Fellow's research: Mapping sticky regions of a cell membrane protein: implications for neurodegenerative diseases


25 Mar 2019

Fellow's research: Mapping sticky regions of a cell membrane protein: implications for neurodegenerative diseases

 

Dr R Mahalakshmi, Intermediate Fellow

Indian Institute of Science Education and Research (IISER), Bhopal

Our recently published study identifies, for the first time, how specific intra-protein interactions can decide fate of the cell by regulating the switch between protein stability and aggregation.

Human Voltage-Dependent Anion Channels (hVDACs) are abundant in the mitochondria, the powerhouse of the cell, as outer membrane proteins. VDACs form pores in the outer mitochondrial membrane and are essential for the bidirectional transport of metabolites and ions across the membrane. Structurally, VDACs are 19-stranded transmembrane beta barrels with a voltage-sensor N-terminal helix. hVDACs interact with a number of proteins in the cell and thus form a robust interactome.

VDACs have been implicated in various neurodegenerative diseases. As consequence of their ability to interact with several proteins, they associate with alpha-synuclein, tau, and superoxide dismutase and give rise to neurodegenerative protein aggregates under adverse cellular conditions. Inhibition of the aggregation process could be an effective approach in delaying the onset of neurodegeneration.

In light of this, we sought out to identify the molecular factors that control the switch of VDACs from forming functional interactomes to forming aggregates resulting in disease states. Understanding this switch will contribute significantly to the design and development of targeted protein aggregation inhibitors.

We mapped the aggregation prone regions of hVDACs using a combinatorial approach that involves a systematic thiol replacement strategy coupled with high throughput spectroscopic and spectropolarimetric methods. We found that docking of the N-terminal helix on the barrel interior controls stability of hVDAC. We also found that the docking region, which is near strands beta7-beta9, is intrinsically aggregation prone. The aggregation propensity of these strands is suppressed by the docking of the N-helix. Dissociation of this N-helix under cellular stress (i.e. oxidative modifications of the protein due to build-up of peroxide and superoxide ions in the cell) or the association of aggregation-prone proteins (such as A-beta peptide, alpha-synuclein, tau, and superoxide dismutase) with the N-helix destabilizes the barrel of hVDAC. In addition, the hVDAC barrels showed aggregation via partially structured conformations. 

Stabilizing helix-barrel interaction (green) versus intrinsically destabilized zones (red) of hVDAC barrels. Cysteine residues are shown as yellow spheres.

Based on these findings, we propose a mechanism for the role of VDACs in the formation of neurodegenerative aggregates in the cell. The finding also allows us to conclude that the design of aggregation inhibitors based on the sequence of the N-terminal helix, for targeting strands beta7-beta9, will work to alleviate the progression of neurodegeneration.

Proposed mechanism of how the switch in hVDAC stability is effected under cellular stress, giving rise to mitochondrial dysfunction and neurodegeneration.

Reference:

Helix–strand interaction regulates stability and aggregation of the human mitochondrial membrane protein channel VDAC3. Ankit Gupta, Radhakrishnan Mahalakshmi. Journal of General Physiology. January 2019

Banner Credits: Structure of the Human Voltage-Dependent Anion Channel. 10.1073/pnas.0808115105