Discovery, Structure and Tentative Functions of a C-terminal propeptide of Vacuolar Potato Lipases (Patatins)

Potato tuber patatins amount to 25-40% of potato tuber protein. They are dimers of ca. 90 kDa with lipase/ esterase activity as shown by gel filtration followed by activity measurements, whereas the subunits are 40-42 kDa including glycans as demonstrated by non-reducing SDS-PAGE and MALDI-TOF-MS (1). It is well-known that patatins are located in the vacuoles of potato tubers. However, the vacuolar targeting signal has never been identified for this storage and defence protein. Proteome data of potato (Solanum tuberosum) tuber juice and of purified potato tuber vacuoles indicated that mature patatins may perhaps lack a ct-propeptide. We have confirmed this by complete mass spectrometric sequencing of a number of patatin variants as well as their N-linked complex-type glycans from the starch-rich cultivar Kuras. For this cultivar full length patatin cDNAs have also been sequenced, as the patatin locus is highly polymorphous. We propose that a six-residues ct-propeptide, -ANKASY-COO^- composes this signal. The crystallographic structure of a recombinant patatin (Rydel et al., 2003, Biochemistry 42, 6696-6708), which included this propeptide thus shows us, for the first time, a putative ligand of the vacuolar sorting receptor and processing enzyme responsible for patatin import and processing.