Single point mutations in the small cytoplasmic loop of ACA8, a plasma membrane Ca2+-ATPase of Arabidopsis thaliana, generate partially deregulated pumps.

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.

Dual mechanism of activation of plant plasma membrane Ca2+-ATPase by acidic phospholipids: evidence for a phospholipid binding site which overlaps the calmodulin-binding site.

The effect of phospholipids on the activity of isoform ACA8 of Arabidopsis thaliana plasma membrane (PM) Ca2+-ATPase was evaluated in membranes isolated from Saccharomyces cerevisiae strain K616 expressing wild type or mutated ACA8 cDNA. Acidic phospholipids stimulated the basal Ca2+-ATPase activity in the following order of efficiency: phosphatidylinositol 4-monophosphate > phosphatidylserine > phosphatidylcholine approximately = phosphatidylethanolamine approximately = 0. Acidic phospholipids increased V(max-Ca2+) and lowered the value of K(0.5-Ca2+) below the value measured in the presence of calmodulin (CaM). In the presence of CaM acidic phospholipids activated ACA8 by further decreasing its K(0.5-Ca2+) value. Phosphatidylinositol 4-monophosphate and, with lower efficiency, phosphatidylserine bound peptides reproducing ACA8 N-terminus (aa 1-116). Single point mutation of three residues (A56, R59 and Y62) within the sequence A56-T63 lowered the apparent affinity of ACA8 for phosphatidylinositol 4-monophosphate by two to three fold, indicating that this region contains a binding site for acidic phospholipids. However, the N-deleted mutant Delta74-ACA8 was also activated by acidic phospholipids, indicating that acidic phospholipids activate ACA8 through a complex mechanism, involving interaction with different sites. The striking similarity between the response to acidic phospholipids of ACA8 and animal plasma membrane Ca2+-ATPase provides new evidence that type 2B Ca2+-ATPases share common regulatory properties independently of structural differences such as the localization of the terminal regulatory region at the N- or C-terminal end of the protein.

The Genetic Basis of Composite Spike Form in Barley and 'Miracle-Wheat'.

Inflorescences of the tribe Triticeae, which includes wheat (Triticum sp. L.) and barley (Hordeum vulgare L.) are characterized by sessile spikelets directly borne on the main axis, thus forming a branchless spike. 'Compositum-Barley' and tetraploid 'Miracle-Wheat' (T. turgidum convar. compositum (L.f.) Filat.) display noncanonical spike-branching in which spikelets are replaced by lateral branch-like structures resembling small-sized secondary spikes. As a result of this branch formation 'Miracle-Wheat' produces significantly more grains per spike, leading to higher spike yield. In this study, we first isolated the gene underlying spike-branching in 'Compositum-Barley,' i.e., compositum 2 (com2). Moreover, we found that COM2 is orthologous to the branched head(t) (bh(t)) locus regulating spike branching in tetraploid 'Miracle-Wheat.' Both genes possess orthologs with similar functions in maize BRANCHED SILKLESS 1 (BD1) and rice FRIZZY PANICLE/BRANCHED FLORETLESS 1 (FZP/BFL1) encoding AP2/ERF transcription factors. Sequence analysis of the bh(t) locus in a collection of mutant and wild-type tetraploid wheat accessions revealed that a single amino acid substitution in the DNA-binding domain gave rise to the domestication of 'Miracle-Wheat.' mRNA in situ hybridization, microarray experiments, and independent qRT-PCR validation analyses revealed that the branch repression pathway in barley is governed through the spike architecture gene Six-rowed spike 4 regulating COM2 expression, while HvIDS1 (barley ortholog of maize INDETERMINATE SPIKELET 1) is a putative downstream target of COM2. These findings presented here provide new insights into the genetic basis of spike architecture in Triticeae, and have disclosed new targets for genetic manipulations aiming at boosting wheat's yield potential.