Cloning and characterisation of a novel P-glycoprotein homologue from barley.

P-glycoproteins are members of a large superfamily of transport proteins (the 'traffic ATPases') that utilize ATP to translocate a wide range of substrates across biological membranes. Using a PCR-based approach, and degenerate oligonucleotides corresponding to conserved motifs, two 300-bp cDNA fragments (pBMDR1 and pBMDR2) with a significant sequence similarity to mammalian P-glycoproteins were amplified from barley (Hordeum vulgare) root poly A+ RNA and used as probes to screen a barley root cDNA library. A single full-length clone pHVMDR2 coding for a polypeptide of 1232 residues (c. 134 kDa) was isolated. Comparison of this barley sequence with Arabidopsis ATPGP1 and human MDR1 and MDR3 P-glycoprotein sequences showed that the barley cDNA has 44%, 37% and 38% amino acid (aa) identity, respectively, with these sequences, and conserved structural features. RNase protection analysis showed that HVMDR2 mRNA is expressed at low levels in both barley roots and leaves. Southern blot analyses indicated that there is a small multigene family related to P-glycoproteins in barley. Possible functions for these barley P-glycoproteins are discussed.