ABSTRACT
The principal targeting signal used in the ubiquitin-proteasome degradation pathway is a homopolymeric, K48-linked polyubiquitin chain: the chain is recognized by a specific factor(s) in the 19S regulatory complex of the 26S proteasome, while the substrate is degraded by the 20S catalytic complex. We have previously presented evidence implicating the side chains of L8, I44, and V70 in the recognition of K48-linked chains. In the crystal structure of tetraubiquitin, these side chains form a repeating, surface-exposed hydrophobic patch. To test the hypothesis that a close-packing interaction involving this patch is important for the chain recognition, residue 8 was mutated to a series of smaller aliphatic amino acids (G, A, V). The effects of these mutations were first investigated in rabbit reticulocyte fraction II; even the severest truncating mutation (L8G) had only a modest inhibitory effect on the degradation of a model substrate (125I-lactalbumin). We show that these steady-state degradation data substantially underestimate the deleterious effects of these mutations on chain recognition by the proteasome, because the recognition step does not contribute to rate limitation in the fraction II system. Much stronger inhibition was observed when chain binding was measured in a competition assay using purified 26S proteasomes, and the change in binding free energy depended linearly on the surface area of the side chain. This behavior is consistent with a mode of binding in which the hydrophobic effect makes a favorable contribution; i.e., one or more L8 side chains is shielded from solvent when the chain binds to the 19S complex. A similar linear dependence of binding energy on side chain area was observed for chain binding to the 19S subunit known as S5a (as assayed using recombinant S5a bound to nickel beads). Octa-ubiquitin (K0.5 = 1.6 microM) bound to S5a 4.2-fold more tightly than tetra-ubiquitin; this is similar to the factor of 5. 8-fold relating the affinities of the same two chains for the 26S proteasome. Altogether, these findings indicate that the interaction of K48-linked chains with the 19S complex is substantially similar to the interaction of chains with isolated S5a. The results further suggest that the hydrophobic patch is part of a minimum element which allows for specific recognition of the polyubiquitin degradation signal by the 26S proteasome.
