摘要:

AbstractRecent studies on the mobility of membrane markers on crawling cells indicate that there is no long‐range centripetal flow of membrane proteins or lipids during cell locomotion. In this article we reflect on the history of ideas about membrane flow in cells, and we discuss how these new findings will shift the focus of research in cell locomotion away from the cell surface to the molecular interactions and dynamics of the actin cytoskeleto

ISSN:0886-1544    

DOI:10.1002/cm.970180402

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractStructural, biochemical, and genetic evidence has demonstrated there are three inner dynein arm subforms, I1, I2, and I3, which differ in organization and composition (see Piperno et al.:J. Cell Biol.110:379–389, 1990). Using dynein extracted fromChlamydomonasouter dynein armless mutant pf28, we have begun to define the structural and functional properties of isolated inner arm subforms. Inner dynein arm I1 was purified either by sucrose density gradient centrifugation or microtubule binding affinity. I1, composed of heavy chains 1α and 1β, sedimented at 21S and selectively bound to and cross‐linked purified microtubules in and ATP‐sensitive manner. Deep etch electron microscopy revealed that the 21S sedimenting fraction contained two‐headed structures in which large globular heads are connected by long, flexible‐stem domains. In contrast, components derived from I2 and I3 sedimented as a mixture of 11S particles with single globular heads which did not bind to purified microtubules. Both the 21S and 11S sedimenting fractions supported microtubule translocation in in vitro motility assays. In 1 mM MgATP the I1‐containing fraction produced very slow microtubulegliding velocities (0.76 μm/sec) compared to the I2, I3‐containing fract

ISSN:0886-1544    

DOI:10.1002/cm.970180403

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractIn response to step‐up as well as step‐down blue or white light stimuli, changes of beat pattern were observed in the two flagella ofChlamydomonas.The front amplitude was either increased or decreased, always in reverse in the two flagella. Again, two opposite combinations of step‐up and step‐down responses were found roughly in parallel to the two types of beat frequency changes. It is shown that positive phototaxis is probably achieved by the first type [called type (+)] and negative phototaxis by the second one [called type (−)]. Comparative measurements have revealed that frequency is not only related to the rate of flagellar movement, but also to the beat pattern. The rate of movement may change in different ways in the recovery and in the effective stroke. Though beat frequency and pattern changes are opposite in the two types, the rates of movement of the two flagella during the effective stroke are not always. In type (−) divergent changes were found in the rates of effective stroke movement, perhaps indicating the involvement of an additiona

ISSN:0886-1544    

DOI:10.1002/cm.970180404

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractCell division inCymatopleurarequires precise and major relocations of the nucleus and chioroplast which have been followed by time‐lapse cinematography and with the electron microscope. These movements are (1) the premitotic nucleus migrating to one end of the cell; (2) after cytokinesis, the daughter nuclei moving back to the cell centre, often oscillating several times while establishing their final location; (3) the single chloroplast folding over and sandwiching the central nucleus; and (4) the folded end of the chloroplast stretching back to fill the empty half of the cell. In all cases, straight, actively moving, transient strands of cytoplasm are associated with the movement of the nucleus and chloroplast, and these often appear to be pulling on the surface or the fold of the chloroplast which undergoes transient distortion.These movements are rapid and colchicine‐sensitive. Ultrastructurally, they appear to be mediated by the prominent microtubule centre (MC) and its associated cytoskeleton of microtubules (MTs) although MTs do not attach directly to either nucleus or chloroplast. The MC is located close to the moving nucleus. Later, it moves ahead of the moving chloroplast and its MTs ensheath the tip. Later still, it is seen embedded in the fold of the chloroplast. In all three situations, MTs from it are seen in the strands of cytoplasm radiating from this area across the vacuole. After these events, the MC resumes its usual interphase situation on the nuclear surf

ISSN:0886-1544    

DOI:10.1002/cm.970180405

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractThe cytoplasmic microtubule system seems to influence the position and structure of nucleoli inDictyostelium discoideumamoebae in several growing and migrating states. For example, nucleoli were usually excluded from the nuclear periphery near the microtubule‐organizing center (MTOC) in all cases; and in migrating adherent cells, more than half the nucleoli were located opposite the MTOC. This localization was disrupted by nocodazole treatment, after which the nucleoli were largely dispersed except near the MTOC. More extensive effects of microtubules on nucleolar structure were seen in aggregating cells. In contrast to the normal oval structure in growing cells, nucleoli took on a different morphology: they protruded from the leading edge of nuclei and elongated to form nozzle‐like structures. Analysis by rapid‐freeze substitution and indirect immunofluorescence showed each nozzle surrounded by more than 10 microtubules; and in the presence of nocodazole, the microtubules shortened as expected and the nozzles disappeared. Between microtubules and the outer nuclear envelope, various‐sized cross‐bridges were seen. The implication that microtubules were associated with the nucleoli in aggregating cells was verified in vitro: nuclei isolated from growing cells contained the MTOC but few if any detectable microtubules; but nuclei from aggregating cells were surrounded by them. These data are consistent with the notion the microtubule system may help regulate the position and conformation of nucleoli during early development ofDict

ISSN:0886-1544    

DOI:10.1002/cm.970180406

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractMitotic spindles isolated from sea urchin eggs can be reactivated to undergo mitotic processes in vitro. Spindles incubated in reactivation media containing sea urchin tubulin and nucleotides undergo pole‐pole elongation similar to that observed in living cells during anaphase‐B. The in vitro behavior of spindles isolated during metaphase and anaphase are compared. Both metaphase and anaphase spindles undergo pole‐pole elongation with similar rates, but only in the presence of added tubulin. In contrast, metaphase but not anaphase spindles increase chromosome‐pole distance in the presence of exogenous tubulin, suggesting that in vitro, tubulin can be incorporated at the kinetochores of metaphase but not anaphase chromosomes. The rate of spindle elongation, ultimate length achieved, and the increase in chromosome‐pole distance for isolated metaphase spindles is related to the concentration of available tubulin. Pole‐pole elongation and chromosome‐pole elongation does not require added adenosine triphosphate (ATP). Guanosine triphosphate (GTP) will support all activities observed. Thus, the force generation mechanism for anaphase‐B in isolated sea urchin spindles is independent of added ATP, but dependent on the availability of tubulin. These results support the hypothesis that the mechanism of force generation for anaphase‐B is linked to the incorporation of tubulin into the mitotic apparatus. (If, in addition, a microtubule‐dependent motor‐protein(s) is acting to generate force, it does not appear to be dependant on ATP as the exc

ISSN:0886-1544    

DOI:10.1002/cm.970180407

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

ISSN:0886-1544    

DOI:10.1002/cm.970180408

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

ISSN:0886-1544    

DOI:10.1002/cm.970180401

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY