title: Preformed Soluble Chemoreceptor Trimers That Mimic Cellular Assembly States and Activate CheA Autophosphorylation.
abstract: Bacterial chemoreceptors associate with the histidine kinase CheA and coupling protein CheW to form extended membrane arrays that receive and transduce environmental signals. A receptor trimers-of-dimers resides at each vertex of the hexagonal protein lattice. CheA is fully activated and regulated when it is integrated into the receptor assembly. To mimic these states in solution, we have engineered chemoreceptor cytoplasmic kinase-control modules (KCMs) based on the Escherichia coli aspartate receptor Tar that are covalently fused and trimerized by a foldon domain (Tar(FO)). Small-angle X-ray scattering, multi-angle light scattering, and pulsed-dipolar electron spin resonance spectroscopy of spin-labeled proteins indicate that the Tar(FO) modules assemble into homogeneous trimers wherein the protein interaction regions closely associate at the end opposite to the foldon domains. The Tar(FO) variants greatly increase the saturation levels of phosphorylated CheA (CheA-P), indicating that the association with a trimer of receptor dimers changes the fraction of active kinase. However, the rate constants for CheA-P formation with the Tar variants are low compared to those for autophosphorylation by free CheA, and net phosphotransfer from CheA to CheY does not increase commensurately with CheA autophosphorylation. Thus, the Tar variants facilitate slow conversion to an active form of CheA that then undergoes stable autophosphorylation and is capable of subsequent phosphotransfer to CheY. Free CheA is largely incapable of phosphorylation but contains a small active fraction. Addition of Tar(FO) to CheA promotes a planar conformation of the regulatory domains consistent with array models for the assembly state of the ternary complex and different from that observed with a single inhibitory receptor. Introduction of Tar(FO) into E. coli cells activates endogenous CheA to produce increased clockwise flagellar rotation, with the effects increasing in the presence of the chemotaxis methylation system (CheB/CheR). Overall, the Tar(FO) modules demonstrate that trimerized signaling tips self-associate, bind CheA and CheW, and facilitate conversion of CheA to an active conformation.
authors.affiliation: †Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Anna R
authors.initials: AR
authors.lastname: Greenswag
authors.affiliation: †Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Xiaoxiao
authors.initials: X
authors.lastname: Li
authors.affiliation: ‡Center for Advanced ESR Studies, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Peter P
authors.initials: PP
authors.lastname: Borbat
authors.affiliation: †Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Dipanjan
authors.initials: D
authors.lastname: Samanta
authors.affiliation: §Division of Microbiology and Molecular Genetics, Loma Linda University, Loma Linda, California 92350, United States.
authors.forename: Kylie J
authors.initials: KJ
authors.lastname: Watts
authors.affiliation: ‡Center for Advanced ESR Studies, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Jack H
authors.initials: JH
authors.lastname: Freed
authors.affiliation: †Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Brian R
authors.initials: BR
authors.lastname: Crane
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000071677:Histidine Kinase; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000072236:Methyl-Accepting Chemotaxis Proteins; D008780:Methyltransferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; C489103:CheW protein, E coli; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D000072236:Methyl-Accepting Chemotaxis Proteins; D011956:Receptors, Cell Surface; C057331:Tar protein, E coli; C000607606:cheY protein, E coli; C513923:CheR protein, E coli; D008780:Methyltransferases; D000071677:Histidine Kinase; C000607608:cheA protein, E coli