Insight Into the Molecular Mechanisms of Membrane Geometry Generation and Recognition

Open Access
Author:
Gill, Richard Lee
Graduate Program:
Biochemistry and Molecular Biology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 27, 2014
Committee Members:
  • Fang Tian, Dissertation Advisor
  • Fang Tian, Committee Chair
  • Ira Joseph Ropson, Committee Member
  • John Michael Flanagan Jr., Committee Member
  • Faoud T Ishmael, Committee Member
  • James Robert Connor, Committee Member
Keywords:
  • Structural Biology
  • Protein-Lipid Interactions
  • NMR
  • Membrane Curvature Sensing
Abstract:
Membrane remodeling is an essential process in cell growth, division, intracellular vesicle transport, and many other essential biological phenomena. Furthermore, membrane geometry has recently been shown to regulate biological activity and determines the final cellular localization of some proteins. The aim of this thesis is to provide structural and molecular insights into the mechanism governing membrane geometry induction and recognition. SpoVM is a small peptide that was recently found to recognize and preferentially localize to the slightly curved outer surface of the forespore (diameter of curvature, R, ~1 μm) during Bacillus subtilis spore development. However, little was known about how this was accomplished. We determined that the SpoVM molecule adopted an atypical amphipathic helical structure in bilayer-like bicelles and that the helix is deeply embedded into the membrane. Our study challenges the current accepted model, which is based on circular dichroism and biochemical data, and provides structural support that SpoVM uses a new molecular mechanism to sense small membrane curvature. Recent studies have linked protein intracellular trafficking to human diseases. In Alzheimer’s Disease (AD), a type I membrane protein referred to as LR11 (SorLA) is a key regulator of the amyloid precursor protein (APP) trafficking. LR11 diverts APP away from amyloidogenic processing to Aβ peptides. The accumulation of Aβ in the brain is generally accepted as the primary cause of AD. We discovered a membrane proximal amphipathic α-helix in LR11 C-terminal domain and, moreover, this helix deforms the membrane mimic liposomes. Since changes in membrane iv geometry are inherent to trafficking events, we postulate that this helix may sense and/or induce the bending of the membrane during vesicle biogenesis to facilitate LR11 intracellular transport. Amphipathic helices play a myriad of functions in mediating protein-lipid interactions. Despite their structural simplicity, several key properties including charge, the physiochemical characteristics of the hydrophobic vs. hydrophilic face, hydrophobicity, helical length, membrane orientation, and insertion depth, can be fine-tuned for their biological functions. Our study highlights the importance to quantitatively dissect these parameters in order to mechanistically understand their functions in membrane remodeling and ultimately to be able to manipulate these properties for therapeutic uses such as in designing antimicrobial and cell penetrating peptides.