Summary Model. The model emphasizes that tmCdc34 is functional in substrate monoubiquitination but has a defect in the synthesis of substrate-attached and free polyubiquitin chains. This emphasis does not mean that tmCdc34 cannot synthesize polyubiquitin chains. Rather, it emphasizes that long polyubiquitin chains cannot be synthesized in the short time frame normally available for their synthesis in vivo. Cdc34 is marked blue; SCFCdc4 is marked navy blue; Sic1 substrate is marked red; the free form of ubiquitin (Ub) is marked white; the ubiquitin-thiolester is marked yellow; the ubiquitin engaged in isopeptide bond (with substrate or other ubiquitin molecules) is marked green. Black arrows indicate a robust (three arrows), or modest (one arrow) synthesis of polyubiquitin chains. Green arrow indicates monoubiquitination.