Nucleotide Sequence of a Transmembrane Protein (TMP-C) cDNA in Arabidopsis thaliana

Channel-like proteins, including water channel proteins, belong to a family of major intrinsic proteins that was originally characterized in bovine lens fiber cells. They have structurally similar properties with six putative membranespanning domains and a well-conserved amino acid sequence in a loop between the second and third membrane-spanning domains. The major intrinsic proteins are thought to be ion channels in cell-cell junctions (Ehring et al., 1990). A bacterial homolog, GlpF, of this family facilitates the transport of glycerol (Heller et al., 1980), and another homolog found in human erythrocytes, CHIP28, is known to be a water channel (Preston et al., 1992). In higher plants, many genes of this family have been reported (Fortin et al., 1987; Yamamoto et al., 1990; Hofte et al., 1992). One of them, yTIP (tonoplast intrinsic protein), was shown to function as a water channel when its mRNA is expressed in Xenopus oocytes (Maurel et al., 1993). Water plays a crucial role both in the maintenance of cell turgor pressure and for the transport of nutrients and ions in plants. Water channel proteins in membranes must be important in regulating the water balance in cells and tissues during wilting by desiccation and during dehydration at the late stage of seed maturation. These processes cannot be explained by simple diffusion of water molecules through the lipid bilayers of membranes. We have isolated a cDNA clone (1100 bp) encoding a TMP homolog from a library from flower buds of Arabidopsis (Table I). The deduced amino acid sequence is very similar to those of TMP-A and TMP-B from Arabidopsis (Shagan and Bar-Zvi, 1993; Shagan et al., 1993). Therefore, we have designated it TMP-C. The TMP-C polypeptide (287 amino acids) is larger than TMP-A and TMP-B by a one-amino acid insertion. The insertion of Thr is found at the 28th amino acid position from the N terminus of the TMP-C polypeptide. TMP-C is 94 and 92% identical in amino acid sequence to TMP-A and TMP-B, respectively. The nucleotide sequence of the coding region of TMP-C shows 82 and 86% identity with TMP-A and TMP-B, respectively. The untranslated reTable 1. Characteristics of A. thaliana TMP-C cDNA