Induction of intracellular and secreted acid phosphatases (APases) is a widespread response of orthophosphate (Pi)-starved (2Pi) plants. APases catalyze Pi hydrolysis from a broad range of phosphomonoesters at an acidic pH. The largest class of nonspecific plant APases is comprised of the purple APases (PAPs). Although the biochemical properties, subcellular location, and expression of several plant PAPs have been described, their physiological functions have not been fully resolved. Recent biochemical studies indicated that AtPAP26, one of 29 PAPs encoded by the Arabidopsis (Arabidopsis thaliana) genome, is the predominant intracellular APase, as well as a major secreted APase isozyme up-regulated by 2Pi Arabidopsis. An atpap26 T-DNA insertion mutant lacking AtPAP26 transcripts and 55-kD immunoreactive AtPAP26 polypeptides exhibited: (1) 9-and 5-fold lower shoot and root APase activity, respectively, which did not change in response to Pi starvation, (2) a 40% decrease in secreted APase activity during Pi deprivation, (3) 35% and 50% reductions in free and total Pi concentration, respectively, as well as 5-fold higher anthocyanin levels in shoots of soil-grown 2Pi plants, and (4) impaired shoot and root development when subjected to Pi deficiency. By contrast, no deleterious influence of AtPAP26 loss of function occurred under Pi-replete conditions, or during nitrogen or potassium-limited growth, or oxidative stress. Transient expression of AtPAP26-mCherry in Arabidopsis suspension cells verified that AtPAP26 is targeted to the cell vacuole. Our results confirm that AtPAP26 is a principal contributor to Pi stress-inducible APase activity, and that it plays an important role in the Pi metabolism of 2Pi Arabidopsis.Orthophosphate (Pi) is an essential plant macronutrient required for many pivotal metabolic processes such as photosynthesis and respiration. However, the massive use of Pi fertilizers in agriculture demonstrates how the free Pi level of many soils is suboptimal for plant growth. The world's reserves of rock phosphate, our major source of Pi fertilizers, are projected to be depleted by the end of this century (Vance et al., 2003). Furthermore, Pi runoff from fertilized fields into nearby surface waters results in environmentally destructive processes such as aquatic eutrophication and blooms of toxic cyanobacteria. Effective biotechnological strategies are needed to engineer Pi-efficient transgenic crops to ensure agricultural sustainability and a reduction in Pi fertilizer overuse. This necessitates a detailed understanding of Pi-starvation-inducible (PSI) gene expression and the complex morphological, physiological, and biochemical adaptations of Pi-deficient (2Pi) plants.A well-documented component of the plant Pi stress response is the up-regulation of intracellular and secreted acid phosphatases (APases; E.C. 3.1.3.2) that catalyze the hydrolysis of Pi from various phosphate monoesters and anhydrides in the acidic pH range (Tran et al., 2010a). APase induction by 2Pi plants has been correlated wit...

Ferlins are large multi-C2 domain membrane proteins involved in membrane fusion and fission events. In this study we investigate the effects binding of the C2 domains of otoferlin, dysferlin and myoferlin have upon the structure of lipid bilayers. Fluorescence measurements indicate that multi-C2 domain constructs of myoferlin, dysferlin and otoferlin change the lipid packing of both small unilamellar vesicles and giant plasma membrane vesicles. The activities of these proteins were enhanced in the presence of calcium, and required negatively charged lipids like phosphatidylserine or phosphatidylglycerol for activity. Experiments on individual domains uncovered functional differences between the C2A domain of otoferlin as compared to dysferlin and myoferlin, and truncation studies suggest that the effects of each subsequent C2 domain on lipid ordering appear additive. Finally, we demonstrate that the activities of these proteins on membranes are insensitive to high salt concentrations, suggesting a non-electrostatic component to the interaction between ferlin C2 domains and lipid bilayers. Together, the data indicate that dysferlin, otoferlin, and myoferlin do not merely passively adsorb to membranes, but actively sculpt lipid bilayers, which would result in highly curved or distorted membrane regions that could facilitate membrane fusion, fission, or recruitment of other membrane trafficking proteins.

Dysferlin is a large membrane protein involved in calcium-triggered resealing of the sarcolemma after injury. Although it is generally accepted that dysferlin is Ca(2+) sensitive, the Ca(2+) binding properties of dysferlin have not been characterized. In this study, we report an analysis of the Ca(2+) and membrane binding properties of all seven C2 domains of dysferlin as well as a multi-C2 domain construct. Isothermal titration calorimetry measurements indicate that all seven dysferlin C2 domains interact with Ca(2+) with a wide range of binding affinities. The C2A and C2C domains were determined to be the most sensitive, with Kd values in the tens of micromolar, whereas the C2D domain was least sensitive, with a near millimolar Kd value. Mutagenesis of C2A demonstrates the requirement for negatively charged residues in the loop regions for divalent ion binding. Furthermore, dysferlin displayed significantly lower binding affinity for the divalent cations magnesium and strontium. Measurement of a multidomain construct indicates that the solution binding affinity does not change when C2 domains are linked. Finally, sedimentation assays suggest all seven C2 domains bind lipid membranes, and that Ca(2+) enhances but is not required for interaction. This report reveals for the first time, to our knowledge, that all dysferlin domains bind Ca(2+) albeit with varying affinity and stoichiometry.

The chloroplast signal recognition particle (cpSRP) and its receptor, chloroplast FtsY (cpFtsY), form an essential complex with the translocase Albino3 (Alb3) during post-translational targeting of light-harvesting chlorophyll-binding proteins (LHCPs). Here, we describe a combination of studies that explore the binding interface and functional role of a previously identified cpSRP43-Alb3 interaction. Using recombinant proteins corresponding to the C terminus of Alb3 (Alb3-Cterm) and various domains of cpSRP43, we identify the ankyrin repeat region of cpSRP43 as the domain primarily responsible for the interaction with Alb3-Cterm. Furthermore, we show Alb3-Cterm dissociates a cpSRP⅐LHCP targeting complex in vitro and stimulates GTP hydrolysis by cpSRP54 and cpFtsY in a strictly cpSRP43-dependent manner. These results support a model in which interactions between the ankyrin region of cpSRP43 and the C terminus of Alb3 promote distinct membrane-localized events, including LHCP release from cpSRP and release of targeting components from Alb3.Mitochondrial inner membranes and chloroplast thylakoid membranes are densely populated with protein complexes vital to the production of metabolic energy. For both membrane systems, biogenesis requires specialized protein sorting and integration systems, which localize nucleus-and organelle-encoded proteins to the target membrane. Consistent with the prokaryotic origin of mitochondria and chloroplasts, protein insertion into their energy-generating membranes is accomplished via the action of Oxa1p and Albino3 (Alb3), respectively, which belong to a family of protein insertases that includes YidC in bacteria (1-6).Although YidC/Oxa1p/Alb3 homologues vary dramatically in length (225-795 residues), all share a conserved hydrophobic core of about 200 residues (2) that extends across five transmembrane domains leaving the C terminus exposed to the cytoplasm, matrix, or stroma, respectively. Complementation studies demonstrated that the core regions of both Oxa1p and Alb3 functionally replace the core of YidC to insert membrane proteins via a "YidC only" pathway (7,8). Similarly, a chimera of YidC fused with the C-terminal ribosome-binding domain of Oxa1p was useful in demonstrating that the core region of YidC can functionally replace the core region of Oxa1p (9). These experimental results show that the core regions of YidC/ Oxa1p/Alb3 are at least partially interchangeable and house the capacity for assisting membrane protein transition into adjacent bilayers. They also support the possibility that a conserved function of the YidC/Oxa1p/Alb3 C terminus is to bind soluble targeting machinery. For example, the hydrophilic C-terminal extension of Oxa1p forms an ␣-helical domain essential for interacting with the ribosome during cotranslational integration (10, 11). Like Oxa1p, Alb3 contains a hydrophilic C-terminal extension that may play a critical role in protein targeting (12, 13). Alb3 works in conjunction with a post-translational chloroplast signal recognition particle (cpSRP)...