SURFACE HYDROPHOBIC RESIDUES OF MULTIUBIQUITIN CHAINS ESSENTIAL FOR PROTEOLYTIC TARGETING

Ubiquitin conjugation is a signal for degradation of eukaryotic protei ns by the 26S protease, Conjugation of a homopolymeric multiubiquitin chain to a substrate lysine residue results in 10-fold faster degradat ion than does conjugation of monoubiquitin, but the molecular basis of enhanced targeting by chains is unknown, We show that ubiquitin resid ues L8, I44, and V70 are critical for targeting, Mutation of pairs of these residues to alanine had little effect on attachment of ubiquitin to substrates but severely inhibited degradation of the resulting con jugates, The same mutations blocked the binding of chains to a specifi c subunit (S5a) of the regulatory complex of the 26S protease, The sid e chains implicated in this binding-L8, I44, and V70-form repeating pa tches on the chain surface, Thus, hydrophobic interactions between the se patches and S5a apparently contribute to enhanced proteolytic targe ting by multiubiquitin chains.