Fig. 6: Structural dynamics of the midnolin–proteasome mediated substrate turnover.

a MIDN may bind to substrates in the cytoplasm, and direct the complexes into the nucleus. The αHelix-C of MIDN binds to RPN1, promoting the interaction between MIDN and the 19S regulatory particle of the proteasome within the nucleus. Subsequently, the Ubl of MIDN binds to the proteasomal component RPN11, positioning the Catch domain above the ATPase ring to initiate substrate degradation. The substrate is translocated into the 20S core particle for degradation. After substrate translocation is complete, the disordered regions of MIDN guide itself into the 20S core particle for degradation. The proteasome then returns to a substrate-free state and continues to perform protein degradation functions within the nucleus. b Conformational dynamics of the AAA+ motor domain during substrate processing. Four sequential states of the human midnolin–26S proteasome reveal the mechanism by which ATP hydrolysis and Mg²⁺ release collaboratively drive substrate translocation. In the model, the AAA+ ATPase subunits are colored differently, with the bound substrate positioned within the central channel, illustrating a helical staircase arrangement and contacts between the RPT subunits and the substrate. Bound ATP is shown in orange, and ADP in cyan. The states are arranged in a cycle according to a logical sequence of events and transitions. Key events during proteasomal substrate degradation, including magnesium ion release, ATP hydrolysis (H), and ADP-to-ATP exchange (E), are indicated.