Nitric oxide (NO) signaling in biology relies on its activating cyclic guanosine monophosphate (cGMP) production by the NO receptor soluble guanylyl cyclase (sGC). sGC must obtain heme and form a heterodimer to become functional, but paradoxically often exists as an immature heme-free form in cells and tissues. Based on our previous finding that NO can drive sGC maturation, we investigated its basis by utilizing a fluorescent sGC construct whose heme level can be monitored in living cells. We found that NO generated at physiologic levels quickly triggered cells to mobilize heme to immature sGC. This occurred when NO was generated within cells or by neighboring cells, began within seconds of NO exposure, and led cells to construct sGC heterodimers and thus increase their active sGC level by several-fold. The NO-triggered heme deployment involved cellular glyceraldehyde-3-phosphate dehydrogenase (GAPDH)–heme complexes and required the chaperone hsp90, and the newly formed sGC heterodimers remained functional long after NO generation had ceased. We conclude that NO at physiologic levels triggers assembly of its own receptor by causing a rapid deployment of cellular heme. Redirecting cellular heme in response to NO is a way for cells and tissues to modulate their cGMP signaling and to more generally tune their hemeprotein activities wherever NO biosynthesis takes place.

The bacteriophage infection cycle plays a crucial role in recycling the world's biomass. Bacteriophages devise various cell lysis systems to strictly control the length of the infection cycle for an efficient phage life cycle. Phages evolved with lysis protein systems, which can control and fine-tune the length of this infection cycle depending on the host and growing environment. Among these lysis proteins, holin controls the first and ratelimiting step of host cell lysis by permeabilizing the inner membrane at an allele-specific time and concentration hence known as the simplest molecular clock. Pinholin S 21 is the holin from phage Φ21, which defines the cell lysis time through a predefined ratio of active pinholin and antipinholin (inactive form of pinholin). Active pinholin and antipinholin fine-tune the lysis timing through structural dynamics and conformational changes. Previously we reported the structural dynamics and topology of active pinholin S 21 68. Currently, there is no detailed structural study of the antipinholin using biophysical techniques. In this study, the structural dynamics and topology of antipinholin S 21 68 IRS in DMPC proteoliposomes is investigated using electron paramagnetic resonance (EPR) spectroscopic techniques. Continuous-wave (CW) EPR line shape analysis experiments of 35 different R1 side chains of S 21 68 IRS indicated restricted mobility of the transmembrane domains (TMDs), which were predicted to be inside the lipid bilayer when compared to the N-and C-termini R1 side chains. In addition, the R1 accessibility test performed on 24 residues using the CW-EPR power saturation experiment indicated that TMD1 and TMD2 of S 21 68 IRS were incorporated into the lipid bilayer where N-and C-termini were located outside of the lipid bilayer. Based on this study, a tentative model of S 21 68 IRS is proposed where both TMDs remain incorporated into the lipid bilayer and N-and C-termini are located outside of the lipid bilayer. This work will pave the way for the further studies of other holins using biophysical techniques and will give structural insights into these biological clocks in molecular detail.

No abstract

Calcium (Ca2þ) dysregulation is a hallmark of heart failure and is characterized by impaired Ca 2þ sequestration into the sarcoplasmic reticulum (SR) by the SR-Ca 2þ -ATPase (SERCA). Regulins, single-pass membrane proteins, bind and inhibit SERCA by allosterically modulating the affinity of its Ca 2þ binding sites. DWarf Open Reading Frame (DWORF) is a 35 amino acid regulin that enhances SERCA2a activity in cardiomyocytes. As a first step to understanding DWORF regulation of SERCA, we used Oriented-Sample Solid-State NMR spectroscopy (OS-ssNMR) and computational methods to determine the structure and topology of DWORF in liquid crystalline lipid bilayers. We found that DWORF adopts a helical structure, with a pronounced kink at the N-terminus. Restrained molecular dynamics calculations using backbone dipolar couplings and chemical shift anisotropy show the dynamic topology of DWORF, with the N-terminal helix spanning tilt angles ranging from 50-60 degrees and C-terminal helix ranging from 30-40 degrees. The two helical domains are anchored to both membrane leaflets by polar residues. This topology differs from that of phospholamban, a main regulator of SERCA, and might explain the opposite effects on the ATPase's apparent affinity for Ca 2þ ions of these two regulins.

No abstract