Direct effects of ionizing radiation on integral membrane proteins. Noncovalent energy transfer requires specific interpeptide interactions

The 12 transmembrane alpha helices (TMHs) of human erythrocyte glucose transporter were individually cut by pepsin digestion as membrane-bound 2.5-3.5-kDa peptide fragments. Radiation-induced chemical degradation of these fragments showed an average target size of 34 kDa. This is 10-12 x larger than the average size of an individual TMH, demonstrating that a significant energy transfer occurs among these TMHs in the absence of covalent linkage. Heating this TMH preparation at 100 degrees C for 15 min reduced the target size to 5 kDa or less, suggesting that the noncovalent energy transfer requires specific helix-helix interactions. Purified phospholamban, a small (6-kDa) integral membrane protein containing a single TMH, formed a pentameric assembly in sodium dodecyl sulfate. The chemical degradation target size of this phospholamban pentamer was 5-6 kDa, illustrating that not all integral membrane protein assemblies permit intersubunit energy transfer. These findings together with other published observations suggest strongly that significant noncovalent energy transfer can occur within the tertiary and quaternary structure of membrane proteins and that as yet undefined proper molecular interactions are required for such covalent energy transfer. Our results with pepsin-digested glucose transporter also illustrate the importance of the interhelical interaction as a predominating force in maintaining the tertiary structure of a transmembrane protein.     

Copolymerization of pNcollagen III and collagen I. pNcollagen III decreases the rate of incorporation of collagen I into fibrils, the amount of collagen I incorporated, and the diameter of the fibrils formed

Previous observations suggested that pNcollagen III, the partially processed form of type III procollagen, coats fibrils of collagen I and thereby helps regulate the diameter of fibrils formed by collagen I. The previous observations, however, did not exclude the possibility that pNcollagen III was deposited on preformed collagen I fibrils after the fibrils were assembled. Here, mixtures of pNcollagen III and collagen I were generated simultaneously by enzymatic cleavage of precursor forms of the proteins. The results demonstrated that pNcollagen III forms true copolymers with collagen I. The presence of pNcollagen III both inhibited the rate at which collagen I assembled into fibrils and decreased the amount of collagen I incorporated into fibrils at steady-state equilibrium. In addition, the results demonstrated that copolymerization of pNcollagen III with collagen I generated fibrils that were thinner than fibrils generated under the same conditions from collagen I alone. Increasing the initial molar ratio of pNcollagen III to collagen I in the solution-phase increased the amount of pNcollagen III copolymerizing with collagen I and progressively decreased the diameter of the fibrils. Therefore, the copolymers were heterogeneous in that the stoichiometry of the two monomers in the fibrils varied. The results are consistent with a model in which pNcollagen III can regulate the diameter of collagen I fibrils by coating the surface of the fibrils and thereby allow tip growth but not lateral growth of the fibrils.     

Fusion of sphingomyelin vesicles induced by proteins from Taiwan cobra (Naja naja atra) venom. Interactions of zwitterionic phospholipids with cardiotoxin analogues

Egg sphingomyelin vesicles were used to assay aggregation/fusion activities of proteins from Taiwan (Naja naja atra) venom to avoid the problem of phospholipase A2 contamination during protein purification. It led to the identification of a new cardiotoxin (CTX) analogue protein (CTX V) with major aggregation/fusion, but few hemolysis, activities. On the contrary, cardiotoxin (CTX III) induced significant hemolysis of human red blood cells but exhibited few aggregation/fusion activities. To study the structure/activity relationship of these CTX-induced processes, the amino acid sequence of CTX V was determined and its aggregation/fusion activity was compared with that of CTX III by transmission electron microscopy, quasielastic laser light scattering, differential scanning calorimetry, and fluorescence spectroscopy. The results show that the CTX-induced fusion process at temperatures slightly above that of the gel to liquid-crystalline phase transition of sphingomyelin vesicles can ultimately convert small sonicated vesicles into large fused vesicles with sizes of 1-2 microns. The abilities of CTX V to induce the leakage of sphingomyelin vesicles content and to cause the fusion of vesicles are approximately 10-fold higher than those of CTX III. Based on the CTX structures determined in the present and other studies, it is suggested that the amino acid residue X within the well conserved sequence of -Cys-Pro-X-Gly-Lys-Gln-Leu-Cys- plays a role in the interaction of CTX with lipid molecules. The lipid phase transition could further enhance the protein-lipid interaction in the process leading to the fusion of vesicles.     

Alteration of N-terminal residues of mature human lysozyme affects its secretion in yeast and translocation into canine microsomal vesicles

Signal sequences play a central role in the initial membrane translocation of secretory proteins. Their functions depend on factors such as hydrophobicity and conformation of the signal sequences themselves. However, some characteristics of mature proteins, especially those of the N-terminal region, might also affect the function of the signal sequences. To examine this possibility, several mutants of human lysozyme modified in the N-terminal region of the mature protein were constructed, and their secretion in yeast as well as in vitro translocation into canine pancreatic microsomes were analyzed using an idealized signal sequence L8 (MR(L)8PLAALG). Our results show the following. (1) Change in the charge at the N-terminal residue of the mature protein does not affect secretion drastically. (2) Substitution of a proline residue at the N terminus prevents cleavage of the signal sequence, although translocation itself is not impaired. (3) Excessive positive charges in the N-terminal region delay translocation of the precursor protein across the membrane. (4) Polar and negatively charged residues introduced into the N-terminal region affect the secretion of the mature protein by preventing its correct folding.     

Identification of sphingomyelin turnover as an effector mechanism for the action of tumor necrosis factor alpha and gamma-interferon. Specific role in cell differentiation.

The biochemical signaling mechanisms involved in transducing the effects of tumor necrosis factor alpha (TNF alpha) and gamma-interferon (gamma-IFN) on leukemia cell differentiation are poorly defined. Recent studies established the existence of a sphingomyelin cycle that operates in response to the action of vitamin D3 on HL-60 cells and that may transduce the effects of vitamin D3 on cell differentiation (Okazaki, T., Bell, R., and Hannun, Y. (1989) J. Biol. Chem. 264, 19076-19080). The effects of TNF alpha and gamma-IFN on sphingomyelin turnover were determined, and the specificity and role of sphingomyelin hydrolysis in HL-60 human promyelocytic leukemia cells with 20% hydrolysis of sphingomyelin at 15 min, 40% hydrolysis at 30-60 min, and return to base line at 2 h. The hydrolyzed sphingomyelin (18 pmol/nmol total phospholipid) was accompanied by the concomitant generation of ceramide (11.2 pmol/nmol total phospholipid). gamma-IFN also caused reversible hydrolysis of sphingomyelin with onset at 1 h and peak effect at 2 h. This sphingomyelin cycle appeared to be specific to the monocytic pathway of HL-60 differentiation, since it was not activated by retinoic acid or dibutyryl cAMP, inducers of granulocytic differentiation, nor with phorbol myristate acetate, an inducer of macrophage-like differentiation. Addition of synthetic ceramide or bacterial sphingomyelinase induced monocytic differentiation of HL-60 cells. Cell-permeable ceramide also caused prompt down-regulation of mRNA for the c-myc protooncogene. The time course of c-myc down-regulation was consistent with the action of ceramide as the mediator of TNF alpha action. These results suggest that sphingomyelin turnover may be an important signaling mechanism transducing the actions of TNF alpha and gamma-IFN with specific function in cell differentiation.     

An approach to the optimization of preparation of bioprosthetic heart valves

The stress and strain states of the valve leaflets during fixation with glutaraldehyde affect their final mechanical parameters. Comparative studies of the stress-strain relationships of aortic valve leaflet strips from fresh, statically and dynamically fixed porcine and human valves were made. Static pressures of 5 mmHg, 16 mmHg, and 95 mmHg result in stress-strain relationships which are in a region between that of fresh porcine and fresh human leaflet strips in the circumferential direction, while they are far from that of fresh porcine tissue (larger strains) in the radial direction. Leaflet strips, fixed under dynamic loading between zero and a predefined maximum load, set at an early post-transition state, give parameters not significantly different from those of human valves.     

Purification and properties of an ethylene-forming enzyme from Pseudomonas syringae pv. phaseolicola PK2.

A novel ethylene-forming enzyme that catalyses the formation of ethylene from 2-oxoglutarate was purified from a cell-free extract of Pseudomonas syringae pv. phaseolicola PK2. It was purified about 2800-fold with an overall yield of 53% to a single band of protein after SDS-PAGE. The purified enzyme had a specific activity of 660 nmol ethylene min-1 (mg protein)-1. The molecular mass of the enzyme was approximately 36 kDa by gel filtration and 42 kDa by SDS-PAGE. The isoelectric point and optimum pH were 5.9 and ca. 7.0-7.5, respectively. There was no homology between the N-terminal amino acid sequence of the ethylene-forming enzyme of Ps. syringae pv. phaseolicola PK2 and the sequence of the ethylene-forming enzyme of the fungus Penicillium digitatum IFO 9372. However, the two enzymes have the following properties in common. The presence of 2-oxoglutarate, L-arginine, Fe2+ and oxygen is essential for the enzymic reaction. The enzymes are highly specific for 2-oxoglutarate as substrate and L-arginine as cofactor. EDTA, Tiron, DTNB [5,5'-dithio-bis(2-nitrobenzoate)] and hydrogen peroxide are all effective inhibitors.     

Interaction of CD2 with its ligand lymphocyte function-associated antigen-3 induces adenosine 3',5'-cyclic monophosphate production in T lymphocytes.

CD2 (T11, the T cell E receptor), a nonpolymorphic 47- to 55-kDa glycoprotein, is a T cell-specific surface protein that plays an important role in T lymphocyte adhesion, signal transduction, and differentiation. A natural ligand of CD2 is lymphocyte function associated Ag-3 (LFA-3 (CD58)), a widely expressed glycoprotein of 50 to 70 kDa. The physiologic interaction of CD2 with LFA-3 functions to increase intercellular adhesion and plays a role in T cell activation. This interaction, however, in the absence of other stimuli, has not previously been shown to induce intracellular signals such as Ca2+ mobilization or IL-2 production. To investigate whether cAMP may play a role in ligand-triggered CD2-mediated signal transduction, we have studied the ability of purified LFA-3 and anti-CD2 mAb to induce changes in intracellular cAMP content in murine Ag-specific T cell hybridomas that stably express wild-type and mutated human CD2 molecules. By using a RIA sensitive to the femtomolar range and specific for cAMP, we demonstrate that purified LFA-3, like anti-CD2 mAb, is capable of inducing marked, transient increases in the intracellular concentration of cAMP. Presentation of purified LFA-3, like anti-CD2 mAb, is capable of inducing marked, transient increases in the intracellular concentration of cAMP. Presentation of purified LFA-3 alone to CD2-expressing hybridoma cells, however, did not stimulate phosphatidylinositol turnover nor IL-2 production. The cytoplasmic domain of CD2 is necessary for these ligand-induced cAMP changes, demonstrating that LFA-3 binding to CD2 transduces a signal to the cell. Experiments using the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine showed that CD2-mediated regulation of cAMP levels occurs primarily by the stimulation of cAMP production rather than by the inhibition of cAMP degradation. These results demonstrate that the interaction of LFA-3 with CD2, in the absence of other stimuli, is capable of initiating intracellular biochemical changes and suggest that CD2/LFA-3 interactions may regulate T cell function at least in part through the generation of intracellular cAMP.     

Apparent convergence (at 2-monoacylglycerol level) of phosphatidic acid and 2-monoacylglycerol pathways of synthesis of chylomicron triacylglycerols

Dietary fats are converted into chylomicron triacylglycerols via the 2-monoacylglycerol and phosphatidic acid pathways of acylglycerol formation. In view of the known positional and fatty acid specificity of the acyltransferases, the triacylglycerol structures resulting from the two pathways would be expected to differ, but this has not been demonstrated. We have performed stereospecific analyses on the chylomicron triacylglycerols from rats fed menhaden oil and the corresponding fatty acid alkyl esters, which would be expected to be assimilated via the monoacylglycerol and the phosphatidic acid pathways, respectively. The results show a remarkable similarity between the two triacylglycerol types in the fatty acid composition of the sn-1 and sn-3 positions, along with marked differences in the composition of the sn-2 positions. The triacylglycerols from rats fed oil retained about 85% of the original fatty acids in the sn-2 position, including a high proportion of the long chain polyunsaturates (e.g., 5-7% 20:5 and 4-5% 22:6). The triacylglycerols from rats fed the alkyl ester contained large amounts of endogenous fatty acids in the sn-2 position (e.g., 18% 16:1, 14% 18:1, 14% 18:2, and 2.5% 20:4), which approximated the composition of the sn-2 position of the presumed phosphatidic acid intermediates. The sn-1 position contained a much higher proportion of polyunsatured fatty acids (e.g., 12-13% 20:5, 5-6% 22:6) than the sn-2 position (e.g. 2-3% 20:5, 0-0.6% 22:6) of triacylglycerols from rats fed the ester. We conclude that the chylomicron triacylglycerols arising via the 2-monoacylglycerol and the phosphatidic acid pathways differ mainly in the composition of the fatty acids in the sn-2 position. The similarity in the acids of the sn-1 and sn-3 positions of the chylomicron triacylglycerols from rats fed oil or ester is consistent with a hydrolysis of the acylglycerol products of the phosphatidic acid pathway to 2-monoacylglycerols prior to reconversion to triacylglycerols via the monoacylglycerol pathway and secretion as chylomicrons.     

Ejaculations induced by p-chloroamphetamine (PCA) in rats, hamsters and mice.

The ejaculatory response induced by p-chloroamphetamine (PCA) in male rats, hamsters and mice was observed during 2 hours after the injection. The animals were treated intraperitoneally with PCA at doses ranging from 0.78125 to 160 mg/kg. The ED50 (effective dose in 50% of animals) values of PCA for the initiation of ejaculation in rats and hamsters were 1.3397 (1.0732-1.6725) and 0.1105 (0.0802-0.1522) mg/kg, respectively. On the other hand, no ejaculation was observed in any mice at any doses examined. So we concluded that there are species differences in the ejaculatory response, induced by PCA, among rats, hamsters and mice.