Molecular Basis of Q-length Selectivity for the MW1 Antibody-Huntingtin Interaction.

Huntington's disease is caused by a polyglutamine (polyQ) expansion in the huntingtin protein. Huntingtin exon 1 (Httex1), as well as other naturally occurring N-terminal hun-tingtin fragments with expanded polyQ are prone to aggrega-tion, forming potentially cytotoxic oligomers and fibrils. Antibodies and other N-terminal huntingtin binders are widely explored as biomarkers and possible aggregation-inhibiting therapeutics. A monoclonal antibody, MW1, is known to preferentially bind to huntingtin fragments with expanded polyQ lengths, but the molecular basis of the polyQ length specificity remains poorly understood. Using solution NMR, electron paramagnetic resonance, and other biophysical methods, we investigated the structural features of the Httex1- MW1 interaction. Rather than recognizing residual alpha-helical structure, which is promoted by expanded Q-lengths, MW1 caused the formation of a new, non-native, conformation in which the entire polyQ is largely extended. This non-native polyQ structure allowed the formation of large mixed Httex1-MW1 multimers (600-2900 kD), when Httex1 with pathogenic Q-length (Q46) was used. We propose that these multivalent, entropically favored interactions, are available only to proteins with longer Q-lengths and represent a major factor governing the Q-length preference of MW1. The present study reveals that it is possible to target proteins with longer Q-lengths without having to stabilize a natively favored conformation. Such mechanisms could be exploited in the design of other Q-length specific binders.