A new class of ubiquitin-Atg8 receptors involved in selective autophagy and polyQ protein clearance

Abstract

Selective ubiquitin-dependent autophagy mediates the disposal of superfluous cellular structures and clears cells of protein aggregates such as polyQ proteins linked to Huntington disease. Crucial selectivity factors of this pathway are ubiquitin-Atg8 receptors such as human SQSTM1/p62, which recognize ubiquitinated cargoes and deliver them to phagophores for degradation. Contrasting previous beliefs, we recently showed that ubiquitin-dependent autophagy is not restricted to higher eukaryotes but also exists in yeast with Cue5, harboring a ubiquitin-binding CUE domain, being a ubiquitin-Atg8 receptor. Notably, the human CUE domain protein TOLLIP is functionally similar to yeast Cue5, indicating that Cue5/TOLLIP (CUET) proteins represent a new and conserved class of autophagy receptors. Remarkably, both Cue5 in yeast and TOLLIP in human cells mediate efficient clearance of aggregation-prone polyQ proteins derived from human HTT/huntingtin. Indeed, TOLLIP is potentially more potent in polyQ clearance than SQSTM1/p62 in HeLa cells, suggesting that TOLLIP, also implicated in innate immunity, may be significant for human health and disease.

Keywords: Atg8; Cue5; TOLLIP; autophagy; huntingtin; ubiquitin.

Figure 1.

Scheme of ubiquitin-Atg8 receptors. S. cerevisiae Cue5 and human TOLLIP (CUET family) proteins possess CUE domains for ubiquitin recognition, whereas human SQSTM1/p62 and NBR1 (SQSTM1/p62 family) recognize ubiquitin via UBA domains. The relative positions of Atg8 (LC3) binding sites (yellow) and additional domains are shown (see text for TOLLIP).