The hydrophobic effect contributes to polyubiquitin chain recognition

Area of Science:

• Biochemistry
• Molecular Biology
• Cellular Biology

Background:

• The ubiquitin-proteasome system (UPS) degrades proteins using a polyubiquitin chain as a targeting signal.
• K48-linked polyubiquitin chains are recognized by the 19S regulatory complex of the 26S proteasome.
• Previous studies implicated specific residues (L8, I44, V70) forming a hydrophobic patch in K48-linked chain recognition.

Purpose of the Study:

• To investigate the role of the hydrophobic patch in K48-linked polyubiquitin chain recognition by the 26S proteasome.
• To determine the contribution of hydrophobic interactions to the binding energy between polyubiquitin chains and proteasome components.
• To assess the importance of specific residues in mediating the interaction between polyubiquitin chains and the 19S proteasome subunit S5a.

Main Methods:

• Site-directed mutagenesis of ubiquitin residue L8 to smaller aliphatic amino acids (G, A, V).
• Assay of protein degradation in rabbit reticulocyte fraction II using a model substrate.
• Measurement of 26S proteasome and purified S5a binding to polyubiquitin chains using competition assays.
• Determination of binding affinities for tetra-ubiquitin and octa-ubiquitin chains.

Main Results:

• Mutations at L8 showed only modest inhibition of substrate degradation in crude extracts, underestimating their impact on chain recognition.
• Competition assays with purified 26S proteasomes revealed significant inhibition, with binding free energy changes correlating linearly with side chain surface area.
• Similar linear relationships were observed for binding to the 19S subunit S5a, suggesting S5a is a major determinant of K48-linked chain recognition.
• Octa-ubiquitin bound more tightly to S5a and 26S proteasome than tetra-ubiquitin, consistent with the role of the hydrophobic patch.

Conclusions:

• The hydrophobic patch, particularly involving L8, is critical for specific recognition of K48-linked polyubiquitin chains by the 19S proteasome complex.
• Hydrophobic interactions significantly contribute to the binding energy of polyubiquitin chains to the proteasome.
• The interaction of K48-linked chains with the 19S complex is largely mediated by S5a and involves a conserved recognition mechanism.
• The identified hydrophobic patch represents a key element for polyubiquitin signal recognition within the 26S proteasome.