Structural transitions in Orb2 prion-like domain relevant for functional aggregation in memory consolidation

Abstract

The recent structural elucidation of ex vivo Drosophila Orb2 fibrils revealed a novel amyloid formed by interdigitated Gln and His residue side chains belonging to the prion-like domain. However, atomic-level details on the conformational transitions associated with memory consolidation remain unknown. Here, we have characterized the nascent conformation and dynamics of the prion-like domain (PLD) of Orb2A using a nonconventional liquid-state NMR spectroscopy strategy based on 13C detection to afford an essentially complete set of 13C, 13C{beta}, 1H and backbone 13CO and 15N assignments. At pH 4, where His residues are protonated, the PLD is disordered and flexible, except for a partially populated -helix spanning residues 55-60. At pH 7, in contrast, His residues are predominantly neutral and the Q/H segments adopt minor populations of helical structure, show decreased mobility and start to self-associate. At pH 7, the His residues also bind Zn++, which promotes further association. These findings represent a remarkable case of structural plasticity, based on which an updated model for Orb2A functional amyloidogenesis is advanced. Highlights{middle dot} The Orb2 prion like domain that forms the structures related to memory consolidation is studied by solution NMR. {middle dot} The amyloidogenic Q/H-rich stretch is disordered and flexible at low pH. {middle dot} Residues 55-60 form a partly populated -helix at pH 4. {middle dot} At pH 7, the Q/H-rich segment also adopts a low population of -helix and rigidifies. {middle dot} Zn++ binding induces associative changes in the Orb2 prion-like domain.