Plasma membrane calcium pump: structure, function and relationships

The plasma membrane Ca-pump (134 kDa) is stimulated by calmodulin and by other treatments (exposure to acidic phospholipids, treatments with proteases, phosphorylation by protein kinases A or C, self-association to form oligomers). It is the product of four genes (in humans), but additional isoforms originate through alternative mRNA spicing. Most of the pump mass protrudes into the cytoplasm with three main units. The calmodulin binding domain is located in the C-terminal protruding unit. The domain is a positively charged segment of about 25 residues. The calcium-activated protease calpain activates the pump by removing its calmodulin binding domain and the portion C-terminal to it. The-resulting 124 KDa fragment has been used to test the suggestion of an autoinhibitory function of the calmodulin binding domain. The latter interacts with two domains of the pump, one located close to the active site in the mid-cytoplasmic protruding unit, the other in the first (N-terminal) protruding unit. The isoforms of the pump show variations in the regulatory domains, e.g., alternative mRNA splicing can eliminate the domain phosphorylated by protein kinase A, or alter the sensitivity of the pump to calmodulin. This occurs by inserting sequences rich in. His between calmodulin binding subdomains A and B. The inserted domain(s) confer pH sensitivity to the binding of calmodulin. Calcium binding sites have been found in acidic regions preceding and following the calmodulin binding domai

Choice of profile for the wings of an airplane. Part I

The choice of the profile for the wings of an airplane is a problem which should be solved by a scientific method based on data obtained by systematic experimentation. The problem, in its present form, may be stated as follows: "To find a profile which has certain required aerodynamic characteristics and which encloses the spars, whose number, dimensions and separating distance are likewise determined by structural considerations." At present, the static test, corresponding to the case of accelerated flight at limited speed, requires the knowledge of the moment of the aerodynamic resultant at the angle of zero lift, and the possibility of controlling the magnitude of the corresponding absolute coefficient within more or less extensive limits

Choice of profile for the wings of an airplane. Part II

This report gives a general method for drawing airplane profiles. This method is useful, but it leads to a somewhat laborious drawing which becomes quite complicated when we take a transformation function having terms of a high degree

Effects of Monovalent and Divalent Cations on Ca2+ Fluxes Across Chromaffin Secretory Membrane Vesicles

Abstract: Bovine chromaffin secretory vesicle ghosts loaded with Na+ were found to take up Ca2+ when incubated in K+ media or in sucrose media containing micromolar concentrations of free Ca2+. Li+- or choline+loaded ghosts did not take up Ca2+. The Ca2+ accumulated by Na+-loaded ghosts could be released by the Ca2+ ionophore A23187, but not by EGTA. Ca2+ uptake was inhibited by external Sr2+, Na +, Li +, or choline +. All the 45Ca2+ accumulated by Na+-dependent Ca2+ uptake could be released by external Na +, indicating that both Ca2+ influx and efflux occur in a Na+-dependent manner. Na + -dependent Ca2+ uptake and release were only slightly inhibited by Mg2+. In the presence of the Na+ ionophore Monensin the Ca2+ uptake by Na +-loaded ghosts was reduced. Ca2+ sequestered by the Na+-dependent mechanism could also be released by external Ca2+ or Sr2+ but not by Mg2+, indicating the presence of a Ca2+/Ca2+ exchange activity in secretory membrane vesicles. This Ca2+/Ca2+ exchange system is inhibited by Mg2+, but not by Sr2+. The Na + -dependent Ca2+ uptake system in the presence of Mg2+ is a saturable process with an apparent Km of 0.28 μM and a Vmax= 14.5 nmol min−1 mg protein−1. Ruthenium red inhibited neither the Na+/Ca2+ nor the Ca2+/Ca2+ exchange, even at high concentrations

A theory of Plasma Membrane Calcium Pump stimulation and activity

The ATP-driven Plasma Membrane Calcium pump or Ca(2+)-ATPase (PMCA) is characterized by a high affinity to calcium and a low transport rate compared to other transmembrane calcium transport proteins. It plays a crucial role for calcium extrusion from cells. Calmodulin is an intracellular calcium buffering protein which is capable in its Ca(2+) liganded form of stimulating the PMCA by increasing both the affinity to calcium and the maximum calcium transport rate. We introduce a new model of this stimulation process and derive analytical expressions for experimental observables in order to determine the model parameters on the basis of specific experiments. We furthermore develop a model for the pumping activity. The pumping description resolves the seeming contradiction of the Ca(2+):ATP stoichiometry of 1:1 during a translocation step and the observation that the pump binds two calcium ions at the intracellular site. The combination of the calcium pumping and the stimulation model correctly describes PMCA function. We find that the processes of calmodulin-calcium complex attachment to the pump and of stimulation have to be separated. Other PMCA properties are discussed in the framework of the model. The presented model can serve as a tool for calcium dynamics simulations and provides the possibility to characterize different pump isoforms by different type-specific parameter sets.Comment: 24 pages, 6 figure