Control of Protein Oligomerization and De-oligomerization on Lipid Membranes

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Ajay Mahalka Name of the doctoral dissertation Control of Protein Oligomerization and De-oligomerization on Lipid Membranes Publisher School of Science Unit Department of Biomedical Engineering and Computational Science Series Aalto University publication series DOCTORAL DISSERTATIONS 220/2013 Field of research Biomedical Engineering and Biophysics Manuscript submitted 4 October 2013 Date of the defence 24 January 2014 Permission to publish granted (date) 26 November 2013 Language English Monograph Article dissertation (summary + original articles) Abstract Oligomerization of protein into amyloid fibrils is central to the pathogenesis of several neurodegenerative disorders. Amyloid fibrillation and the cytotoxic actions of amyloids are membrane-associated processes. The interactions of amyloid-forming proteins with lipids at the membrane surface accelerate fibrillation and induce membrane permeabilization. Oligomerization also plays a functional role in antimicrobial defense and controls the catalytic activity of phospholipase A2 (PLA2). The protein oligomerization and amyloid formation can be modulated by heat shock protein 70 (Hsp70). Thus, the aim of the present work was to study membrane-associated protein oligomerization and its modulation by Hsp70 on the phospholipid model membrane system. Sequence analyses revealed that antimicrobial peptides (AMPs) contained sequence motifs that showed propensities for self-assembly, aggregation, and oligomerization into amyloid fibrils. The presence of such oligomerizationmediating sequences was characteristic of amyloidogenic cytotoxic proteins, including gelsolin involved in familial Finnish type amyloidosis (FAF). 1-Palmitoyl-2-(9'-oxo-nonanoyl)-snglycero-3-phosphocholine (PoxnoPC), an oxidized phospholipid, accelerated fibrillation of the core amyloidogenic segment of gelsolin.The PoxnoPC-mediated fibrillation of gelsolin was dependent on both the concentration and the aggregation state of PoxnoPC. Fibril growth followed simple nucleation-dependent kinetics with the formation of transient prefibrillar oligomers in the lag phase. Subsequently, in order to understand the functional role of membrane-associated Hsp70, we studied lipid-Hsp70 interactions. The association of Hsp70 with phospholipid membranes was highly dependent on their lipid compositions. Hsp70 associated with phosphatidylcholine bilayers and penetrated into the hydrocarbon region. In contrast to the above data, in the presence of negatively charged phospholipids, Hsp70 bound peripherally to membrane surfacesby extended phospholipid anchorage. A specific pH-dependent association of Hsp70 with bis(monoacylglycero)phosphate, an acidic phospholipid enriched in the inner lysosomal membrane, activated lysosomal acid sphingomyelinase and promoted cell survival. We also showed that the Hsp70 sustained the hydrolytic activity of PLA2 by modulating the oligomerization and transformation of PLA2 into amyloid fibers. Hsp70 attenuated the lysophosphatidylcholine-induced inhibition and amyloid formation of PLA2 in an ATPdependent manner. Finally, an oligomerization-mediating sequence in PLA2 was identified. Synthetic peptides corresponding to amyloidogenic, aggregation-promoting regions inhibited the hydrolytic activity of PLA2.