MdAP, a novel protein in apple, is associated with the major allergen Mal d 1

Mal d 1, an 18-kDa intracellular pathogenesis-related protein (PR-10), has been known since long as the major apple allergen in Middle and Northern Europe. However, its biological function, as that of many other PR-10 proteins, is still unknown. In order to identify proteins putatively interacting with Mal d 1, an expression library of Malus domestica was screened using the yeast-two-hybrid (Y2H) system. A novel protein binding to two isoforms of Mal d 1 being used as 'bait' was isolated. The deduced amino acid sequence from the corresponding full-length cDNA of the predicted Mal d 1-Associated-Protein (MdAP) do not display any homology to known proteins, but shares 45% identity with a 'hypothetical protein' in Arabidopsis thaliana. Southern analysis of the apple genome indicated that MdAP, comprising 190 amino acids, is encoded by a single gene. The expression pattern of the 1-kb MdAP transcript resembled the expression profile of the different Mal d 1 isoforms in various apple organs, however at a much lower level. Furthermore, a huge variation in transcription levels of Mal d 1 isoforms was observed in apple tissue. For both, Mal d 1 and MdAP highest amounts of mRNAs were measured in ripe fruits and significantly lower amounts in vegetative tissue by quantitative reverse transcription-polymerase chain reaction (RT-PCR).