title: Expression, purification and characterization of recombinant human choline acetyltransferase: phosphorylation of the enzyme regulates catalytic activity.
abstract: Choline acetyltransferase synthesizes acetylcholine in cholinergic neurons and, in humans, may be produced in 82- and 69-kDa forms. In this study, recombinant choline acetyltransferase from baculovirus and bacterial expression systems was used to identify protein isoforms by two-dimensional SDS/PAGE and as substrate for protein kinases. Whereas hexa-histidine-tagged 82- and 69-kDa enzymes did not resolve as individual isoforms on two-dimensional gels, separation of wild-type choline acetyltransferase expressed in insect cells revealed at least nine isoforms for the 69-kDa enzyme and at least six isoforms for the 82-kDa enzyme. Non-phosphorylated wild-type choline acetyltransferase expressed in Escherichia coli yielded six (69 kDa) and four isoforms (82 kDa) respectively. Immunofluorescent labelling of insect cells expressing enzyme showed differential subcellular localization with the 69-kDa enzyme localized adjacent to plasma membrane and the 82-kDa enzyme being cytoplasmic at 24 h. By 64 h, the 69-kDa form was in cytoplasm and the 82-kDa form was only present in nucleus. Studies in vitro showed that recombinant 69-kDa enzyme was a substrate for protein kinase C (PKC), casein kinase II (CK2) and alpha-calcium/calmodulin-dependent protein kinase II (alpha-CaM kinase), but not for cAMP-dependent protein kinase (PKA); phosphorylation by PKC and CK2 enhanced enzyme activity. The 82-kDa enzyme was a substrate for PKC and CK2 but not for PKA or alpha-CaM kinase, with only PKC yielding increased enzyme activity. Dephosphorylation of both forms of enzyme by alkaline phosphatase decreased enzymic activity. These studies are of functional significance as they report for the first time that phosphorylation enhances choline acetyltransferase catalytic activity.
authors.affiliation: Department of Physiology, Medical Sciences Building, University of Western Ontario, London, Ontario, Canada N6A 5C1.
authors.forename: T
authors.initials: T
authors.lastname: Dobransky
authors.forename: W L
authors.initials: WL
authors.lastname: Davis
authors.forename: G H
authors.initials: GH
authors.lastname: Xiao
authors.forename: R J
authors.initials: RJ
authors.lastname: Rylett
mesh_terms: D000818:Animals; D016367:Baculoviridae / Q000378:metabolism; D015153:Blotting, Western; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D047390:Casein Kinase II; D002384:Catalysis; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002795:Choline O-Acetyltransferase / Q000096:biosynthesis* / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D002847:Chromatography, Agarose; D003593:Cytoplasm / Q000378:metabolism; D015180:Electrophoresis, Gel, Two-Dimensional; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry; D007313:Insecta; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D010766:Phosphorylation; D020033:Protein Isoforms; D011493:Protein Kinase C / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000302:isolation & purification; D013997:Time Factors
chemical_list: D020033:Protein Isoforms; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; D002795:Choline O-Acetyltransferase; D047390:Casein Kinase II; D017346:Protein Serine-Threonine Kinases; D011493:Protein Kinase C; C517657:CAMK2A protein, human; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D017871:Calcium-Calmodulin-Dependent Protein Kinases