Three-dimensional reconstruction of tubulin in zinc-induced sheets: II. Consequences of removal of microtubule associated proteins

The three-dimensional structure of zinc-induced tubulin sheets freed of microtubule associated proteins has been determined to 20 Å resolution by electron microscopy and image reconstruction. The determination was carried out with porcine brain tubulin separated from microtubule associated proteins by phosphocellulose chromatography. Negatively stained samples were tilted using the goniometer stage of the electron microscope to provide images of the tubulin sheets ranging in tilt from ?60 ° to +60 °. The micrographs were digitized and subjected to a cross-correlation analysis to compensate for smooth curvature of the lattice in the sheets. For each angle of tilt, an average unit cell was obtained from the cross-correlation analysis and subsequently a Fourier transform was computed for inclusion in the three-dimensional Fourier data set. The transforms of 47 tilted images plus the average of five untilted sheets were combined and an inverse Fourier transform was applied to give a threedimensional reconstruction of the microtubule associated protein-free tubulin sheets. Comparison of the protofilament structure in these sheets with the previously published protofilament structure of zinc-induced tubulin sheets containing microtubule associated proteins reveals a number of consequences of the removal of microtubule associated proteins. (1) The extensive internal contact along the protofilament observed in microtubule associated protein-containing tubulin sheets is maintained in microtubule associated protein-free tubulin sheets. (2) In projection, the protofilaments in microtubule associated protein-free tubulin sheets are 2.2 Å closer together than in microtubule associated protein-tubulin sheets. (3) The deviations of adjacent protofilaments from the plane of the sheets when viewed end-on are more pronounced in the absence of microtubule associated proteins. Differences are also observed at the level of individual tubulin subunits. In particular, the distinct cleft which was found in one class of subunits in tubulin sheets with microtubule associated proteins is absent in the microtubule associated protein-free tubulin sheets. The loss of this cleft and some changes in the shape of the tubulin subunits upon removal of microtubule associated proteins suggest a possible site for the interaction of tubulin with microtubule associated proteins.     

The influence of microtubule associated proteins on the production of polymorphic products from tubulin and microtubules

Crude preparations of microtubule-associated proteins (MAPs), as well as purified MAP 2, influence the structure of products assembled from purified tubulin at low pH values. At pH 6.2, only 12% of the assembled products were microtubules (MTs) when assembly was conducted in 10% DMSO; 88% were large sheets of protofilaments. In the absence of DMSO, 28% of the structures were MTs. As the content of MAPs in the assembly reaction was increased, the proportion of MTs increased to 87% at a MAP/tubulin ($\text{w}\text{w}$) ratio of 0.67 in the presence of DMSO and to 98% at a MAP/tubulin ($\text{w}\text{w}$) ratio of 0.33 in the absence of DMSO. Purified MAP 2 was as effective as crude MAP preparations in promoting MT formation at pH 6.2. MTs formed from purified tubulin and MAP 2 were transformed into spirals of protofilaments upon the addition of Vinblastine (VLB). Spirals were also formed when VLB was added to a mixture of tubulin and MAP 2 at 4 ° C. It thus appears that MAP 2 is a causative factor in initiating spiral formation in the presence of VLB.     

Three-dimensional reconstruction of tubulin in zinc-induced sheets.

The three-dimensional structure of porcine brain tubulin in planar sheets formed in the presence of zinc has been determined to a resolution of approximately 20 Å by electron microscopy and image reconstruction on negatively stained samples. The samples were prepared with a mica floatation technique, which yields tubulin sheets with 36 reciprocal space maxima on lattice lines at 21, 28, 42 and 84 Å^?1 in Fourier transforms of digitized images. In order to obtain three-dimensional data, sheets were tilted with the goniometer stage of the electron microscope to provide images at various angles between 0 ° and ± 60 °. Transforms of 33 tilted images plus the transform of untilted sheets based on an average of nine untilted images were combined to give the third dimension of reciprocal space ($\text{z}{}^1$). These data, were expressed in terms of the phases and amplitudes along the $\text{z}{}^1$ lattice line for each of the 36 maxima observed in untilted samples, as well as five additional lattice lines which have zero-amplitudes in the non-tilted central section of the three-dimensional transform. Home of these zero-amplitudes arise from systematic absences which are due to a 2-fold screw axis relating adjacent protofilaments of tubulin in the zinc-induced sheets. Thus in the three-dimensional reconstructions of the sheets a polarity of the protofilaments is apparent, with adjacent protofilaments aligned in opposite directions to give an antiparallel pattern, in contrast to normal microtubules composed of protofilaments in parallel alignment. Two classes of morphological units, each with a mass corresponding to a molecular weight of about 55,000, are found to alternate along the protofilaments. These distinct morphological units are identified as the α and β subunits of tubulin, confirming the representation of tubulin as an αβ heterodimer. Furthermore, the extensive internal contact between subunits within a dimer can readily be distinguished from the less extensive contact between dimer units. Such differences in contacts were not apparent in the earlier two-dimensional reconstructions. In addition, areas of excluded stain joining one class of subunits to the subunits of the other class in adjacent protofilaments have been resolved for tubulin polymerized in zinc-induced sheets. Of the two classes of subunits one is distinguished by a prominent cleft. Identification of which class of subunits is α and which is β is not yet possible.     

Reassembly of flagellar B (alpha beta) tubulin into singlet microtubules: consequences for cytoplasmic microtubule structure and assembly

B(alpha beta) tubulin was obtained from a homogeneous class of microtubules, the incomplete B subfiber of sea urchin sperm flagellar doublet microtubules, by thermal fractionation. The thermally derived soluble B tubulin fraction (100, 000 g-h) repolymerizes in vitro, yielding microtubule-like structures. The microtubule-associated protein (MAP) composition and certain assembly parameters of thermally derived B tubulin are different from those reported for sonication- derived flageller tubulin and purified vertebrate tubulin. The "microtubules" reassembled from thermally prepared B tubulin are composed of 12-15 protofilaments (73% possess 14 protofilaments). A certain number possess a single "adlumenal component" applied to their inside walls, regardless of the number of protofilaments. Following the first cycle of polymerization, 81% of the B tubulin and essentially 100% of the MAPs remain cold insoluble. Evidence suggests that B tubulin assembles faithfully into a B lattice, creating a j seam between two protofilaments that are laterally bonded in a A-lattice configuration. The significance of these seams is discussed in relation to the mechanism of microtubule assembly, the stability of observed ribbons of protofilaments, and the three-dimensional organization of microtubule-associated components.     

Differential binding regulation of microtubule-associated proteins MAP1A, MAP1B, and MAP2 by tubulin polyglutamylation

The major neuronal post-translational modification of tubulin, polyglutamylation, can act as a molecular potentiometer to modulate microtubule-associated proteins (MAPs) binding as a function of the polyglutamyl chain length. The relative affinity of Tau, MAP2, and kinesin has been shown to be optimal for tubulin modified by approximately 3 glutamyl units. Using blot overlay assays, we have tested the ability of polyglutamylation to modulate the interaction of two other structural MAPs, MAP1A and MAP1B, with tubulin. MAP1A and MAP2 display distinct behavior in terms of tubulin binding; they do not compete with each other, even when the polyglutamyl chains of tubulin are removed, indicating that they have distinct binding sites on tubulin. Binding of MAP1A and MAP1B to tubulin is also controlled by polyglutamylation and, although the modulation of MAP1B binding resembles that of MAP2, we found that polyglutamylation can exert a different mode of regulation toward MAP1A. Interestingly, although the affinity of the other MAPs tested so far decreases sharply for tubulins carrying long polyglutamyl chains, the affinity of MAP1A for these tubulins is maintained at a significant level. This differential regulation exerted by polyglutamylation toward different MAPs might facilitate their selective recruitment into distinct microtubule populations, hence modulating their functional properties.     

Structure of the tubulin dimer in zinc-induced sheets

The structure of tubulin has been studied in projection by minimum beam electron microscopy and image processing of negatively stained zinc-induced sheets. The reconstructed images include data to 15 Å resolution.We report here a clear and reproducible 82 Å repeat arising from the arrangement of heterodimers in sheet aggregates of tubulin. This repeat is only observed in diffraction patterns from images recorded by minimum beam methods (10 to 20 e/Ų) and arises from small, but consistent, structural differences between two similar subunits believed to represent the two chemical species of tubulin monomer (M_r, 55,000). At higher electron doses (100 to 200 e/Ų), the additional information is lost or very much reduced, and only a repeat of 41 Å is observed, owing to the loss of distinction between monomers in the tubulin heterodimer.The sheets are composed of 49 Å wide, polar protofilaments, similar to those observed in microtubules; however, the interprotofilament packing is completely different in the two structures. In these sheets, adjacent protofilaments point and face in opposite directions; i.e. they are related by dyad-screw axes normal to the protofilament axes and in the plane of the sheet. Thus, the zinc-induced sheets are crystals of space group P2₁, with cell dimensions of about 97 Å × 82 Å, containing one tubulin heterodimer per asymmetric unit.Reconstructed images of four individual sheets, and their average, show the arrangement and shapes of the two heterodimers contained in each unit cell. The structure and packing of heterodimers in sheets are compared to those in opened out microtubules where all protofilaments point and face in the same direction.     

Structural studies on porcine brain tubulin in extended sheets

Structural studies have been conducted on porcine brain tubulin, assembled into microtubule-related structures, by electron microscopy in conjunction with optical diffraction and image reconstruction techniques. By minimizing background noise and sample damage, we have improved the resolution on negatively stained samples, extending the data from the previous limit of a 42 Å layer line to an additional layer line at 21 Å. The new reflections confirm the basic surface lattice proposed from the earlier studies and extend the structural features that can be assigned to individual tubulin molecules. Data are obtained for microtubules in standard buffers for both the helical form and flat sheets of up to 13 protofilaments. When zinc is added to the preparations, sheets with more than 13 protofilaments are formed and the extended lattices provide more reflections on both the 42 Å and 21 Å layer lines, as well as the equator. The lattice in the presence of zinc differs considerably from the normal lattice, with adjacent protofilaments staggered by 21 Å, compared to the staggering of adjacent filaments of about 10 Å in the absence of zinc. There is also a distinct pairing of adjacent protofilaments in the zinc-induced sheets. Initial studies with the Unwin-Henderson method on unstained zinc-tubulin sheets suggest that the adjacent protofilaments may be related by a dyad axis, either perpendicular or parallel to the protofilament axes.     

HURP wraps microtubule ends with an additional tubulin sheet that has a novel conformation of tubulin

HURP is a newly discovered microtubule-associated protein (MAP) required for correct spindle formation both in vitro and in vivo. HURP protein is highly charged with few predicted secondary and tertiary folding domains. Here we explore the effect of HURP on pure tubulin, and describe its ability to induce a new conformation of tubulin sheets that wrap around the ends of intact microtubules, thereby forming two concentric tubes. The inner tube is a normal microtubule, while the outer one is a sheet composed of tubulin protofilaments that wind around the inner tube with a 42.5° inclination. We used cryo-electron microscopy and unidirectional surface shadowing to elucidate the structure and conformation of HURP-induced tubulin sheets and their interaction with the inner microtubule. These studies clarified that HURP-induced sheets are composed of anti-parallel protofilaments exhibiting P2 symmetry. HURP is a unique MAP that not only stabilizes and bundles microtubules, but also polymerizes free tubulin into a new configuration.     

Promotion of MAP/MAP interaction by taxol

The effects of taxol on microtubule-associated proteins of high molecular weight (MAPs) were studied in vitro. After negative staining, microtubules reconstituted in the presence of taxol from preparations of partially purified tubulin and MAPs, besides being bundled, displayed prominent elongated or globular extensions without apparent regularity. These extensions, but not the tubulin polymer, were heavily decorated after immuno-gold-labeling using antibodies to MAP-1 and MAP-2. Microtubules reconsituted in the absence of taxol showed a much more regular, and apparently helical, arrangement of MAPs along their surfaces. The formation of polymeric structures was also observed when preparation of MAPs free of tubulin were incubated with taxol. In this case in addition to large network-type aggregates with little apparent substructure, more regular structures seemingly consisting of approximately 5-nm-thick filaments arrayed in parallel were observed. Taxol-induced MAP aggregation occurred rapidly and was directly proportional to the concentration of protein, as revealed by optical density measurements. It is concluded that taxol, aside from promoting the assembly of tubulin and stabilizing microtubules, promotes MAP/MAP interaction.     

Microtubules and microtubule-associated proteins from the nematode Caenorhabditis elegans: periodic cross-links connect microtubules in vitro

The nematode Caenorhabditis elegans should be an excellent model system in which to study the role of microtubules in mitosis, embryogenesis, morphogenesis, and nerve function. It may be studied by the use of biochemical, genetic, molecular biological, and cell biological approaches. We have purified microtubules and microtubule-associated proteins (MAPs) from C. elegans by the use of the anti-tumor drug taxol (Vallee, R. B., 1982, J. Cell Biol., 92:435-44). Approximately 0.2 mg of microtubules and 0.03 mg of MAPs were isolated from each gram of C. elegans. The C. elegans microtubules were smaller in diameter than bovine microtubules assembled in vitro in the same buffer. They contained primarily 9-11 protofilaments, while the bovine microtubules contained 13 protofilaments. The principal MAP had an apparent molecular weight of 32,000 and the minor MAPs were 30,000, 45,000, 47,000, 50,000, 57,000, and 100,000-110,000 mol wt as determined by SDS-gel electrophoresis. The microtubules were observed, by electron microscopy of negatively stained preparations, to be connected by stretches of highly periodic cross-links. The cross-links connected the adjacent protofilaments of aligned microtubules, and occurred at a frequency of one cross-link every 7.7 +/- 0.9 nm, or one cross-link per tubulin dimer along the protofilament. The cross-links were removed when the MAPs were extracted from the microtubules with 0.4 M NaCl. The cross-links then re-formed when the microtubules and the MAPs were recombined in a low salt buffer. These results strongly suggest that the cross-links are composed of MAPs.     

A single protofilament is sufficient to support unidirectional walking of Dynein and Kinesin

Cytoplasmic dynein and kinesin are two-headed microtubule motor proteins that move in opposite directions on microtubules. It is known that kinesin steps by a 'hand-over-hand' mechanism, but it is unclear by which mechanism dynein steps. Because dynein has a completely different structure from that of kinesin and its head is massive, it is suspected that dynein uses multiple protofilaments of microtubules for walking. One way to test this is to ask whether dynein can step along a single protofilament. Here, we examined dynein and kinesin motility on zinc-induced tubulin sheets (zinc-sheets) which have only one protofilament available as a track for motor proteins. Single molecules of both dynein and kinesin moved at similar velocities on zinc-sheets compared to microtubules, clearly demonstrating that dynein and kinesin can walk on a single protofilament and multiple rows of parallel protofilaments are not essential for their motility. Considering the size and the motile properties of dynein, we suggest that dynein may step by an inchworm mechanism rather than a hand-over-hand mechanism.     

Virions and membrane proteins of the potato X virus interact with microtubules and enables tubulin polymerization in vitro

A study was made of the in vitro interactions of virions and the coat protein (CP) of the potato virus X (PVX) with microtubules (MT). Both virions and CP cosedimented with taxol-stabilized MT. In the presence of PVX CP, tubulin polymerized to produce structures resistant to chilling. Electron microscopy revealed the aberrant character of the resulting tubulin polymers (protofilaments and their sheets), which differed from MT assembled in the presence of cell MAP2. In contrast, PVX virions induced the assembly of morphologically normal MT sensitive to chilling. Virions were shown to compete with MAP2 for MT binding, suggesting an overlap for the MT sites interacting with MAP2 and with PVX virions. It was assumed that PVX virions interact with MT in vivo and that, consequently, cytoskeleton elements participate in intracellular compartmentalization of the PVX genome.     

A taxol-dependent procedure for the isolation of microtubules and microtubule-associated proteins (MAPs)

The effect of the antimitotic drug taxol on the association of MAPs (microtubule-associated proteins) with microtubules was investigated. Extensive microtubule assembly occurred in the presence of Taxol at 37 degrees C. at 0 degrees C, and at 37 degrees C in the presence of 0.35 M NaCl, overcoming the inhibition of assembly normally observed under the latter two conditions. At 37 degrees C and at 0 degrees C, complete assembly of both tubulin and the MAPs was observed in the presence of Taxol. However, at elevated ionic strength, only tubulin assembled, forming microtubules devoid of MAPs. The MAPs could also be released from the surface of preformed microtubules by exposure to elevated ionic strength. These properties provided the basis for a rapid new procedure for isolating microtubules and MAPs of high purity from small amounts of biological material. The MAPs could be recovered by exposure of the microtubules to elevated ionic strength and subjected to further analysis. Microtubules and MAPs were prepared from bovine cerebral cortex (gray matter) and from HeLa cells. MAP 1, MAP2, and the tau MAPs, as well as species of Mr = 28,000 and 30,000 (LMW, or low molecular weight, MAPs) and a species of Mr = 70,000 were isolated from gray matter. Species identified as the 210,000 and 125,000 mol wt HeLa MAPs were isolated from HeLa cells. Microtubules were also prepared for the first time from white matter. All of the MAPs identified in gray matter preparations were identified in white matter, but the amounts of individual MAP species differed. The most striking difference in the two preparations was a fivefold lower level of MAP 2 relative to tubulin in white matter than in gray. The high molecular weigh MAP, MAP1, was present in equal ratio to tubulin in white and gray matter. These results indicate that MAP 1 and MAP2, as well as other MAP species, may have a different cellular or subcellular distribution.     

Virions and the Coat Protein of the Potato Virus X Interact with Microtubules and Induce Tubulin Polymerization In Vitro

A study was made of the in vitro interactions of virions and the coat protein (CP) of the potato virus X (PVX) with microtubules (MT). Both virions and CP cosedimented with taxol-stabilized MT. In the presence of PVX CP, tubulin polymerized to produce structures resistant to chilling. Electron microscopy revealed the aberrant character of the resulting tubulin polymers (protofilaments and their sheets), which differed from MT assembled in the presence of cell MAP2. In contrast, PVX virions induced the assembly of morphologically normal MT sensitive to chilling. Virions were shown to compete with MAP2 for MT binding, suggesting an overlap for the MT sites interacting with MAP2 and with PVX virions. It was assumed that PVX virions interact with MT in vivo and that, consequently, cytoskeleton elements participate in intracellular compartmentalization of the PVX genome.     

Different assembly properties of cod, bovine, and rat brain microtubules.

Assembly properties of cod, bovine, and rat brain microtubules were compared. Estramustine phosphate, heparin, poly-L-aspartic acid, as well as NaCl, inhibited the assembly and disassembled both bovine and rat microtubules by inhibition of the binding between tubulin and MAPs. The assembly of cod brain microtubules was in contrast only marginally affected by these agents, in spite of a release of the MAPs. The results suggest that cod tubulin has a high intrinsic ability to assemble. This was confirmed by studies on phosphocellulose-purified cod tubulin, since the critical concentration for assembly was independent of the presence or absence of MAPs. The results show therefore that cod brain tubulin has, in contrast to bovine and rat brain tubulins, a high propensity to assembly under conditions which normally require the presence of MAPs. Even if cod MAPs, which have an unusual protein composition, were not needed for the assembly of cod microtubules, they were able to induce assembly of bovine brain tubulin. Both cod and bovine MAPs bound to cod microtubules, and bovine MAP1 and MAP2 bound to, and substituted at least the 400 kDa cod protein. This suggests that the tubulin-binding sites and the assembly-stimulatory ability of MAPs are common properties of MAPs from different species, independent of the tubulin assembly propensity.     

The pool of map kinase associated with microtubules is small but constitutively active.

Mitogen-activated protein kinase (MAPK) is activated by many kinds of stimuli and plays an important role in integrating signal transduction cascades. MAPK is present abundantly in brain, where we have studied its association with microtubules. Immunofluorescence of primary hippocampal neurons revealed that MAPK staining co-localized with microtubules and biochemical analyses showed that MAPK co-purified with microtubules. Approximately 4% of MAPK in cytosolic extracts was associated with microtubules, where it was associated with both tubulin and microtubule-associated proteins (MAPs) fractions. Further fractionation of MAPs suggested that a portion of MAPK is associated with MAP2. An association with MAP2 was also demonstrated by co-immunoprecipitation and in vitro binding experiments. A similar association was shown for the juvenile MAP2 isoform, MAP2C. The pool of MAPK associated with microtubules had a higher activity relative to the nonassociated pool in both brain and proliferating PC12 cells. Although MAPK was activated by nerve growth factor in PC12 cells, the activity of microtubule-associated MAPK did not further increase. These results raise the possibility that microtubule-associated MAPK operates through constitutive phosphorylation activity to regulate microtubule function in neurons.     

Effects of tubulin acetylation and tubulin acetyltransferase binding on microtubule structure

Tubulin undergoes posttranslational modifications proposed to specify microtubule subpopulations for particular functions. Most of these modifications occur on the C-termini of tubulin and may directly affect the binding of microtubule-associated proteins (MAPs) or motors. Acetylation of Lys-40 on α-tubulin is unique in that it is located on the luminal surface of microtubules, away from the interaction sites of most MAPs and motors. We investigate whether acetylation alters the architecture of microtubules or the conformation of tubulin, using cryo–electron microscopy (cryo-EM). No significant changes are observed based on protofilament distributions or microtubule helical lattice parameters. Furthermore, no clear differences in tubulin structure are detected between cryo-EM reconstructions of maximally deacetylated or acetylated microtubules. Our results indicate that the effect of acetylation must be highly localized and affect interaction with proteins that bind directly to the lumen of the microtubule. We also investigate the interaction of the tubulin acetyltransferase, αTAT1, with microtubules and find that αTAT1 is able to interact with the outside of the microtubule, at least partly through the tubulin C-termini. Binding to the outside surface of the microtubule could facilitate access of αTAT1 to its luminal site of action if microtubules undergo lateral opening between protofilaments.     

Selective purification of microtubule-associated proteins 1 and 2 from rat brain using poly(L-aspartic acid)

A rapid and selective purification procedure for microtubule-associated protein (MAP) 1 and MAP 2 has been established. This procedure is based upon the fact that poly(L-aspartic acid) (PLAA) can specifically remove MAP 1 from microtubules polymerized by taxol (Nakamura et al., 1989, J. Biochem. 106, 93-97). MAP 1 released by PLAA was further purified by column chromatography on phosphocellulose and Bio-Gel A-15m. The purified MAP 1 contained MAPs 1A and 1 B. From microtubules devoid of MAP 1, MAP 2, consisting of MAPs 2A and 2B, could also be isolated by exposure to high ionic strength solutions in the presence of taxol without heat treatment. Both MAPs 1 and 2 cosedimented with microtubules consisting of purified tubulin.     

Binding specificities of purified porcine brain alpha- and beta-tubulin subunits and of microtubule-associated proteins 1 and 2 examined by electron microscopy and solid-phase binding assays

In this study, the molecular interaction of separated alpha- and beta-tubulin with purified microtubule-associated protein 1 (MAP 1) and MAP 2 was studied using electron microscopy and solid-phase binding assays with 125I-radiolabeled proteins. Electron microscopy of proteins recovered from sodium dodecyl sulfate polyacrylamide gels and subsequently incubated in various combinations under conditions promoting tubulin polymer formation revealed that both subunits have binding sites for MAP 1 as well as MAP 2. Overlays of nitrocellulose-transblotted MAPs with electrophoretically separated tubulin subunits eluted from gels confirmed these results. In overlays of nitrocellulose-immobilized tubulin subunits with gel-eluted MAP 2, self-association of MAP 2, but no binding to tubulin was detected. However, overlays with MAP 1 and MAP 2 purified under nondenaturing conditions revealed binding of both MAPs to beta-tubulin. In addition, these experiments demonstrated binding of both MAPs to MAP 2 and to the neurofilament proteins NF 70, NF 150 and NF 200. It is concluded that both alpha- and beta-tubulin possess binding sites for MAP 1 as well as MAP 2, but that the accessibility and/or binding affinity of these sites are strongly dependent on the tertiary structure of proteins. The demonstrated in vitro binding of MAP 1 and MAP 2 to all three neurofilament proteins as well as to MAP 2 confirms their presumed role as cytoskeletal linking proteins.     

ZipA is a MAP-Tau homolog and is essential for structural integrity of the cytokinetic FtsZ ring during bacterial cell division

The first visible event in prokaryotic cell division is the assembly of the soluble, tubulin-like FtsZ GTPase into a membrane-associated cytokinetic ring that defines the division plane in bacterial and archaeal cells. In the temperature-sensitive ftsZ84 mutant of Escherichia coli, this ring assembly is impaired at the restrictive temperature causing lethal cell filamentation. Here I present genetic and morphological evidence that a 2-fold higher dosage of the division gene zipA suppresses thermosensitivity of the ftsZ84 mutant by stabilizing the labile FtsZ84 ring structure in vivo. I demonstrate that purified ZipA promotes and stabilizes protofilament assembly of both FtsZ and FtsZ84 in vitro and cosediments with the protofilaments. Furthermore, ZipA organizes FtsZ protofilaments into arrays of long bundles or sheets that probably represent the physiological organization of the FtsZ ring in bacterial cells. The N-terminal cytoplasmic domain of membrane-anchored ZipA contains sequence elements that resemble the microtubule-binding signature motifs in eukaryotic Tau, MAP2 and MAP4 proteins. It is postulated that the MAP-Tau-homologous motifs in ZipA mediate its binding to FtsZ, and that FtsZ-ZipA interaction represents an ancient prototype of the protein-protein interaction that enables MAPs to suppress microtubule catastrophe and/or to promote rescue.