Structural transitions in conserved, ordered Beclin 1 domains essential to regulating autophagy [Cell Biology]

Beclin 1 (BECN1) is a key regulator of autophagy, a critical catabolic homeostasis pathway that involves the sequestration of cytoplasmic components by multilayered vesicles called autophagosomes, followed by lysosomal fusion and degradation. BECN1 is a core component of class III phosphatidylinositol-3-kinase complexes responsible for autophagosome nucleation. Without heterologous binding partners, BECN1 forms an antiparallel homodimer via its coiled-coil domain (CCD). However, the last 16 CCD residues, composing an overlap helix (OH), have been crystallized in two mutually exclusive states: either as part of the CCD or packed against the C-terminal lower case β-α-repeated, autophagy-specific domain (BARAD). Here, using circular dichroism (CD) spectroscopy, isothermal titration calorimetry (ITC), and small-angle X-ray scattering (SAXS), we show that in the homodimeric state, the OH transitions between these two different packing states, with the predominant state comprising the OH packed against the BARAD, contrary to expectations based on known BECN1 interactions with heterologous partners. We confirmed this observation by comparing the impact of mutating four residues that mediate packing of the OH against both the CCD and BARAD on structure and stability of the CCD, the OH+BARAD, and the two-domain CCD-BARAD. Lastly, we used cellular assays demonstrating that mutation of these OH-interface residues abrogates starvation-induced up-regulation of autophagy, but does not affect basal autophagy. In summary, we have identified a BECN1 helical region that transitions between packing as part of either one of two conserved domains, i.e. the CCD or the BARAD. Our findings have important implications for the relative stability of autophagy-inactive and autophagy-active BECN1 complexes.

from # All Medicine by Alexandros G. Sfakianakis via Alexandros G.Sfakianakis on Inoreader http://ift.tt/2vUmVoW
via IFTTT