SPPL2a Inhibitor Binding Induces `TM6a` Disorder-to-Order Transition, Guiding Alzheimer's Drug Design.

Alzheimer's disease (AD) drug development has faced setbacks, notably the clinical trial failures of non-selective γ-secretase inhibitors. This has redirected focus towards more specific targets like signal peptide-peptidase-like 2a (SPPL2a), an intramembrane protease critical for processing TMEM106B, a protein implicated in AD pathology. Understanding the dynamic mechanisms of SPPL2a's substrate entry and inhibitor recognition is crucial to developing selective therapeutics that avoid the broad side effects seen with earlier γ-secretase inhibitors.

Researchers performed the first microsecond-time scale, all-atom molecular dynamics simulations of human SPPL2a in both its apo and inhibitor-bound states. This computational study also included a comparative analysis with the γ-secretase catalytic subunit, PS1. The simulations aimed to characterize the dynamic mechanisms governing substrate entry and inhibitor recognition, providing an atomistic view of protein conformational changes upon inhibitor binding.

Inhibitor binding to SPPL2a fundamentally remodels its interhelical contact network and quenches conformational sampling, leading to a significant disorder-to-order transition in the TM6a region, where helicity increased from 10% to 77%. The simulations revealed that SPPL2a is highly dynamic. They characterized the lateral gate dynamics of TM2, showing that inhibitor binding dramatically alters the protein's flexibility. A unique inhibitor-stabilized lipid hotspot was identified at Trp189. Furthermore, the study demonstrated that SPPL2a utilizes a distributed polar network for binding energy, a mechanism distinct from the concentrated aspartate-driven affinity observed in PS1. These findings provide a quantitative atomistic blueprint of SPPL2a dynamics, highlighting key structural differences that can be exploited for selective drug design.