Hakami Zanjani, A. A. | Search bioRxiv

bioRxiv2025

A Novel Model for Proton Transport Mediated by Uncoupling Protein 1

Uncoupling Protein 1 (UCP1) is a mitochondrial protein which drives thermogen-esis in brown adipose tissue. UCP1 facilitates the dissipation of the proton gradient as heat and plays a critical role in energy expenditure and metabolic regulation. We employ advanced molecular simulations and mutagenesis to reveal the mechanism of UCP1-mediated proton and fatty acid (FA) transport. We demonstrate that FAs bind spontaneously to UCP1s central substrate-binding site. In the binding site, a proton transfer to the FA is facilitated by a key aspartate residue (D28) and a coordinating water molecule. The protonated FA exits UCP1 through a well defined pathway, and releases its proton into the mitochondrial matrix. UCP1 then facilitates the return of deprotonated FAs to the intermembrane space. Nucleotide binding disrupts this mechanism by inducing conformational changes in the transmembrane helices and ob-structing the FA return pathway. Our mechanism explains every step of the transport cycle, is supported by simulation and biochemical data, and explains a diverse set of biochemical data about the transport mechanisms in UCP1 and its analogues: ANT, UCP2 and UCP3.

biophysics↗

bioRxiv2020

Annexin A4 trimers are recruited by high membrane curvatures in Giant Plasma Membrane Vesicles

The plasma membrane of eukaryotic cells consists of a crowded environment comprised of a high diversity of proteins in a complex lipid matrix. The lateral organization of membrane proteins in the plasma membrane (PM) is closely correlated with biological functions such as endocytosis, membrane budding and other processes which involve protein mediated shaping of the membrane into highly curved structures. Annexin A4 (ANXA4) is a prominent player in a number of biological functions including plasma membrane repair. Its binding to membranes is activated by Ca2+ influx and it is therefore rapidly recruited to the cell surface near rupture sites where Ca2+ influx takes place. However, the free edges near rupture sites can easily bend into complex curvatures and hence may accelerate recruitment of curvature sensing proteins to facilitate rapid membrane repair. To analyze the curvature sensing behavior of curvature inducing proteins in crowded membranes, we quantifify the affinity of ANXA4 monomers and trimers for high membrane curvatures by extracting membrane nanotubes from giant plasma membrane vesicles (GPMVs). ANXA4 is found to be a sensor of negative membrane curvatures. Multiscale simulations furthermore predicted that ANXA4 trimers generate membrane curvature upon binding and have an affinity for highly curved membrane regions only within a well defined membrane curvature window. Our results indicate that curvature sensing and mobility of ANXA4 depend on the trimer structure of ANXA4 which could provide new biophysical insight into the role of ANXA4 in membrane repair and other biological processes.

biophysics↗