Ca2+ gradient and drugs reveal different binding sites for Pi and Mg2+ in phosphorylation of the sarcoplasmic reticulum ATPase.

The first step towards ATP synthesis by the Ca2-ATPase of sarcoplasmic reticulum is the phosphorylation of the enzyme by Pi. Phosphoenzyme formation requires both Pi and Mg2+. At 35 degrees C, the presence of a Ca2+ gradient across the vesicle membrane increases the apparent affinity of the ATPase for Pi more than 10-fold, whereas it had no effect on the apparent affinity for Mg2+. In the absence of a Ca2+ gradient, the phosphorylation reaction is inhibited by both K+ and Na+ at all Mg2+ concentrations used. However, in the presence of 1 mM Mg2+ and of a transmembrane Ca2+ gradient, the reaction is still inhibited by Na+, but the inhibition promoted by K+ is greatly decreased. When the Mg2+ concentration is raised above 2 mM, the enzyme no longer discriminates between K+ and Na+, and the phosphorylation reaction is equally inhibited by the two cations. Trifluoperazine, ruthenium red and spermidine were found to inhibit the phosphorylation reaction by different mechanisms. In the absence of a Ca2+ gradient, trifluoperazine competes with the binding to the enzyme of both Pi and Mg2+, whereas spermidine and ruthenium red were found to compete only with Mg2+. The data presented suggest that the enzyme has different binding sites for Mg2+ and for Pi.     

Modulation of human dermal fibroblast extracellular matrix metabolism by the lymphokine leukoregulin

The effect of leukoregulin, a 50-kD lymphokine with unique antitumor properties, was studied in vitro on several fibroblast functions. Leukoregulin did not inhibit fibroblast proliferation, as measured by cell enumeration and [3H]thymidine incorporation, and had no cytotoxic effect in terms of increased membrane permeability detected by trypan blue exclusion, two of the major leukoregulin actions on tumor cells. Leukoregulin induced a dose-dependent decrease in collagen synthesis, demonstrated by decreased [3H]proline incorporation into collagenase-digestible protein, as early as 6 h after the addition of the lymphokine to human fibroblasts. Leukoregulin inhibited the synthesis of both type I and type III collagen, as measured by SDS-PAGE and by specific radioimmunoassay. Neutralizing antibodies to interleukin-1 alpha, interleukin-1 beta, tumor necrosis factor-alpha, and interferon-gamma failed to alter the effect of leukoregulin on collagen synthesis, attesting that leukoregulin action was not due to contamination by these cytokines. Inhibition of collagen synthesis occurred concomitantly with increased secretion of prostaglandin E2 and a transient rise in intracellular cyclic AMP content, peaking at 6 h. However, blocking prostaglandin synthesis with indomethacin did not counteract inhibition of collagen synthesis by leukoregulin, demonstrating independence of this action of leukoregulin from cyclooxygenase metabolites. Leukoregulin also stimulated glycosaminoglycan production in a dose-dependent manner, affecting the synthesis of hyaluronic acid as the major fibroblast-derived extracellular glycosaminoglycan. In addition, secretion of neutral proteases (collagenase, elastase, caseinase) was increased. These observations indicate that leukoregulin is able to regulate synthesis of molecules critical to the deposition of the extracellular matrix by nontransformed nonmalignant fibroblasts.     

EMMPRIN/CD147, an MMP modulator in cancer, development and tissue repair

Matrix metalloproteinases (MMPs) play a central role in normal tissue remodeling and disease, they regulate tumor microenvironment and their expression is increased in most human cancers. Targeting their activity remains a major challenge. Their production and activation is tightly regulated by complex mechanisms that include cytokines and growth factors, cell-matrix and cell-cell interactions. The observations of increased MMP level at the epithelio-stromal interface led to the identification of EMMPRIN/CD147, a membrane spanning molecule highly expressed in tumor cells, that stimulates MMPs production in neighboring fibroblasts. Later studies have shown that EMMPRIN can also induce MMP in the same population of cells. Elevated EMMPRIN level was detected in numerous malignant tumors and has been correlated with tumor progression in experimental and clinical conditions. The presence and modulation of EMMPRIN in normal tissues associated with increased MMP expression suggests that this EMMPRIN-mediated MMP induction could be a common mechanism in non-tumoral physiological and/or pathological situations. Targeting EMMPRIN in cancer and other pathological conditions such arthritis and ulceration appears a promising future therapeutic strategy, but requires a better understanding of its mode of action and regulation. Potential regulators that influence EMMPRIN level and its MMP inducing activity include growth factors, hormones, glycosylation and membrane shedding. This review will discuss the recent findings concerning these diverse regulatory mechanisms in various physiological and pathological situations.     

Conformational states and thermodynamics of alpha-lactalbumin bound to membranes: a case study of the effects of pH, calcium, lipid membrane curvature and charge

The study of the conformational changes of bovine alpha-lactalbumin, switching from soluble states to membrane-bound states, deepens our knowledge of the behaviour of amphitropic proteins. The binding and the membrane-bound conformations of alpha-lactalbumin are highly sensitive to environmental factors, like calcium and proton concentrations, curvature and charge of the lipid membrane. The interactions between the protein and the membrane result from a combination of hydrophobic and electrostatic interactions and the respective weights of these interactions depend on the physicochemical conditions. As inferred by macroscopic as well as residue-level methods, the conformations of the membrane-bound protein range from native-like to molten globule-like states. However, the regions anchoring the protein to the membrane are similar and restricted to amphiphilic alpha-helices. H/(2)H-exchange experiments also yield residue-level data that constitute comprehensive information providing a new point of view on the thermodynamics of the interactions between the protein and the membrane.     

Does fusion of domains from unrelated proteins affect their folding pathways and the structural changes involved in their function? A case study with the diphtheria toxin T domain

We investigated whether the structural and functional behaviors of two unrelated protein domains were modified when fused. The IgG-binding protein ZZ derived from staphylococcal protein A was fused to the N- and/or C-terminus of the diphtheria toxin transmembrane domain (T). T undergoes a conformational change from a soluble native state at neutral pH to a molten globule-like state at acidic pH, leading to its interaction with membranes. We found that this molten globule state was not connected to the GdnHCl-induced unfolding pathway of T. The pH-induced transition of T, and also the unfolding of T and ZZ at neutral and acidic pH, were unchanged whether the domains were isolated or fused. The position of ZZ, however, influenced the solubility of T near its pK(i). SPR measurements revealed that T has a high affinity for membranes, isolated or within the fusion proteins (K(D)< 10(-11) M). This work shows that in the case of T and ZZ, the fusion of protein domains with different stabilities does not alter the structural changes involved in folding and function. This supports the use of T as a soluble membrane anchor.     

Cd2+- or Hg2+-binding proteins can replace the Cu+-chaperone Atx1 in delivering Cu+ to the secretory pathway in yeast

Copper delivery to Ccc2--the Golgi Cu+-ATPase--was investigated in vivo, replacing the Cu+-chaperone Atx1 by various structural homologues in an atx1-Delta yeast strain. Various proteins, displaying the same ferredoxin-like fold and (M/L)(T/S)CXXC metal-binding motif as Atx1 and known as Cu+-, Cd2+- or Hg2+-binding proteins were able to replace Atx1. Therefore, regardless of their original function, these proteins could all bind copper and transfer it to Ccc2, suggesting that Ccc2 is opportunistic and can interact with many different proteins to gain Cu+. The possible role of electrostatic potential surfaces in the docking of Ccc2 with these Atx1-homologues is discussed.