HYPK scaffolds the Nedd8 and LC3 proteins to initiate formation of autophagosome around polyneddylated huntingtin exon1 aggregates

Abstract

Selective autophagy of protein aggregates is necessary for maintaining the cellular proteostasis. Several regulatory proteins play critical roles in this process. Here, we report that the huntingtin interacting protein K (HYPK) modulates the autophagic degradation of poly-neddylated huntingtin exon1 aggregates. HYPK functions as a scaffolding protein that binds to the Nedd8 and LC3 proteins. The C-terminal ubiquitin-associated (UBA) domain of HYPK binds to the Nedd8, whereas an N-terminal tyrosine-type (Y-type) LC3 interacting region (LIR) of HYPK binds to the LC3. Several conserved amino acids in the UBA domain of HYPK are necessary to mediate the efficient binding of HYPK to Nedd8. The autophagy inducing properties of HYPK are manifested by the increased lipidation of LC3 protein, increased expression of beclin-1 and ATG-5 proteins, and generation of puncta-like granules of LC3 in the HYPK overexpressing cells. Association of the ‘H-granules’ of HYPK with the poly-neddylated huntingtin exon1 aggregates results in the formation of autophagosome around the huntingtin exon1 aggregates, thereby clearing the aggregates by aggrephagy. Poly-neddylation of huntingtin exon1 is required for its autophagic degradation by HYPK. Thus, overexpression of Nedd8 also increases the basal level of cellular autophagy, other than maintaining the autophagy flux. The poly-neddylation dependent autophagic clearance of huntingtin exon1 by HYPK leads to better cell physiology and survival. Taken together, our study describes a novel mechanism of HYPK mediated autophagy of poly-neddylated huntingtin exon1 aggregates.

Direct Conjugation of NEDD8 to the N-Terminus of a Model Protein Can Induce Degradation

“…Alternatively, the misfolding of the substrate we detect in our model protein may create a situation that favors the removal of the misfolded protein by the autophagy pathway. While a few recent reports have demonstrated proteasome-mediated degradation of NEDDylated substrates [ 47 , 48 , 49 ], there is less evidence in the literature that NEDDylated proteins can be degraded by autophagy [ 86 ]. Future work will have to carefully evaluate if autophagy-mediated degradation of naturally NEDDylated proteins occurs or if this is an artifact of the misfolded nature of the model substrate.…”

The Many Potential Fates of Non-Canonical Protein Substrates Subject to NEDDylation

“…Indeed Ghosh et al have identified that the aggregate-prone form of Htt is NEDDylated and interacts with the UBA domain of huntingtin interacting protein K (HYPK). HYPK also interacts with the autophagy factor LC3 and together, induce the degradation of aggregated Huntington protein via the autophagy pathway [79]. Kim et al have recently shown that under stressful conditions which promote atypical NEDDylation in the cytoplasm, HDAC6 can directly interact with non-canonical targets of NEDDylation and induce aggregate formation [80].…”