![]() ![]() Consequently, deletion of the α2 N-terminal tail attenuates proteolysis despite the appearance of an open gate state. Phenylalanine at this position in the α2 subunit comes in direct contact with the translocating substrate. The third residue (X) of this YD(X) motif aligns with the open channel. The lone exception is the α1–α2 pair leaving a gap in the ring circumference. Specifically, the open state is stabilized by a hydrogen bond between an invariant tyrosine (Y) in each subunit with a conserved aspartate (D) in its counterclockwise neighbor. By truncating or mutating each of the participating α N-termini, we report that whereas only a few N-termini are important for maintaining the closed gate, all seven N-termini participate in the open gate. ![]() The attachment of the activators or regulatory particles rearranges the blocking α subunit N-termini facilitating the entry of substrates. In the resting state, the N-termini of neighboring α subunits form a gate blocking access to the channel. Access to this chamber is through a narrow channel defined by the seven outer α subunits. The proteolytic active sites of the 26S proteasome are sequestered within the catalytic chamber of its 20S core particle (CP). ![]()
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