title: Single point mutations in the small cytoplasmic loop of ACA8, a plasma membrane Ca2+-ATPase of Arabidopsis thaliana, generate partially deregulated pumps.
abstract: ACA8 is a type 2B Ca(2+)-ATPase having a regulatory N terminus whose auto-inhibitory action can be suppressed by binding of calmodulin (CaM) or of acidic phospholipids. ACA8 N terminus is able to interact with a region of the small cytoplasmic loop connecting transmembrane domains 2 and 3. To determine the role of this interaction in auto-inhibition we analyzed single point mutants produced by mutagenesis of ACA8 Glu(252) to Asn(345) sequence. Mutation to Ala of any of six tested acidic residues (Glu(252), Asp(273), Asp(291), Asp(303), Glu(302), or Asp(332)) renders an enzyme that is less dependent on CaM for activity. These results highlight the relevance in ACA8 auto-inhibition of a negative charge of the surface area of the small cytoplasmic loop. The most deregulated of these mutants is D291A ACA8, which is less activated by controlled proteolysis or by acidic phospholipids; the D291A mutant has an apparent affinity for CaM higher than wild-type ACA8. Moreover, its phenotype is stronger than that of D291N ACA8, suggesting a more direct involvement of this residue in the mechanism of auto-inhibition. Among the other produced mutants (I284A, N286A, P289A, P322A, V344A, and N345A), only P322A ACA8 is less dependent on CaM for activity than the wild type. The results reported in this study provide the first evidence that the small cytoplasmic loop of a type 2B Ca(2+)-ATPase plays a role in the attainment of the auto-inhibited state.
authors.affiliation: Dipartimento di Biologia L. Gorini, Università di Milano, Istituto di Biofisica del Consiglio Nazionale delle Ricerche, Sezione di Milano, via G. Celoria 26, Milano 20133, Italy.
authors.forename: Tiziana
authors.initials: T
authors.lastname: Fusca
authors.forename: Maria Cristina
authors.initials: MC
authors.lastname: Bonza
authors.forename: Laura
authors.initials: L
authors.lastname: Luoni
authors.forename: Silvia
authors.initials: S
authors.lastname: Meneghelli
authors.forename: Claudia Adriana
authors.initials: CA
authors.lastname: Marrano
authors.forename: Maria Ida
authors.initials: MI
authors.lastname: De Michelis
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D002147:Calmodulin / Q000235:genetics / Q000378:metabolism; D002462:Cell Membrane / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics; D015971:Gene Expression Regulation, Enzymologic; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D017354:Point Mutation; D011485:Protein Binding; D016415:Sequence Alignment
chemical_list: D029681:Arabidopsis Proteins; D002147:Calmodulin; C508150:Aca8 protein, Arabidopsis; D000252:Calcium-Transporting ATPases