Insights into membrane binding of PROPPINs and Reconstitution of mammalian autophagic conjugation systems

by Ricarda Busse
Doctoral thesis
Date of Examination:2013-01-08
Date of issue:2013-06-24
Advisor:Dr. Karin Kühnel
Referee:Dr. Karin Kühnel
Referee:Prof. Dr. Michael Thumm
Persistent Address: http://dx.doi.org/10.53846/goediss-3900

 

 

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Abstract

English

Autophagy is a degradation pathway conserved in eukaryotes. Upon induction of autophagy a double layered membrane is formed de novo and engulfs the cytosolic content. After fusion of the membrane, an autophagosome vesicle is formed, which then fuses with the vacuole (or lysosome) where its content is degraded. PROPPINs, β-propeller proteins that bind polyphosphoinositides, play a role in autophagy and phosphoinositide binding depends on a conserved FRRG motif. The three yeast PROPPINs Atg18, Atg21 and Hsv2 are involved in different subtypes of autophagy. In this study, I purified different Atg18, Atg21 and Hsv2 PROPPIN homologs and showed that they bind specifically to PI3P and PI(3,5)P2 using protein-liposome co-flotation assays. Recently, we published the first structure of the PROPPIN Hsv2. Based on our structure I performed mutagenesis studies to probe phosphoinositide binding of Hsv2. I analyzed phosphoinositide binding of the alaninine mutants with liposome flotation assays. I identified conserved residues essential for binding right and left of the FRRG motif, indicating the presence of two phosphoinositide binding sites, which was an unexpected finding. Using ITC measurements I then confirmed the binding stoichiometry of two phosphoinositides to one Hsv2 molecule and determined the binding affinities of PROPPINs to both PI3P and PI(3,5)P2 incorporated in small unilamellar vesicles. Phosphoinositide binding of S. cerevisiae Hsv2 is pH dependent. Acidic environment increases and basic environment decreases the affinity. In addition, I showed the involvement of loop 6CD in membrane binding. Mutagenesis analysis of loop 6CD residues revealed that membrane insertion is dependent on both ionic and hydrophobic interactions. Two ubiquitin-like conjugation systems modifying Atg8 (in mammals MAP1LC3) and Atg12 are essential for autophagy. Homologs of the canonical ubiquitin conjugation system, E1- and E2-like enzymes, are involved in the conjugation of Atg8 and Atg12 to their specific targets phosphatidylethanolamine and Atg5, respectively. A in vivo reconstitution system for the two human ubiquitin-like conjugation systems Atg12 and MAP1LC3 was established using the MultiBac baculovirus expression system in insect cells. This allowed full length expression of the involved proteins and purification of the Atg5-Atg12 conjugate and lipidated MAP1LC3 in small yields.
Keywords: PROPPINs; Atg18; Atg21; Hsv2; phosphoinositides; beta-propeller; autophagy
 
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