A POTATO CDNA-ENCODING A HOMOLOG OF MAMMALIAN MULTIDRUG-RESISTANT P-GLYCOPROTEIN

A homologue of the multidrug resistance (MDR) gene was obtained while screening a potato stolen tip cDNA expression library with S-35-labele d calmodulin. The mammalian MDR gene codes for a membrane-bound P-glyc oprotein (170-180 kDa) which imparts multidrug resistance to cancerous cells. The potato cDNA (PMDR1) codes for a polypeptide of 1313 amino acid residues (ca. 144 kDa) and its structural features are very simil ar to the MDR P-glycoprotein. The N-terminal half of the PMDR1-encoded protein shares striking homology with its C-terminal half, and each h alf contains a conserved ATP-binding site and six putative transmembra ne domains. Southern blot analysis indicated that potato has one or tw o MDR-like genes. PMDR1 mRNA is constitutively expressed in all organs studied with higher expression in the stem and stolen tip. The PMDR1 expression was highest during tuber initiation and decreased during tu ber development.