title: Functional consequences of alterations to hydrophobic amino acids located at the M4S4 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum.
abstract: Site-specific mutagenesis was used to investigate the functional roles of amino acids in the relatively hydrophobic sequence Ile-Thr-Thr-Cys-Leu-Ala-320, located at the M4S4 boundary of the sarcomplasmic reticulum Ca(2+)-ATPase. Each of the residues was replaced with either a less hydrophogic, a polar, or a charged residue. Mutants Ile-315----Arg and Leu-319----Arg were devoid of any Ca2+ transport function or ATPase activity, while the mutant Thr-317----Asp retained about 5 and 7% of the wild-type Ca2+ transport and ATPase activities, respectively. These three mutants were able to form the ADP-sensitive phosphoenzyme intermediate (E1P) by reaction with ATP, but this intermediate decayed very slowly to the ADP-insensitive phosphoenzyme intermediate (E2P). In the mutants Ile-315----Arg and Leu-319----Arg, the level of E2P formed in the backward reaction with inorganic phosphate was extremely low, but hydrolysis of E2P occurred at a normal rate. These mutants, in addition, displayed a higher apparent affinity for Ca2+ than the wild-type enzyme. In the mutants Ile-315----Ser and Ile-315----Asp, the Ca2+ transport and ATPase activities were moderately reduced to 30-40% of the wild-type activities, but normal affinities for Ca2+, Pi, and ATP were retained, as was the low affinity modulatory effect of ATP. Mutation of Thr-316 to Asp, Thr-317 to Ala, Cys-318 to Ala and Ala-320 to Arg had little or no effect on Ca2+ transport or ATPase activities. Introduction of two negative and one positive charge by triple mutation of the Ile-Thr-Thr-317 sequence created a mutant enzyme that, although completely inactive, was inserted into the membrane, consistent with a location of these residues on the cytoplasmic side of the M4S4 interface. Our findings suggest that the amphipathic character of the S4 helix and/or the distribution of charges in S4 is important for the stability of the E2P intermediate.
authors.affiliation: Danish Biomembrane Research Centre, Institute of Physiology, University of Aarhus.
authors.forename: B
authors.initials: B
authors.lastname: Vilsen
authors.forename: J P
authors.initials: JP
authors.lastname: Andersen
authors.forename: D H
authors.initials: DH
authors.lastname: MacLennan
mesh_terms: D000595:Amino Acid Sequence; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D000252:Calcium-Transporting ATPases; D002118:Calcium