The mechanism for the lysis pathway of double-stranded DNA bacteriophages involves a small hole-forming class of membrane proteins, the holins. This study focuses on a poorly characterized class of holins, the pinholin, of which the S 21 protein of phage φ21 is the prototype. Here we report the first in vitro synthesis of the wildtype form of the S21 pinholin, S 21 68, and negativedominant mutant form, S 21 IRS, both prepared using solid phase peptide synthesis and studied using biophysical techniques. Both forms of the pinholin were labeled with a nitroxide spin label and successfully incorporated into both bicelles and multilamellar vesicles which are membrane mimetic systems. Circular dichroism revealed the two forms were both >80% alpha helical, in agreement with the predictions based on the literature. The molar ellipticity ratio [θ] 222 / [θ] 208 for both forms of the pinholin was 1.4, suggesting a coiled-coil tertiary structure in the bilayer consistent with the proposed oligomerization step in models for the mechanism of hole formation. 31 P solid-state NMR spectroscopic data on pinholin indicate a strong interaction of both forms of the pinholin with the membrane headgroups. The 31 P NMR data has an axially symmetric line shape which is consistent with lamellar phase proteoliposomes lipid mimetics.

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Bacteriophages have evolved with an efficient host cell lysis mechanism to terminate the infection cycle and release the new progeny virions at the optimum time, allowing adaptation with the changing host and environment. Among the lytic proteins, holin controls the first and rate-limiting step of host cell lysis by permeabilizing the inner membrane at an allele-specific time known as "holin triggering". Pinholin S 21 is a prototype holin of phage Φ21 which makes many nanoscale holes and destroys the proton motive force, which in turn activates the signal anchor release (SAR) endolysin system to degrade the peptidoglycan layer of the host cell and destruction of the outer membrane by the spanin complex. Like many others, phage Φ21 has two holin proteins: active pinholin and antipinholin. The antipinholin form differs only by three extra amino acids at the N-terminus; however, it has a different structural topology and conformation with respect to the membrane. Predefined combinations of active pinholin and antipinholin fine-tune the lysis timing through structural dynamics and conformational changes. Previously, the dynamics and topology of active pinholin and antipinholin were investigated (Ahammad et al. JPCB 2019, 2020) using continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy. However, detailed structural studies and direct comparison of these two forms of pinholin S 21 are absent in the literature. In this study, the structural topology and conformations of active pinholin (S 21 68) and inactive antipinholin (S 21 68 IRS ) in DMPC (1,2-dimyristoyl-snglycero-3-phosphocholine) proteoliposomes were investigated using the four-pulse double electron−electron resonance (DEER) EPR spectroscopic technique to measure distances between transmembrane domains 1 and 2 (TMD1 and TMD2). Five sets of interlabel distances were measured via DEER spectroscopy for both the active and inactive forms of pinholin S 21 . Structural models of the active pinholin and inactive antipinholin forms in DMPC proteoliposomes were obtained using the experimental DEER distances coupled with the simulated annealing software package Xplor-NIH. TMD2 of S 21 68 remains in the lipid bilayer, and TMD1 is partially externalized from the bilayer with some residues located on the surface. However, both TMDs remain incorporated in the lipid bilayer for the inactive S 21 68 IRS form. This study demonstrates, for the first time, clear structural topology and conformational differences between the two forms of pinholin S 21 . This work will pave the way for further studies of other holin systems using the DEER spectroscopic technique and will give structural insight into these biological clocks in molecular detail.

We undertook a comprehensive assessment of odorant molecules, using an odorant-centric approach to understand the structural and electronic nature of these compounds. We tested odorants that were implicated in activating (strongly, moderately or weakly) functionally characterized olfactory receptors. The conformational space of these odorants were explored in vacuo; this was followed by unrestricted and targeted docking of these odorants in computational models of variants of these olfactory receptors (wild type and mutated). We also carried out extensive QSAR and 1d-and 3d-SDAR assessments of odorants known to activate ORs to identify nuanced electronicstructural features that are potentially responsible for odorant interactions. The use of drug-design ideations in chemoreception is seminal. 271-PosRegulation of Proton Transport in Tetrameric UCP2 by an Intramolecular Salt-Bridge Network