Cyrus R. Safinya , University of California-Santa Barbara, DMR 1101900

1. Stacking of Short DNA Controls Membrane Shape Evolution: The Gyroid Cubic-to-Inverted Hexagonal Transition in Lipid–Short DNA Assemblies Cyrus R. Safinya, University of California-Santa Barbara, DMR 1101900 Recent work from our group described a novel gyroid cubic QIIG,sRNA phase, which incorporates short functional RNA molecules within two intertwined and “continuously curved” water channels stabilized by lipids (see Fig. 1, Right, based on synchrotron x-ray scattering; J. Am. Chem. Soc.2010, 132, 16841; Langmuir 2011, 27, 7691). In the current studies we investigated the instability in the gyroid QIIG phase (precipitating a change in membrane shape) after complexation with short-DNA (sDNA) molecules with three types of end structure (“sticky” A-T (adenine–thymine) overhangs; blunt; and “nonsticky” T-T overhangs). Understanding the mechanisms which underlie membrane shape evolution is important because shape enables function in nature (e.g. the shape change leading to vesicle budding is used to transport molecules to distinct locations in cells). Figure 1. Modulation of the phase behavior (i.e. membrane shape) of Lipid–short DNA (sDNA) complexes by the stacking state of DNA rods (green). The schematic depicts the two observed phases, the inverted hexagonal phase (HIIC, left, with straight cylindrical micelles encapsulating DNA rods) and the gyroid cubic phase (QIIG, sDNA, right, with two highly curved water channels coated by lipids incorporating sDNA), and the conditions for their formation. Long sDNAs and sDNA stacks exceeding a critical length (LC) form complexes in the HIIC phase. The length of the sDNA stacks is controlled by temperature and end structure, which govern the end-to-end interactions. Decreasing the length below LC, by increasing the temperature or weakening end-to-end interactions by tuning the end structure, leads to formation of the QIIG, sDNA phase. (Adapted from Leal, C.; Ewert, K. K.; Bouxsein, N. F.; Shirazi, R. S.; Li, Y.; Safinya, C. R.: Stacking of Short DNA Induces the Gyroid Cubic-to-Inverted Hexagonal Phase Transition in Lipid–DNA Complexes. Manuscript under review) Our findings present a new paradigm for membrane shape control where coupling to short-DNA enables switching between the gyroid (QIIG) and the inverted hexagonal (HIIC) phase. X-ray scattering shows that the switch results from tunable end-to-end sDNA stacking interactions (see Fig. 1, manuscript under review). This is in contrast to shape remodeling in cells which is driven by association of lipids with curvature generating proteins.

2. Broader Impacts: Education and Outreach Research TrainingCyrus R. Safinya, University of California-Santa Barbara, DMR 1101900 Education: Undergraduate and graduate students, and postdoctoral scholars with backgrounds inmaterials science, physics, chemistry, and biology, are educated in methods to discover nature’s rules for assembling molecular building blocks in distinct shapes and sizes for particular functions. The learned concepts enable development of advanced materials for applications Outreach/Participation of undergraduate/underrepresented students: Bianca Salas (top photo) is an undergraduate student at UCSB working towards her B.S. degree in Chemistry and Biochemistry at UC-Santa Barbara. Her project is centered around understanding the mechanisms of self-assembly of positively charged liposomes (closed membrane shells) and anionic DNA. Her lipid-DNA complexes have the ability to cross the outer membrane of cells and deliver exogenous genes. She is currently mentored by Ramsey Majzoub(a physics PhD graduate student, top photo). The middle photo shows EkeneAkabike(left) standing next to Peter Chung (a physics PhD graduate student). Ekene is an undergraduate Chemistry and Biochemistry student mentored by Peter. Her experiments are designed to understand the biophysical functions of tau (a microtubule-associated-protein). Tau is involved in a range of biological functions, in particular, modulation of the dynamics of slow polymerization of tubulin (into microtubules, MTs) and rapid depolymerization of MTs.Ekene’s project involves both the preparation of tau protein (from plasmid expression) and the self-assembly of MTs in the presence of tau. Our group also plays host to first year graduate students who are in the very early stages of their research careers. The bottom photo shows NeerajaVenkateswaran(from the Biomolecular Science and Engineering program at UCSB) standing next to her mentor Dr. Bruno Silva (a Marie Curie Postdoctoral Scholar). Neeraja and Bruno were developing microfluidics methods for studying the response of liquid crystal phases to confinement and flowing conditions in the microfluidics chamber environment. (For more information see http://www.mrl.ucsb.edu/safinyagroup/undergrads.htm)