Identification of BECN1 and ATG14 coiled-coil interface residues important for starvation-induced autophagy

Abstract

Autophagy, an essential eukaryotic homeostasis pathway, enables sequestration of unwanted, damaged or harmful cytoplasmic components in vesicles called autophagosomes, enabling subsequent lysosomal degradation and nutrient recycling. Autophagosome nucleation is mediated by Class III phosphatidylinositol 3-kinase complexes that include two key autophagy proteins, BECN1/Beclin 1 and ATG14/BARKOR, which form parallel heterodimers via their coiled-coil domains (CCDs). Here we present the 1.46 Å X-ray crystal structure of the anti-parallel, human BECN1 CCD homodimer, which represents BECN1 oligomerization outside the autophagosome nucleation complex. We use circular dichroism and small-angle X-ray scattering (SAXS) to show that the ATG14 CCD is significantly disordered, but becomes more helical in the BECN1:ATG14 heterodimer, although it is less well-folded than the BECN1 CCD homodimer. SAXS also indicates that the BECN1:ATG14 heterodimer is more curved than other BECN1-containing CCD dimers, which has important implications for the structure of the autophagosome nucleation complex. A model of the BECN1:ATG14 CCD heterodimer that agrees well with the SAXS data shows that BECN1 residues at the homodimer interface are also responsible for homodimerization, enabling us to identify ATG14 interface residues. Lastly, we verify the role of BECN1 and ATG14 interface residues in binding by assessing the impact of point mutations of these residues on coimmunoprecipitation of the partner, and demonstrate that these mutations abrogate starvation-induced up-regulation of autophagy, but do not impact basal autophagy. Thus, this research provides insights into structures of the BECN1 CCD homodimer and the BECN1:ATG14 CCD heterodimer, and identifies interface residues important for BECN1:ATG14 heterodimerization and for autophagy.

Graphical abstract