A novel antimicrobial protein isolated from potato (Solanum tuberosum) shares homology with an acid phosphatase

A novel antimicrobial protein (AP₁) was purified from leaves of the potato (Solanum tuberosum, variety MS-42.3) with a procedure involving ammonium sulphate fractionation, molecular sieve chromatography with Sephacryl S-200 and hydrophobic chromatography with Butyl-Sepharose using a FPLC system. The inhibition spectrum investigation showed that AP₁ had good inhibition activity against five different strains of Ralstonia solanacearum from potato or other crops, and two fungal pathogens, Rhizoctonia solani and Alternaria solani from potato. The full-length cDNA encoding AP₁ has been successfully cloned by screening a cDNA expression library of potato with an anti-AP₁ antibody and RACE (rapid amplification of cDNA ends) PCR. Determination of the nucleotide sequences revealed the presence of an open reading frame encoding 343 amino acids. At the C-terminus of AP₁ there is an ATP-binding domain, and the N-terminus exhibits 58% identity with an/the acid phosphatase from Mesorhizobium loti. SDS/PAGE and Western blotting analysis suggested that the AP₁ gene can be successfully expressed in Escherichia coli and recognized by an antibody against AP₁. Also the expressed protein showed an inhibition activity the same as original AP₁ protein isolated from potato. We suggest that AP₁ most likely belongs to a new group of proteins with antimicrobial characteristics in vitro and functions in relation to phosphorylation and energy metabolism of plants.