A Role for the Proteasome Alpha2 Subunit N-Tail in Substrate Processing.

Area of Science:

• Molecular Biology
• Proteostasis
• Enzymology

Background:

• The 26S proteasome's active sites are enclosed within the 20S core particle (CP).
• Substrate entry is controlled by a narrow channel regulated by seven outer alpha (α) subunits.
• The N-termini of these α subunits form a gate, blocking access in the resting state.

Purpose of the Study:

• To investigate the role of individual α subunit N-termini in regulating proteasome gate function.
• To elucidate the structural mechanisms governing substrate access and translocation.
• To understand how N-terminal modifications affect proteolysis.

Main Methods:

• Truncation and mutation of individual α subunit N-termini.
• Analysis of gate dynamics and substrate translocation.
• Structural and functional characterization of the 20S proteasome.

Main Results:

• Specific N-termini are crucial for maintaining the closed gate, while all seven participate in gate opening.
• A conserved YD(X) motif, involving hydrogen bonds between tyrosine (Y) and aspartate (D), stabilizes the open gate.
• The α2 subunit's N-terminal phenylalanine (F) directly interacts with translocating substrates, and its deletion impairs proteolysis.

Conclusions:

• The interlacing N-terminal YD(X) motifs are critical regulators of both proteasome gating and substrate translocation.
• The α2 N-terminal tail plays a unique role in substrate translocation, independent of gate opening.
• Understanding these mechanisms provides insights into proteasome function and regulation.