Polyalanine disease mutations impair UBA6-dependent ubiquitination

Abstract

Expansion mutations in polyalanine stretches are now associated with a growing number of human diseases with common genotypes and similar phenotypes. These similarities prompted us to query the normal function of physiological polyalanine stretches, and investigate whether a common molecular mechanism is involved in these diseases. Here, we show that UBA6, an E1 ubiquitin-activating enzyme, recognizes a polyalanine stretch within its cognate E2 ubiquitin-conjugating enzyme, USE1. Aberrations in this polyalanine stretch reduced ubiquitin transfer to USE1 and downstream target, the E3 ubiquitin ligase, E6AP. Intriguingly, we identified competition for the UBA6-USE1 interaction by various proteins with polyalanine expansion mutations in the disease state. In mouse primary neurons, the deleterious interactions of expanded polyalanine proteins with UBA6, alter the levels and ubiquitination-dependent degradation of E6AP, which in turn affected the levels of the synaptic protein, Arc. These effects could be observed in induced pluripotent stem cell-derived autonomic neurons from patients with polyalanine expansion mutations. Our results suggest a shared mechanism for such mutations, which may contribute to the congenital malformations seen in polyalanine diseases.