Secreted arabinogalactan protein from salt-adapted tobacco BY-2 cells appears to be glycosylphosphatidyl inositol-anchored and associated with lipophilic moieties

ABSTRACT

Arabinogalactan proteins (AGPs) are plant extracellular proteoglycans associated with the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. This moiety is thought to be cleaved by phospholipase for secretion. Salt-adapted tobacco BY-2 cells were reported to secrete large amounts of AGPs into the medium. To investigate this mechanism, we expressed a fusion protein of tobacco sweet potato sporamin and AGP (SPO-AGP) in BY-2 cells and analyzed its fate after salt-adapting the cells. A two-phase separation analysis using Triton X-114 indicated that a significant proportion of SPO-AGP in the medium was recovered in the detergent phase, suggesting that this protein is GPI-anchored. Differential ultracentrifugation and a gradient density fractionation implicated extracellular vesicles or particles with SPO-AGP in the medium. Endogenous AGP secreted from salt-adapted and nontransgenic BY-2 cells behaved similarly to SPO-AGP. These results suggest that a part of the secreted AGPs from salt-adapted tobacco BY-2 cells are GPI-anchored and associated with particles or vesicles.