polyserine region is a site for phosphorylation [20,21]. The lysine-rich signature motif is also found at the
carboxy terminus. There is another conserved motif, DEYGNP, that is found one or two times near the
amino terminus. However, gene products with the consensus lysine-rich repeat have been identified that
do not have the polyserine region, referred to as type II in Figure 1. These genes are made up of the con-
sensus lysine-rich repeat alternating with glycine-rich repeats. The glycine-rich repeats do not have a con-
sensus sequence; they are characterized only by the abundance of glycine residues. These genes have been
identified in wheat [22,23] and in alfalfa [24]. Interestingly, the alfalfa genes are induced by low-tem-
perature treatments but not by drought stress or by ABA application [24]. In wheat, the gene, which en-
codes a 39-kDa protein, is induced preferentially by low-temperature treatments.
- Groups 3 and 5
Group 3 proteins, represented by D-7 from cotton, and group 5 proteins, represented by D-29 from cot-
ton, contain repeated tracts of 11 amino acids [4]. Although the 11-mer repeat has diverged among dif-
ferent species, a functional consensus was derived based on the polarity, charge, or methylation of the
amino acid (Figure 1). The periodicity of 11 indicates that there may be an amphiphilic -helix formed
by polar and apolar amino acids aligned on different faces of the -helix [3,4,25]. The D-7 protein is
found uniformly in the mature cotton embryo, calculated to be from 200 to 300 M in the cytoplasm im-
mediately prior to desiccation [26]. Interestingly, in Craterostigma plantagineum, a protein with a simi-
lar 11-mer repeat was found to be localized in the chloroplast [16]. The abundance of these proteins has
been used to rule out several possible functions—these proteins are not expected to function as enzymes,
structural (architectural) proteins, regulatory proteins, or as ion or water transport proteins [26]. The pre-
diction has been made that these proteins function in the sequestration of ions during cellular dehydration
[25].
- Group 4
It has been discovered that another gene that is expressed in drying cotton seeds accumulates to high lev-
els [26]. This protein, D-113, has a homologue in tomato, LE25 [11,27], whose mRNA is expressed in
vegetative tissues in response to drought stress, and elevated levels of endogenous ABA are required for
its expression [14]. Members of the D-113 family are biased toward alanine residues and contain the ran-
dom coil–promoting residues glycine and threonine. It is predicted that the amino-terminal portion of the
molecule is made up of an -helix (Figure 1). The remainder of the molecule has no predicted structure
[19]. It has been proposed that D-113 behaves as a surrogate water film in the desiccated state, stabiliz-
ing the intracellular surface of seeds [26]. It is uncertain if this would also be a role for D-113–like pro-
teins in vegetative tissues of plants such as tomato because these tissues are not capable of surviving des-
iccation.
- LT178, 65/RD29 Family
An additional family of genes, which have thus far been identified only in Arabidopsis, are also induced
by water deficit–based stresses and ABA [28–30]. There are two genes in Arabidopsisthat are adjacent
to each other in the genome. The deduced amino acid sequences result in 77.9- and 64.5-kDa proteins that
are overwhelmingly hydrophilic [30]. These genes were identified by three different laboratories and have
been named lti78andlti65[28],rd29Aandrd29B[29,30], and cor78[31]. Thus far, no specific predic-
tions about the function of these proteins have been made.
- Glycine-Rich Proteins
A family of genes in alfalfa has been characterized which are regulated by ABA, low temperature, and
drought. The gene products are predicted to be hydrophilic and are characterized by glycine-rich repeats
[31–33]. There are no specific predictions for their role other than a general role in protection from cel-
lular dehydration.
- KIN1 and KIN2/COR6.6
Another protein family was identified during low-temperature stress in Arabidopsisand is encoded by at
least two genes in Arabidopsis[34–36]. These proteins are of low molecular weight (6.6 kDa), are rich in
alanine, glycine, and lysine, and are largely hydrophilic. These genes are also induced by ABA treatments
[35]. The kin2gene is expressed strongly in response to drought stress and salinity [36]. A low degree of
738 BRAY