FIGURE 5.

Model of the ubiquitin dependent degradation process of orphan protein Fas2, which has lost its partner Fas1. In the absence of Fas1, Fas2 is still organized in an intermediate complex that is recognized by the ubiquitin ligase Ubr1, ubiquitinated with the help of the E2 enzymes Ubc2 and Ubc4, and afterwards disassembled by the Cdc48 machinery for final proteasomal degradation. Degradation of orphan Fas2 requires the presence of the Hsp70 chaperone Ssa1, which keeps Fas2 in a soluble state for efficient ubiquitination and degradation by the proteasome. Its function in the ubiquitination process by attaching Fas2 to Ubr1 via an yet unknown partner is also possible.