AN ULTRASTRUCTURAL STUDY OF PLANT SPERMATOGENESIS : Spermatogenesis in Nitella

Spermatogenesis in the charophyte Nitella has been followed in antheridia prepared for light and electron microscopy. The antheridial filament cells contain paired centrioles which are similar in structure and behavior to the centrioles of animal cells. In the early spermatid, the centrioles undergo an initial elongation at their distal ends and become joined by a spindle-shaped fibrous connection. At the same time, their proximal ends are closely associated with the development of a layer of juxtaposed microtubules which will form the microtubular sheath. The architectural arrangement of these microtubules suggests that they constitute a cytoskeletal system, forming a framework along which the mitochondria and plastids become aligned and along which the nucleus undergoes extensive elongation and differentiation. The microtubular sheath persists in the mature sperm. During mid-spermatid stages, the centrioles give rise to the flagella and concomitantly undergo differentiation to become the basal bodies. The Golgi apparatus goes through a period of intensive activity during mid-spermatid stages, then decreases in organization until it can no longer be detected in the late spermatid. An attempt is made to compare similarities between plant and animal spermiogenesis.     

Plant microtubule studies: past and present

Here, I briefly review historical and morphological aspects of plant microtubule studies in land plants. Microtubules are formed from tubulins, and the polymeric configurations appear as singlet, doublet, and triplet microtubules. Doublet microtubules occur in the axoneme of cilia and flagella, and triplet microtubules occur in the basal bodies and centrosomes. Doublet and triplet microtubules are lost in all angiosperms and some gymnosperms that do not possess flagellated sperm. In land plants with flagellated sperm, centriolar centrosomes transform into basal bodies during spermatogenesis. In flowering plants, however, most male gametes (sperm) are conveyed to eggs without the benefit of cilia or flagella; thus, higher plants lack centriolar centrosome and doublet and triplet microtubules. The loss of centriolar centrosomes from the life cycle of flowering plants may have influenced the evolution of the plant microtubule system. Comparison of mitotic apparatuses in basal land plants and flowering plants illuminates the evolutionary transition from the centriolar microtubule system to the acentriolar microtubule system.     

Loss of microtubules and alteration of glycoprotein migration in organ cultures of mouse intestine exposed to nocodazole or colchicine

Summary Explants from mouse jejunum were cultured for 3–7 h in the absence (control) or presence of colchicine (100 gm/ml) or nocodazole (10 g/ml). In recovery experiments, expiants were cultured in fresh medium for an additional period. To label glycoproteins, ³H-fucose was added during the last 3 or 6 h of the initial culture or recovery period. Subcellular fractionation studies revealed that colchicine and nocodazole inhibited migration of labelled glycoproteins to the brush border (P₂) by 40–45%. Radioautographic studies of absorptive cells showed that colchicine and nocodazole inhibited labelling of the microvillous border by 67% and 87%, while labelling of the basolateral plasma membrane increased by 114% and 275%. Immunocytochemical studies revealed that both colchicine and nocodazole caused the virtual disappearance of the microtubular network in the absorptive cells. It is possible that some glycoproteins normally destined for the microvillous border are rerouted to the basolateral membrane. The observed loss of microtubules after drug treatment suggests that microtubules may play a role in the intracellular migration of membrane glycoproteins. Additional support for this concept is provided by the fact that in recovery experiments the distribution of label returned to control values after the microtubular network became re-established.     

Diameter Oscillation of Axonemes in Sea-Urchin Sperm Flagella

The 9 + 2 configuration of axonemes is one of the most conserved structures of eukaryotic organelles. Evidence so far has confirmed that bending of cilia and flagella is the result of active sliding of microtubules induced by dynein arms. If the conformational change of dynein motors, which would be a key step of force generation, is occurring in a three-dimensional manner, we can easily expect that the microtubule sliding should contain some transverse component, i.e., a motion in a direction at a right angle to the longitudinal axis of axonemes. Using a modified technique of atomic force microscopy, we found such transverse motion is actually occurring in an oscillatory manner when the axonemes of sea-urchin sperm flagella were adhered onto glass substrates. The motion was adenosine triphosphate-dependent and the observed frequency of oscillation was similar to that of oscillatory sliding of microtubules that had been shown to reflect the physiological activity of dynein arms (S. Kamimura and R. Kamiya. 1989. Nature. 340:476–478; 1992. J. Cell Biol. 116:1443–1454). Maximal amplitude of the diameter oscillation was around 10 nm, which was within a range of morphological change observed with electron microscopy (F. D. Warner. 1978. J. Cell Biol. 77:R19–R26; N. C. Zanetti, D. R. Mitchell, and F. D. Warner. 1979. J. Cell Biol. 80:573–588).     

An outer arm dynein light chain acts in a conformational switch for flagellar motility

A system distinct from the central pair–radial spoke complex was proposed to control outer arm dynein function in response to alterations in the mechanical state of the flagellum. In this study, we examine the role of a Chlamydomonas reinhardtii outer arm dynein light chain that associates with the motor domain of the γ heavy chain (HC). We demonstrate that expression of mutant forms of LC1 yield dominant-negative effects on swimming velocity, as the flagella continually beat out of phase and stall near or at the power/recovery stroke switchpoint. Furthermore, we observed that LC1 interacts directly with tubulin in a nucleotide-independent manner and tethers this motor unit to the A-tubule of the outer doublet microtubules within the axoneme. Therefore, this dynein HC is attached to the same microtubule by two sites: via both the N-terminal region and the motor domain. We propose that this γ HC–LC1–microtubule ternary complex functions as a conformational switch to control outer arm activity.     

MICROTUBULE BIOGENESIS AND CELL SHAPE IN OCHROMONAS : II. The Role of Nucleating Sites in Shape Development

The proposal made in the preceding paper that the species-specific shape of Ochromonas is mediated by cytoplasmic microtubules which are related to two nucleating sites has been experimentally verified. Exposure of cells to colchicine or hydrostatic pressure causes microtubule disassembly and a correlative loss of cell shape in a posterior to anterior direction. Upon removal of colchicine or release of pressure, cell shape regenerates and microtubules reappear, first in association with the kineto-beak site concomitant with regeneration of the anterior asymmetry, and later at the rhizoplast site concomitant with formation of the posterior tail. It is concluded that two separate sets of cytoplasmic tubules function in formation and maintenance of specific portions of the total cell shape. On the basis of the following observations, we further suggest that the beak and rhizoplast sites could exert control over the position and timing of the appearance, the orientation, and the pattern of microtubule distribution in Ochromonas. (a) the two sites are accurately positioned in the cell relative to other cell organelles; (b) in regenerating cells microtubules reform first at these sites and appear to elongate to the cell posterior; (c) microtubules initially reappear in the orientation characteristic of the fully differentiated cell; (d) the two sets of tubules are polymerized at different times, in the same sequence, during reassembly or resynthesis of the microtubular system. Experiments using cycloheximide, after a treatment with colchicine, have demonstrated that Ochromonas cannot reassume its normal shape without new protein synthesis. This suggests that microtubule protein once exposed to colchicine cannot be reassembled into microtubules. Pressure-treated cells, on the other hand, reassemble tubules and regenerate the normal shape in the presence or absence of cycloheximide. The use of these two agents in analyzing nucleating site function and the independent processes of synthesis and assembly of microtubules is discussed.     

Polymerization of tubulin in the presence of colchicine or podophyllotoxin. Formation of a ribbon structure induced by guanylyl-5'-methylene diphosphonate

GTP-dependent in vitro polymerization of rat brain microtubular protein is inhibited to 50% by substoichiometric concentrations of the antimitotic drugs colchicine (0.12 mol/mol of tubulin) and podophyllotoxin (0.14 mol/mol of tubulin). Substitution of pp(CH₂)pG² for GTP, however, results in an extensive microtubular protein polymerization at such concentrations. In the presence of pp(CH₂)pG, suprastoichiometric concentrations of podophyllotoxin (19 mol/mol of tubulin) are required to inhibit the polymerization process by 50%. Colchicine is very ineffective since 3 × 10⁵ moles/mole of tubulin are required to give a 50% inhibition. Electron microscopical analysis shows that the polymers formed by microtubular protein in the presence of suprastoichiometric concentrations of drugs are not the normal short microtubules typical of pp(CH₂)pG-driven polymerization, but are ribbons with three or four protofilaments. The colchicine content of the harvested ribbons has been measured directly and found to be approximately 0.8 moles colchicine/mole of tubulin. Treatment of microtubular protein with substoichiometric concentrations of drugs results in an increase in the number of protofilaments forming the ribbons. Many of the ribbons can close into morphologically normal microtubules when microtubular protein is treated with only 0.05 moles of either colchicine or podophyllotoxin per mole of tubulin.     

The motor activity of mammalian axonemal dynein studied in situ on doublet microtubules

Flagellar dynein generates forces that produce relative shearing between doublet microtubules in the axoneme; this drives propagated bending of flagella and cilia. To better understand dynein's role in coordinated flagellar and ciliary motion, we have developed an in situ assay in which polymerized single microtubules glide along doublet microtubules extruded from disintegrated bovine sperm flagella at a pH of 7.8. The exposed, active dynein remain attached to their respective doublet microtubules, allowing gliding of individual microtubules to be observed in an environment that allows direct control of chemical conditions. In the presence of ATP, translocation of microtubules by dynein exhibits Michaelis-Menten type kinetics, with V(max) = 4.7 +/- 0.2 microm/s and K(m) = 124 +/- 11 microM. The character of microtubule translocation is variable, including smooth gliding, stuttered motility, oscillations, buckling, complete dissociation from the doublet microtubule, and occasionally movements reversed from the physiologic direction. The gliding velocity is independent of the number of dynein motors present along the doublet microtubule, and shows no indication of increased activity due to ADP regulation. These results reveal fundamental properties underlying cooperative dynein activity in flagella, differences between mammalian and non-mammalian flagellar dynein, and establish the use of natural tracks of dynein arranged in situ on the doublet microtubules of bovine sperm as a system to explore the mechanics of the dynein-microtubule interactions in mammalian flagella.     

EXTRACELLULAR MICROTUBULES : The Origin, Structure, and Attachment of Flagellar Hairs in Fucus and Ascophyllum Antherozoids

Mastigonemes (Flimmer) from the sperm of Ascophyllum and Fucus were found to consist of a tripartite structure—a ca. 2000-A tapered basal region, a closed microtubular shaft, and a group of terminal filaments. Each of these regions appears to be constructed of globular subunits with a center-to-center distance of about 45 A. The mastigoneme microtubule is of smaller diameter (170–190 A) than cytoplasmic microtubules in these or other plant cells. During the initial stages of flagellar ontogeny, structures similar to mastigonemes (presumptive mastigonemes) are found within membrane-limited sacs in the cytoplasm or within the perinuclear space. Mastigonemes at this time are generally not found on the flagellar surface. Later, when the anterior flagellum acquires mastigonemes, the presumptive mastigonemes are absent from the cytoplasm. The regularity of attachment of mastigonemes to the flagellar surface suggests that specific attachment sites are constructed on the plasma membrane during flagellar ontogeny. No evidence for penetration of the mastigoneme through the plasma membrane was obtained. The origin and structure of mastigonemes are discussed in relation to reports of the origin and structure of other microtubular systems.     

The effects of cytochalasin B and colchicine on the morphogenesis of mitochondria in Drosophila hydei during meiosis and early spermiogenesis

Summary Morphogenesis of mitochondria in male germ cells in cultivated cytocysts begins in early prophase I at which time mitochondria thicken and become ordered along the spindle apparatus during meiosis. At the end of the second meiotic division they aggregate to form the Nebenkern.In the presence of colchicine or cytochalasin B mitochondria are able to begin differentiation, although the correct course of meiosis is not guaranteed. In medium supplemented with colchicine they undergo normal thickening but do not aggregate, in a pattern known from untreated cultures. This may indicate that microtubules are involved in the aggregation process of mitochondria as colchicine is known to inhibit microtubule formation. Moreover, in cell cultures treated with cytochalasin B mitochondrial aggregation does occur; it is concluded that microfilaments, which are sensitive to cytochalasin B, do not play a detectable role in the aggregation of mitochondria.     

The sperm of Matsucoccus feytaudi (Insecta,Coccoidea): Can the microtubular bundle be considered as a true flagellum?

In the present work the spermiogenesis and sperm structure of Matsucoccus feytaudi, a primary pest of the maritime pine in southern eastern Europe, is studied. In addition to the already known characteristics of coccid sperm, such as the absence of the acrosome and mitochondria, and the presence of a bundle of microtubules responsible for sperm motility, a peculiar structure from which the microtubule bundle takes origin is described. Such a structure – a short cylinder provided with a central hub surrounded by several microtubules with a dense wall – is regarded as a Microtubule Organizing Centre (MTOC). During spermiogenesis, quartets of fused spermatids are formed; from each spermatid, a bundle of microtubules, generated by the MTOC, projects from the cell surface. Each cell has two centrioles, suggesting the lack of a meiotic process and the occurrence of parthenogenesis. At the end of the spermiogenesis, when the cysts containing bundles of sperm are formed, part of the nuclear material together with the MTOC structure is eliminated. Based on the origin of the microtubular bundle from the MTOC, the nature of the bundle as a flagellum is discussed.     

Studies on the mechanism of action of isopropyl N-phenyl carbamate

The binding of [¹⁴C]isopropyl N-phenyl carbamate (IPC) to microtubular protein isolated from chick brains, and the effect of isopropyl N-phenyl carbamate (IPC) on the in vitro reassembly of microtubules was investigated. While [¹⁴C]colchicine binds to microtubular protein, [¹⁴C]IPC does not. Concentrations from 1 × 10⁻⁴ M to 1 × 10⁻³ M IPC do not prevent in vitro repolymerization of microtubular protein. IPC (1 × 10⁻⁴ M) does not affect the rate of reassembly of microtubules. We conclude that IPC does not exert its effect through an interaction with microtubular protein; we suggest that IPC probably interacts with microtubule organizing centers.     

Motor apparatus in human spermatozoa that lack central pair microtubules

Electron microscopic examination of the spermatozoa from a man suffering from asthenozoospermia (poor or low sperm motility) showed that approximately 92% of the sperm flagella lacked central pair microtubules but possessed dynein arms and radial spokes while a small percentage of the spermatozoa had complete flagella. The characteristics of the motor apparatus of the spermatozoa and the effects of caffeine on the sperm motility were examined, as were the reactivation of demembranated spermatozoa and the sliding of doublet microtubules. Almost all spermatozoa were immotile in a Tyrode solution while only a small percentage of spermatozoa showed slow forward movement or feeble flagellar vibration, whereas addition of caffeine to the sperm suspension induced forward swimming of approximately half of the spermatozoa. The reactivation of demembranated spermatozoa with MgATP(2-) could not succeed because of disintegration of the demembranated flagella. However, when the demembranated spermatozoa were exposed to MgATP(2-) and then treated with elastase, the microtubular doublets of approximately half the number of the flagella slid from the end or middle of the flagella. These results suggest that the motor apparatus in the sperm flagella that lack the central pair microtubules is functionally assembled and intrinsically capable of undergoing flagellar movement but not strong enough to beat normally.     

THE MECHANISM OF ACTION OF COLCHICINE : Colchicine Binding Properties of Sea Urchin Sperm Tail Outer Doublet Tubulin

The thermal depolymerization procedure of Stephens (1970. J. Mol. Biol. 47:353) has been employed for solubilization of Strongylocentrotus purpuratus sperm tail outer doublet microtubules with the use of a buffer during solubilization which is of optimal pH and ionic strength for the preservation of colchicine binding activity of chick embryo brain tubulin. Colchicine binding values were corrected for first-order decay during heat solubilization at 50°C (t_½ = 5.4 min) and incubation with colchicine at 37°C in the presence of vinblastine sulfate (t_½ = 485 min). The colchicine binding properties of heat-solubilized outer doublet tubulin were qualitatively identical with those of other soluble forms of tubulin. The solubilized tubulin (mol wt, 115,000) bound 0.9 ± 0.2 mol of colchicine per mol of tubulin, with a binding constant of 6.3 x 10⁵ liters/mol at 37°C. The colchicine binding reaction was both time and temperature dependent, and the binding of colchicine was prevented in a competitive manner by podophyllotoxin (K_i = 1.3 x 10^-6 M). The first-order decay of colchicine binding activity was substantially decreased by the addition of the vinca alkaloids, vinblastine sulfate or vincristine sulfate, thus demonstrating the presence of a vinca alkaloid binding site(s) on the outer doublet tubulin. Tubulin contained within the assembled microtubules did not decay. Intact outer doublet microtubules bound less than 0.001 mol of colchicine per mol of tubulin contained in the microtubules, under conditions where soluble tubulin would have bound 1 mol of colchicine per mol of tubulin (saturating concentration of colchicine, no decay of colchicine binding activity). The presence of colchicine had no effect on the rate of solubilization of outer doublet microtubules during incubation at 37°C. Therefore, the colchicine binding site on tubulin is blocked (not available to bind colchicine) when the tubulin is in the assembled outer doublet microtubules.     

THE EFFECT OF COLCHICINE ON MYOGENESIS IN VIVO IN RANA PIPIENS AND RHODNIUS PROLIXUS (HEMIPTERA)

The effect of colchicine on myogenesis in vivo has been studied in the regenerating tadpole tail of the frog, Rana pipiens, and in the abdominal molting muscles of a blood-sucking bug, Rhodnius prolixus Stål. Colchicine is shown to disrupt microtubules in the differentiating muscle cells of both these organisms. The disruption of microtubules is correlated with a loss of longitudinal anisometry in the myoblasts and myotubes of the regeneration blastema in the tadpole tail. Before colchicine treatment, the myotubes contain longitudinally oriented myofibrils. After colchicine treatment, rounded, multinucleate myosacs containing randomly oriented myofibrils are present. It is suggested that the primary function of microtubules in myogenesis in the Rana pipiens tadpole is the maintenance of cell shape. The abdominal molting muscles of Rhodnius undergo repeated phases of differentiation and dedifferentiation of the sarcoplasm. However, the longitudinal anisometry of the muscle fibers is maintained in all phases by the attachments of the ends of the fibers to the exoskeleton, and microtubule disruption does not alter cell shape. The orientation of the developing myofibrils is also unaltered, indicating that the microtubules do not directly align or support the myofibrils in this system.     

Flagellar microtubule dynamics in Chlamydomonas: cytochalasin D induces periods of microtubule shortening and elongation; and colchicine induces disassembly of the distal, but not proximal, half of the flagellum

To study the mechanisms responsible for the regulation of flagellar length, we examined the effects of colchicine and Cytochalasin D (CD) on the growth and maintenance of Chlamydomonas flagella on motile wild type cells as well as on pf 18 cells, whose flagella lack the central microtubules and are immobile. CD had no effect on the regeneration of flagella after deflagellation but it induced fully assembled flagella to shorten at an average rate of 0.03 microns-min. Cells remained fully motile in CD and even stubby flagella continued to move, indicating that flagellar shortening did not selectively disrupt machinery necessary for motility. To observe the effects of the drug on individual cells, pf 18 cells were treated with CD and flagella on cells were monitored by direct observation over a 5-hour period. Flagella on control pf 18 cells maintained their initial lengths throughout the experiment but flagella on CD-treated cells exhibited periods of elongation, shortening, and regrowth suggestive of the dynamic behavior of cytoplasmic microtubules observed in vitro and in vitro. Cells behaved individually, with no two cells exhibiting the same flagellar behavior at any given time although both flagella on any single cell behaved identically. The rate of drug-induced flagellar shortening and elongation in pf 18 cells varied from 0.08 to 0.17 microns-min-1, with each event occurring over 10-60-min periods. Addition of colchicine to wild type and pf 18 cells induced flagella to shorten at an average rate of 0.06 microns-min-1 until the flagella reached an average of 73% of their initial length, after which they exhibited no further shortening or elongation. Cells treated with colchicine and CD exhibited nearly complete flagellar resorption, with little variation in flagellar length among cells. The effects of these drugs were reversible and flagella grew to normal stable lengths after drug removal. Taken together, these results show that the distal half to one-third of the Chlamydomonas flagellum is relatively unstable in the presence of colchicine but that the proximal half to two-thirds of the flagellum is stable to this drug. In contrast to colchicine, CD can induce nearly complete flagellar microtubule disassembly as well as flagellar assembly. Flagellar microtubules must, therefore, be inherently unstable, and flagellar length is stabilized by factors that are sensitive, either directly or indirectly, to the effects of CD.     

CELL ELONGATION IN THE CULTURED EMBRYONIC CHICK LENS EPITHELIUM WITH AND WITHOUT PROTEIN SYNTHESIS : Involvement of Microtubules

Previous studies have shown that cells in the 6-day old embryonic chick lens epithelium elongate in tissue culture. In the present study, the time course of elongation during the 1st day of cultivation has been examined histologically. Cultured epithelia were also treated with cycloheximide or colchicine in order to determine if cell elongation depends on new protein synthesis and on the utilization of microtubules, respectively. In the first 5 hr of culture, the mean cell length increased from 11 µ to 21 µ. Subsequently, elongation was slower; the mean cell length was 28 µ after 24 hr in culture. Continuous exposure to cycloheximide did not inhibit the initial doubling of cell length, but did prevent further elongation. By contrast, colchicine inhibited elongation almost immediately. When added after the cell length had doubled, cycloheximide and colchicine each inhibited further elongation; the treated cells remained columnar. Radioautographic and electrophoretic tests showed that protein synthesis was not appreciably affected by colchicine, but was suppressed by cycloheximide. Electron microscopic examination revealed that microtubules oriented along surface membranes were present in epithelia cultured with serum alone and with cycloheximide, but not in those incubated with colchicine. These results indicate that the early stages of cell elongation in the cultured lens epithelium require an initial assembly and organization of preexisting microtubular elements and that continued elongation depends, in addition, on the de novo synthesis of protein, possibly microtubule protein.     

Actin-microtubule interactions in the algaNitella: analysis of the mechanism by which microtubule depolymerization potentiates cytochalasin's effects on streaming

Summary In the characean algaNitella, depolymerization of microtubules potentiates the inhibitory effects of cytochalasins on cytoplasmic streaming. Microtubule depolymerization lowers the cytochalasin B and D concentrations required to inhibit streaming, accelerates inhibition and delays streaming recovery. Because microtubule depolymerization does not significantly alter³H-cytochalasin B uptake and release, elevated intracellular cytochalasin concentrations are not the basis for potentiation. Instead, microtubule depolymerization causes actin to become more sensitive to cytochalasin. This increased sensitivity of actin is unlikely to be due to direct stabilization of actin by microtubules, however, because very few microtubules colocalize with the subcortical actin bundles that generate streaming. Furthermore, microtubule reassembly, but not recovery of former transverse alignment, is sufficient for restoring the normal cellular responses to cytochalasin D. We hypothesize that either tubulin or microtubule-associated proteins, released when microtubules depolymerize, interact with the actin cytoskeleton and sensitize it to cytochalasin.Abbreviations APW artificial pond water - Ca_c cytoplasraic free calcium concentration - DMSO dimethyl sulfoxide - MT microtubule-minus - MT+ microtubule-plus.     

Extending the Microtubule/Microfibril Paradigm : Cellulose Synthesis Is Required for Normal Cortical Microtubule Alignment in Elongating Cells

The cortical microtubule array provides spatial information to the cellulose-synthesizing machinery within the plasma membrane of elongating cells. Until now data indicated that information is transferred from organized cortical microtubules to the cellulose-synthesizing complex, which results in the deposition of ordered cellulosic walls. How cortical microtubules become aligned is unclear. The literature indicates that biophysical forces, transmitted by the organized cellulose component of the cell wall, provide a spatial cue to orient cortical microtubules. This hypothesis was tested on tobacco (Nicotiana tabacum L.) protoplasts and suspension-cultured cells treated with the cellulose synthesis inhibitor isoxaben. Isoxaben (0.25–2.5 μm) inhibited the synthesis of cellulose microfibrils (detected by staining with 1 μg mL⁻¹ fluorescent dye and polarized birefringence), the cells failed to elongate, and the cortical microtubules failed to become organized. The affects of isoxaben were reversible, and after its removal microtubules reorganized and cells elongated. Isoxaben did not depolymerize microtubules in vivo or inhibit the polymerization of tubulin in vitro. These data are consistent with the hypothesis that cellulose microfibrils, and hence cell elongation, are involved in providing spatial cues for cortical microtubule organization. These results compel us to extend the microtubule/microfibril paradigm to include the bidirectional flow of information.     

Structural Basis for the Association of MAP6 Protein with Microtubules and Its Regulation by Calmodulin

Microtubules are highly dynamic αβ-tubulin polymers. In vitro and in living cells, microtubules are most often cold- and nocodazole-sensitive. When present, the MAP6/STOP family of proteins protects microtubules from cold- and nocodazole-induced depolymerization but the molecular and structure determinants by which these proteins stabilize microtubules remain under debate. We show here that a short protein fragment from MAP6-N, which encompasses its Mn1 and Mn2 modules (MAP6(90–177)), recapitulates the function of the full-length MAP6-N protein toward microtubules, i.e. its ability to stabilize microtubules in vitro and in cultured cells in ice-cold conditions or in the presence of nocodazole. We further show for the first time, using biochemical assays and NMR spectroscopy, that these effects result from the binding of MAP6(90–177) to microtubules with a 1:1 MAP6(90–177):tubulin heterodimer stoichiometry. NMR data demonstrate that the binding of MAP6(90–177) to microtubules involve its two Mn modules but that a single one is also able to interact with microtubules in a closely similar manner. This suggests that the Mn modules represent each a full microtubule binding domain and that MAP6 proteins may stabilize microtubules by bridging tubulin heterodimers from adjacent protofilaments or within a protofilament. Finally, we demonstrate that Ca²⁺-calmodulin competes with microtubules for MAP6(90–177) binding and that the binding mode of MAP6(90–177) to microtubules and Ca²⁺-calmodulin involves a common stretch of amino acid residues on the MAP6(90–177) side. This result accounts for the regulation of microtubule stability in cold condition by Ca²⁺-calmodulin.