Proposed model for a stepwise ubiquitination and degradation mechanism of Petunia hybrida S-RNases. Self and nonself S-RNases (SR and NR) and their region (R) I ubiquitinated forms can enter the cytoplasm through the pollen-tube membrane. The R I ubiquitinated S-RNases could be degraded by the 26S proteasome, whereas their unubiquitinated forms are identified by SLFs. SR and its cognate SLF repel each other and results in an inability of SCF^SLF to ubiquitinate SR. By contrast, NR is attracted by nonself SLF for R II ubiquitination by SCF^SLF and its subsequent degradation by the 26S proteasome. Subsequently, internal R III could be further exposed for ubiquitination, leading to degradation of NR by the 26S proteasome or other unknown pathways. In addition, S-RNase compartmentalisation could occur in the vacuole and contribute to its sequestration. I, II, and III: three ubiquitination regions in S-RNases^[ref].