Figure 6.

The SIB1 protein undergoes UPS-mediated proteolysis. Five-day-old transgenic sib1 plants expressing pSIB1-driven GFP-tagged SIB1 were first treated with 1 mm SA for 4 h. Then, these seedlings were treated with 100 μm MG132 for the indicated times. A, MG132 inhibits SIB1 degradation. Samples were harvested at the indicated time points for immunoblot analysis. SA indicates 4 h of SA treatment (first sample on the left), and the other samples (additional hours of SA treatment as indicated) were prepared in the presence or absence of MG132. B, Exogenous SA promotes the ubiquitinylation of SIB1. SIB1-GFP proteins extracted from 6-h MG132-treated and nontreated samples were first enriched with an IP assay. The SIB1-GFP and ubiquitinylated SIB1-GFP proteins were identified by immunoblot analysis using anti-GFP (αGFP) and anti-ubiquitin (αUb) antibody, respectively.