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Relationship to the Directionality of Flagellar Microtubule Assembly

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Relationship to the Directionality of Flagellar Microtubule Assembly Published September 1, 1977 FLAGELLAR ELONGATION AND SHORTENING IN CHLA MYD OMONAS III. Structures Attached to the Tips of Flagellar Microtubules and Their Relationship to the Directionality of Flagellar Microtubule Assembly WILLIAM L. DENTLER and JOEL L. ROSENBAUM From the Department of Biology, Yale University, New Haven, Connecticut 06520. Dr. Dentler's Downloaded from current address is the Department of Physiology and Cell Biology, MeCollum Laboratories, The University of Kansas, Lawrence, Kansas 66044. ABSTRACT Two structures on the distal ends of Chlamydomonas flagellar microtubules are described. One of these, the central microtubule cap, attaches the distal ends of jcb.rupress.org the central pair microtubules to the tip of the flagellar membrane. In addition, filaments, called distal filaments, are observed attached to the ends of the A- tubules of the outer doublet microtubules. Inasmuch as earlier studies suggested that flagellar elongation in vivo occurs principally by the distal addition of subunits and because it has been shown that brain tubulin assembles in vitro primarily onto on September 10, 2014 the distal ends of both central and outer doublet microtubules, the presence of the cap and distal filaments was quantitated during flagellar resorption and elonga- tion. The results showed that the cap remains attached to the central microtubules throughout flagellar resorption and elongation. The cap was also found to block the in vitro assembly of neurotubules onto the distal ends of the central microtu- bules. Conversely, the distal filaments apparently do not block the assembly of neurotubules onto the ends of the outer doublets. During flagellar elongation, the distal ends of the outer doublets are often found to form sheets of protofilaments, similar to those observed on the elongating ends of neurotubules being assembled in vitro. These results suggest that the outer doublet microtubules elongate by the distal addition of subunits, whereas the two central microtubules assemble by the addition of subunits to the proximal ends. KEY WORDS flagella microtubules by amputating the flagella of certain protozoans microtubule assembly 9Chlamydomonas and algae, allowing them to regenerate their fla- membranes gella to half-length, and then adding a radioactive precursor, such as tritiated leucine, to the cells It has been shown by pulse labeling and light and until flagellar regeneration was completed. After electron microscope autoradiography that flagella autoradiography, most of the incorporated radio- (21, 22) and flagellar microtubules (34) assemble active precursor was observed to be in the distal in vivo principally by the addition of subunits to (tip) half of the flagellum (21, 22) or flagellar their distal ends. These observations were made microtubules (34), indicating that elongation oc- THE JOURNAL OF CELL BIOLOGY 9 VOLUME 74, 1977 9pages 747-759 747 Published September 1, 1977 curred by distal addition of subunits; however, gella terminate in a pair of filaments, called distal there was always some labeled precursor (ca. 20- filaments, which apparently do not prevent the 35%) incorporated into the proximal half of the distal assembly of brain tubulin onto the A-tubules flagellum or flagellar microtubules (21, 22, 34) of the outer doublets. Observations of the distal even after prolonged chases with the unlabeled ends of the outer doublet microtubules during precursor. It was not known whether this proximal flagellar regeneration show them frequently to ter- label represented microtubule assembly by the minate in sheets of protofilaments, similar to those proximal addition of subunits, or whether the la- observed on the ends of brain microtubules being bel was present in some microtubule-associated assembled in vitro (7, 10, 15, 16, 27). These structures added after microtubule assembly (see results suggest that the flagellar outer doublet mi- reference 34). crotubules elongate by the distal addition of sub- With the development of methods for the in units, whereas the central microtubules elongate vitro assembly of brain tubulin (3, 33), it was by the proximal addition of subunits. found that brain tubulin would assemble onto flag- A preliminary report of some of the data pre- ellar microtubules in vitro (1, 2) and that the sented in this paper has been published (8). assembly occurred principally onto the distal ends of the flagellar outer doublet and central microtu- MATERIALS AND METHODS Downloaded from bules (1,2). However, if the tubulin concentration Cell Culture was sufficiently high, neurotubules also assembled onto the proximal, nonfavored ends of the flagel- Chlamydomonas reinhardtii (strain 21 gr) were grown in 125-ml flasks to a cell density of 0.8-1.0 x 10~ cells/ lar microtubules but at a rate five to eight times ml in medium I of Sager and Granick (25). The cells slower than the assembly onto the distal ends (2). were grown with constant aeration at 25~ on a cycle of The relevance of this in vitro proximal assembly to 13 h of light and 11 h of dark. Cells were harvested by flagellar assembly during regeneration in vivo was centrifugation in 50-ml conical centrifuge tubes (IEC jcb.rupress.org unknown, as no information was available on the Model HN centrifuge, rotor number 958, 350 g, 3-5 concentration of tubulin in the cells being used. rain, room temperature) and were resuspended in fresh However, the possibility of proximal assembly oc- culture medium. The cells were aerated during the ex- curring in vivo was considered to be small be- periment either by bubbling air through the medium or cause, in addition to the fact that the distal ends of by stirring the culture with a magnetic stirring bar. both the central and outer doublet microtubules Preparation of Axonemes for on September 10, 2014 were favored for the in vitro assembly of brain tubulin, the outer doublet microtubules do not Electron Microscopy have free proximal ends but are extensions of the To attach flagella to the electron microscope grids, the basal body microtubules. On the other hand, the grids were treated with poly-L-lysine by a modification of proximal ends of the two central microtubules are the method of Mazia et al. (18). Electron microscope free, ending in or just above the basal cup (axos- grids were coated with a Formvar film, shadowed with tome) of the basal body (9, 20). Therefore, the carbon, and glow-discharged for 1-2 min in a vacuum evaporator. The glow-discharged films were then over- possibility could not be excluded that the central laid with a solution of 0.1% poly-L-lysine in water for 5- microtubules assembled in vivo from their proxi- 10 min, washed under a stream of double-distilled water, mal ends at high local tubulin concentrations even and air dried. A drop containing Chlamydomonas cells though their distal ends were favored for the in was then applied to the grid and the grid was drained by vitro assembly of brain tubulin. touching it to filter paper. The grid was then inverted In this study, it is shown that the distal ends of over a ~irop of deflagellation solution (5 mM MgSO4, the two central microtubules insert into a cap, 0.35 mM DTT, 0.5 mM EDTA, 10 mM HEPES, pH called the central microtubule cap, which is at- 7.5, 0.005-0.05% Nonidet P-40 [Shell Chemical Co., tached to the distal tip of the flagellar membrane. N. Y.]) for 5-20 s at room temperature. The concentra- It is also shown that the cap remains attached to tion of detergent and the time of extraction varied with the concentration of cells and various Chlamydomonas the central microtubules during flagellar regenera- mutants; extraction conditions were adjusted so the min- tion and resorption and that the cap prevents the imum treatment, which resulted in the removal of cell addition of brain tubulin subunits onto the distal bodies from the flagella (and from the grid) and the ends of the central microtubules. In addition, the dissolution of most of the flagellar membranes without results in this report show that the A-tubules of disruption of the structures at the flagellar tips, was used. the outer doublet microtubules of full-grown fla- After detergent treatment, the grid was rinsed with 2-5 748 THE JOURNAL OF CELL BIOLOGY 9VOLUME 74, 1977 Published September 1, 1977 drops of 2% aqueous uranyl acetate and air dried. A to remove the flagellar membrane varied with the procedure similar to this has recently been used by Sale number of cells attached to the grid and the partic- and Satir (26) to visualize the organization of radial ular strain of Chlamydomonas used. After deter- spokes in cilia. Tetrahymena gent treatment, most of the flagellar axonemes Flagellar Resorption and Regeneration remained intact except for the distal tips, which were slightly splayed (Fig. 1 B, C). The structure Chlamydomonas were induced to resorb their flagella of the microtubules is similar to that reported by by the high-salt method developed by Lefebvre et al. 1 in others (1, 2, 13, 31): dynein arms were attached which the flagella could be synchronously shortened along the A-tubule of each outer doublet and the from an average length of 12-14 to 2-3 tzm in 3 h. Flagellar regeneration subsequently was induced by har- projections attached to the central microtubules vesting the cells and resuspending them in fresh medium. were often superimposed to form an apparent Samples of cells were taken at intervals during flagellar helical coil (see references 5 and 31). Examination resorption and regeneration and were treated as de- of the tips of the axonemes at higher magnification scribed above to observe the flagellar microtubules. In (Figs. 1 C, 2, 3) revealed structures at the ends of addition, samples of cells were fixed in 2% glutaraide- the microtubules that had not been observed be- hyde in polymerization mixture (PM) and their flagellar fore in negatively stained axonemes.
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