title: Two-photon cross-correlation analysis of intracellular reactions with variable stoichiometry.
abstract: We successfully demonstrate the effectiveness of two-photon fluorescence cross-correlation spectroscopy (TPCCS) to study the complex binding stoichiometry of calmodulin (CaM) and Ca(2+)/CaM-dependent protein kinase II (CaMKII). Practical considerations are made for developing an intracellular cross-correlation assay, including characterization of the fluorescent molecules involved, calibration procedures of the setup, and optimal measurement conditions. Potential pitfalls and artifacts are discussed, and the complex stoichiometry of the molecular system is accounted for by a new experimental and theoretical framework for TPCCS. Our tailored model accommodates up to 12 red-labeled CaMs binding to a single green-labeled dodecameric CaMKII holoenzyme and accounts for the probability distributions of bound ligand as well as the respective changes in fluorescence emission upon binding. The model was experimentally demonstrated both in solution and in living cells by analyzing the binding of Alexa 633(C2)CaM to eGFP-CaMKII under different biochemical conditions known to induce the basal, activated, and autophosphorylated forms of the enzyme. Key binding parameters, such as binding degree, concentrations of reactants, and binding affinities, were determined under varying conditions with certain assumptions. TPCCS thus offers the unique ability to test our biochemical understanding of protein dynamics in the intracellular milieu.
authors.affiliation: Institute of Biophysics, BioTec, Dresden University of Technology, Germany.
authors.forename: Sally A
authors.initials: SA
authors.lastname: Kim
authors.forename: Katrin G
authors.initials: KG
authors.lastname: Heinze
authors.forename: Kirsten
authors.initials: K
authors.lastname: Bacia
authors.forename: M Neal
authors.initials: MN
authors.lastname: Waxham
authors.forename: Petra
authors.initials: P
authors.lastname: Schwille
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000737:chemistry* / Q000378:metabolism*; D002147:Calmodulin / Q000737:chemistry* / Q000378:metabolism*; D002460:Cell Line; D004789:Enzyme Activation; D049452:Green Fluorescent Proteins / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D018613:Microscopy, Confocal; D008954:Models, Biological; D017785:Photons; D011485:Protein Binding; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D013050:Spectrometry, Fluorescence / Q000379:methods*; D014162:Transfection
chemical_list: D002147:Calmodulin; D008024:Ligands; D011993:Recombinant Fusion Proteins; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases