Biology subjects

Muok, A. R.

Publications and source records attributed to Muok, A. R..

4 recordsLinked to original sources

bioRxiv2026

Signaling mechanism of the transmembrane energy receptor Aer

The E. coli aerotaxis receptor Aer is a bacterial chemoreceptor that senses intracellular redox changes via an N terminal PAS domain bound to a flavin adenine dinucleotide (FAD) cofactor. Distinct from canonical methyl-accepting chemotaxis proteins (MCPs) such as Tar/Tsr, Aer lacks a periplasmic ligand-binding domain and adaptive methylation, transmitting conformational signals laterally from the PAS domain to the HAMP domain and the methylation helix-like cap (MHL cap) of the kinase control domain (KCD). To elucidate the Aer signalling mechanism, we determined cryo electron microscopy (cryo EM) structures of full length Aer in oxidized flavin quinone (kinase on) and reduced semiquinone (kinase off) states. Structural comparison reveals redox linked rearrangements of the FAD binding pocket, reorientation of PAS-HAMP interactions, and strikingly altered MHL cap stability. PAS-MHL-cap contact in the oxidized state compresses the receptor and stabilized proximal KCD helices, whereas reduction disrupts these contacts, increasing KCD flexibility. To probe distal effects on KCD architecture, we performed nanodisc reconstitution and pulse dipolar ESR spectroscopy on spin labelled positions along the four helix bundle. Distance distributions indicate redox dependent changes in helix separation, particularly at the C terminal MHL2 region, consistent with PAS driven loosening of KCD packing in kinase off states. These data support a model in which FAD redox chemistry reorganizes flavin pocket residues that in turn subtly alter PAS conformation to influence PAS-HAMP and PAS-MHL-cap packing and hence KCD conformational stability. The findings reveal an Aer specific signaling axis distinct from periplasmic ligand binding MCPs that has adapted MCP architecture for lateral PAS input and cytoplasmic redox sensing.

biochemistry↗

bioRxiv2022

A new class of protein sensor links spirochete pleomorphism, persistence, and chemotaxis

Pathogenic spirochetes can alter their morphologies and behaviors to infect and survive within their hosts. Previous reports demonstrate that the formation of so-called round bodies and biofilms, and chemotaxis are involved in spirochete pathogenesis. Here, in the spirochete Treponema denticola, we report a direct link between these cellular states that involves a new class of protein sensor (CheWS) with hitherto unclear function. Using cryo-EM methods, protein modeling, bioinformatics, genetics methods, and behavioral assays we demonstrate that spirochetes regulate these behaviors in response to the small molecule s-adenosylmethionine (SAM) via a SAM sensor that is anchored to chemotaxis arrays. CheWS influences chemotaxis, biofilm and round body formation under nonstressed conditions by a novel sporulation-like mechanism. Taken together, we establish an improved model for round body formation, we discovered a direct link between this SAM sensor and changes in cellular states, as well as characterized a new sensor class involved in chemotaxis.

microbiology↗

bioRxiv2020

Microbial piggy-back: how Streptomyces spores are transported by motile soil bacteria

Streptomycetes are sessile, soil-dwelling bacteria that produce diverse metabolites that impact plant health and the behavior of microbial communities. Emerging studies have demonstrated that Streptomyces spores are distributed through a variety of mechanisms, but it remains unclear how spores are transported to their preferred micro-environments, such as plant roots. Here, we show that Streptomyces spores are capable of utilizing the motility machinery of other soil bacteria and are transported on the centimeter scale. Motility assays and microscopy studies reveal that Streptomyces spores are transported to plant tissues by interacting directly with the flagella of both gram-positive and gram-negative bacteria. Genetics experiments demonstrate that this form of motility, called piggy-backing, is facilitated by conserved structural proteins present on the surface of Streptomyces spores. These results demonstrate that non-motile bacteria are capable of utilizing the motility machinery of other microbes to complete necessary stages of their lifecycle, and that this mode of transport may be ubiquitous in nature.

microbiology↗

bioRxiv2020

Spirochetes produce ordered chemoreceptor arrays of unusual composition, arrangement, and symmetry to compensate for a highly curved membrane

Abstract/SummaryThe prokaryotic chemotaxis system is arguably the best-understood signaling pathway in biology, but most insights have been obtained from only a few model organisms and many studies have relied on artificial systems that alter membrane curvature1-3. In all previously described species, chemoreceptors organize with the histidine kinase (CheA) and coupling protein (CheW) into a hexagonal (P6 symmetry) extended array that is considered universal among archaea and bacteria4,5. Here, for the first time, we report an alternative symmetry (P2) of the chemotaxis apparatus that emerges from a strict linear organization of CheA in Treponema denticola cells, which possesses arrays with the highest native curvature investigated thus far. Using cryo-ET, we reveal that the Td chemoreceptor arrays assume a truly unusual arrangement of the supra-molecular protein assembly that has likely evolved to accommodate the high membrane curvature. The arrays have several additional atypical features, such as an extended dimerization domain of CheA and a variant CheW-CheR-like fusion protein that is critical for maintaining an ordered chemosensory apparatus in an extremely curved cell. Furthermore, the previously characterized Td oxygen sensor ODP influences array integrity and its loss substantially orders CheA. These results suggest a greater diversity of the chemotaxis signaling system than previously thought and demonstrate the importance of examining transmembrane systems in vivo to retain native membrane curvature.

microbiology↗