Evolving Notch polyQ tracts reveal possible solenoid interference elements

Abstract

ABSTRACTPolyglutamine (polyQ) tracts in regulatory proteins are extremely polymorphic. As functional elements under selection for length, triplet repeats are prone to DNA replication slippage and indel mutations. Many polyQ tracts are also embedded within intrinsically disordered domains, which are less constrained, fast evolving, and difficult to characterize. To identify structural principles underlying polyQ tracts in disordered regulatory domains, here I analyze deep evolution of metazoan Notch polyQ tracts, which can generate alleles causing developmental and neurogenic defects. I show that Notch features polyQ tract turnover that is restricted to a discrete number of conserved “polyQ insertion slots”. Notch polyQ insertion slots are: (i) identifiable by an amphipathic “slot leader” motif; (ii) conserved as an intact C-terminal array in a 1-to-1 relationship with the N-terminal solenoid-forming ankyrin repeats (ARs); and (iii) enriched in carboxamide residues (Q/N), whose sidechains feature dual hydrogen bond donor and acceptor atoms. Correspondingly, the terminal loop and β-strand of each AR feature conserved carboxamide residues, which would be susceptible to folding interference by hydrogen bonding with residues outside the ARs. I thus suggest that Notch polyQ insertion slots constitute an array of AR interference elements (ARIEs). Notch ARIEs would dynamically compete with the delicate serial folding induced by adjacent ARs. Huntingtin, which harbors solenoid-forming HEAT repeats, also possesses a similar number of polyQ insertion slots. These results strongly suggest that intrinsically disordered interference arrays featuring carboxamide and polyQ enrichment are coupled proteodynamic modulators of solenoids.SIGNIFICANCENeurodegenerative disorders are often caused by expanded polyglutamine (polyQ) tracts embedded in the disordered regions of regulatory proteins, which are difficult to characterize structurally. To identify functional principles underlying polyQ tracts in disordered regulatory domains, I analyze evolution of the Notch protein, which can generate polyQ-related alleles causing neurodevelopmental defects. I show that Notch evolves polyQ tracts that come and go in a few conserved “polyQ insertion slots”. Several features suggest these slots are ankyrin repeat (AR) interference elements, which dynamically compete with the delicate solenoid formed by Notch. Huntingtin, whose polyQ expansions causes Huntington’s Disease in humans, also has solenoid-forming modules and polyQ insertion slots, suggesting a common architectural principle underlies solenoid-forming polyQ-rich proteins.