Cold stress changes the concanavalin A-positive glycosylation pattern of proteins expressed in the basal parts of rice leaf sheaths

Post-translational modifications such as glycosylation are important for changing the properties and functions of proteins. To analyze the importance of glycosylation during cold stress in rice, a proteomics approach was used. Proteins extracted from the basal part of rice leaf sheaths were separated by two-dimensional polyacrylamide gel electrophoresis, and subjected to lectin blot analysis using concanavalin A. From a total of 250 detected proteins, 22 reacted with the lectin, suggesting that they were N-glycosylated proteins. To determine how N-glycosylation of these proteins is affected by cold stress, rice seedlings were incubated at 5°C for 48 h, and proteins extracted from the basal parts of leaf sheaths were analyzed by the lectin blot assay. Cold stress changed the reactivity toward the lectin for 12 of the 22 glycoproteins. The identity of the 12 proteins was determined by protein sequencing and mass spectrometry with the majority of these glycoproteins being categorized as involved in energy production. Furthermore, calreticulin, one of the 12 glycoproteins, was also phosphorylated as a result of cold stress. These results indicate that cold stress of the basal parts of rice leaf sheaths changes the glycosylation and phosphorylation profiles of calreticulin, a key protein that regulates the quality control of other proteins.