摘要:

The binding of Glyceraldehyde−3−phosphate dehydrogenase (GAPDH; E.C. 1.2.1.12) to microtubules causes the microtubules to assemble into large bundles. This bundling can be considered as a further step in the assembly of supramolecular structures.The rate of bundle formation, after addition of GAPDH to preformed microtubules, is not dependent on the GAPDH concentration and reflects bundling kinetics.Bundle disassembly can be studied by the addition of 1 mM adenosine 5’−(&bgr;, −imidotri−phosphate) (AMPPNP) to bundled microtubules, and is extremely fast.Bundling reduces the rate of association of tubulin dimers to microtubules, as well as the dissocation from the microtubles. Both rates are reduced to the same extent. This is in agreement with the fact that the critical concentration of tubulin is practically not influenced by the binding of the enzyme.Adding microtubule associated proteins (at I=0.1 M) does not appreciably influence the affinity for GAPDH, but reduces bundle formation possibly for sterical reasons.

ISSN:0094-243X    

DOI:10.1063/1.40608

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

InXenopus laevisoocytes EF−1&agr; is concentrated in a small region of the cytoplasm, known as the mitochondrial mass or Balbiany body. When the mitochondrial mass disperses in early vitellogenic oocytes EF−1&agr; becomes evenely distributed in the cytoplasm. The local concentration of EF‐1&agr; in oocytes probably results from the association of EF‐1&agr; and EF‐1&bgr; with tubulin. Three observations are in favor of this interpretation. First, a large fraction of EF−1&agr; inX.laevisoocytes occurs as a complex with EF−1&bgr;&ggr;. Second, purified EF‐1&ggr; has a strong affinity for tubulin. Third, tubulin is extremely concentrated in the mitochondrial mass and disperses exactly at the same stage of oogenesis as EF−1&agr;. A similar association has been recently postulated between tubulin and an EF‐1&agr;‐like protein located in the centrosphere of sea urchin eggs.

ISSN:0094-243X    

DOI:10.1063/1.40585

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The role of the microtubular marginal band (MT‐MB) in the mature nucleated chicken erythrocyte is still not well understood. We show here, in stopped flow experiments, that the kinetics of swelling of the chicken red blood cells, subsequent to a fast decreases in ionic strength, are identical at 37 °C and 4 °C, i.e. in the presence or in the absence of MT‐MB. This suggests that the MT‐MB does not provide the chicken erythrocyte with significant protection against osmotic stresses. Instead, MT‐MB may be involved in the development and maintenance of the peculiar elliposoidal shape of the chicken erythrocyte. If either one of these functions occursin vivo, MT‐MB should involve close, specific and strong interactions with the chicken red blood cell membrane. We report the identification of two sets of proteins which may represent specific sites for anchoring the MT‐MB on the chicken erythrocyte plasma membrane.

ISSN:0094-243X    

DOI:10.1063/1.40586

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Permeabilization of the plasma membrane of Chinese hamster ovary cells by application of external electric field pulses can be described as a three step process of induction, expansion and annihilation of transient permeated structures. This phenomenon is associated to an increase in microvilli, density at the cell surface. In the case of cells into contact, the resealing of the permeated state can lead to the formation of polycaryons to which is associated to a reorganisation of microtubules into the cells. This observation shows an alteration of the cytoskeleton organization as a consequence of cell electropulsation. Annihilation of the permeated state depends on several physical parameters of the technique, but, in opposition to the two first steps of induction and expansion, it appears to be under the control of the cytoskeleton.These results suggest that microtubules play an important role in the process. Although it is still difficult to describe the exact role of the cytoskelteon in electropermeabilization, its interaction with plasma membrane makes it a candidate as a regulatory element.

ISSN:0094-243X    

DOI:10.1063/1.40587

出版商:AIP    

年代:1991

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.40588

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The internal dynamics of actin filaments and the rheological properties of transiently and chemically cross‐linked actin networks were studied by quasielastic light scattering and a new type of rheometer. Evidence is provided that the actin solutions exhibit typical universal properties as solutions of high molecular weight polymers exhibiting Rouse‐Zimm behavior. A number of power laws is established and the persistence length of actin filaments is determined as &lgr;≊0.3 &mgr;m.

ISSN:0094-243X    

DOI:10.1063/1.40589

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The cytoplasm of motile cells contains a dynamic system of filamentous protein polymers that endow the cell with elasticity permitting it to maintain its shape in the presence of mechanical forces encounteredin vivo. Part of this cytoskeleton is composed of filaments of polymerized actin. Remodeling of this network is required for cell motility and cytoplasmic restructuring, and the reversible polymerization of actin per se has been suggested to cause morphologic changes such as cell ruffling and pseudopd extension. Changes in the degree of polymerization of acting and in the association of actin filaments into supramolecular structures are often associated with cell activation. Such activation is initiated by extracellular signals that bind to receptors which are often coupled by G‐proteins to the production of intracellular second messangers. Cytoplasmic gel‐sol transitions therefore can occur by formation and dissolution of actin networks, mediated by a variety of actin‐binding proteins which are regulated by intracellular signalling molecules such as Ca2+and polyphosphoinositides.The effects of three actin binding proteins: profilin, gelsolin and ABP (Tilamin) on the polymerization of actin and the viscoelasticity of the resulting networks measuredin vitrosuggest possible roles of these proteinsin vivo. In particular, gelsolin, which activated by Ca2+to sever and cap actin filaments, and released from filament ends by PIP2, appears to be a likely candidate for regulation of gel‐sol transitions in response to cell activation. Recent results demonstrate that the hydrolysis of ATP that occurs following actin polymerization also influences the structure of the resulting filament. In addition being regulated by acting‐binding proteins, the viscoelasticity of actin networks is also affected by the presence of the other two classes of cytoplasmic protein polymers, microtubules and intermediate filaments.

ISSN:0094-243X    

DOI:10.1063/1.40599

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Three mutants of &bgr;‐skeletal tropomyosin were produced by genetic engineering. To test the role of hydrophobic interactions invovled on the dimer formation we performed thermal and urea‐mediated denaturation of these mutants in various solvent conditions. In the light of our results we propose some hypotheses concerning interaction of &agr;‐helices in tropomyosins.

ISSN:0094-243X    

DOI:10.1063/1.40600

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The ATPases of activated and relaxed rabbit psoas myofibrils were studied by the rapid flow quench method in a solvent of near physiological pH and ionic strength. Both types of myofibrils bind and hydrolyze ATP with transient kinetics very similar to those found with myosin. But the kcat of activated myofibrils was 100× that with the relaxed myofibrils. Relaxed myofibrils and myosin could not be distinguished kinetically.

ISSN:0094-243X    

DOI:10.1063/1.40601

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

3 to 5 days‐old embryonic rat skeletal muscle cells (myotubes) present electrically‐induced [CA2+]ii transients uncoupled to any measurable contraction (quiescent myotubes). Acetylcholine (10−5M) induces a transient variation of [Ca2+]iwhose spatial maps, at the beginning of the oscillation, is similar to electrically and caffeine‐induced [Ca2+]itransients. Later (after 1 sec), the spatial distribution of [Ca2+]iis modified, suggesting a different mechanism undering the [Ca2+]iincrease.

ISSN:0094-243X    

DOI:10.1063/1.40602

出版商:AIP    

年代:1991

数据来源: AIP