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Age-Induced Protein Modifications and Increased Proteolysis in Potato Seed-Tubers1

G.N. Mohan Kumar, Robert L. Houtz, and N. Richard Knowles*

Department of Agricultural, Food and Nutritional Science, 4-10 Agriculture/Forestry Center, University of Alberta, Edmonton, Alberta, Canada T6G 2P5 (G.N.M.K., N.R.K.); and Department of Horticulture and Landscape Architecture, University of Kentucky, Lexington, Kentucky 40546-0091 (R.L.H.)

Long-term aging of potato (Solanum tuberosum) seed-tubers resulted in a loss of patatin (40 kD) and a cysteine-proteinase inhibitor, potato multicystatin (PMC), as well as an increase in the activities of 84-, 95-, and 125-kD proteinases. Highly active, additional proteinases (75, 90, and 100 kD) appeared in the oldest tubers. Over 90% of the total proteolytic activity in aged tubers was sensitive to trans-epoxysuccinyl-L-leucylamido (4-guanidino) butane or leupeptin, whereas pepstatin was the most effective inhibitor of proteinases in young tubers. Proteinases in aged tubers were also inhibited by crude extracts or purified PMC from young tubers, suggesting that the loss of PMC was responsible for the age-induced increase in proteinase activity. Nonenzymatic oxidation, glycation, and deamidation of proteins were enhanced by aging. Aged tubers developed "daughter" tubers that contained 3-fold more protein than "mother" tubers, with a polypeptide profile consistent with that of young tubers. Although PMC and patatin were absent from the older mother tubers, both proteins were expressed in the daughter tubers, indicating that aging did not compromise the efficacy of genes encoding PMC and patatin. Unlike the mother tubers, proteinase activity in daughter tubers was undetectable. Our results indicate that tuber aging nonenzymatically modifies proteins, which enhances their susceptibility to breakdown; we also identify a role for PMC in regulating protein turnover in potato tubers.

1   This research was funded by the Natural Sciences and Engineering Research Council of Canada (grant to N.R.K.) and by the U.S. Department of Energy (grant no. DE-FG05-92ER26075 to R.L.H.).
*   Corresponding author; e-mail rknowles{at}afns.ualberta.ca; fax 1-403-492-4265.

Plant Physiol. (1999) 119: 89-100
Copyright Clearance Center:   0032-0889/99/119//12
© 1999 American Society of Plant Physiologists




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