys) at positions 6 and 8; and negative charges or amide residues at positions 3, 7, and 11 (Table I). MLG3 has nine conserved repeat units, one degenerate repeat, and a truncated repeat. The maize Lea group 3 protein also shares strong homologies with the wheat and barley proteins in both C-terminal and N-terminal peptide sequences. Data base searches did not reveal any known function for these peptide sequences, although their conservation suggests that they are of structural importance. The predicted mass (22.9 kD) is less than the 27 to 29 kD estimated by SDS-PAGE mobility (Thomann et al., 1992). This discrepancy, found for other Lea 3 proteins (Ried and Walker-Simmons, 19931, has been attributed to anomalous electrophoretic behavior due to the proteins' unique composition and charge characteristics (Hong et al., 1988).