Colchicine-binding sites of brain tubulins from an antarctic fish and from a mammal are functionally similar, but not identical: implications for microtubule assembly at low temperature

The p53 inhibitor, pifithrin-α, disrupts microtubule organization, arrests growth, and induces polyploidy in the rainbow trout gill cell line, RTgill-W1

Pifithrin-α (PFT-α) blocks p53-dependent transcription and is an example of the many drugs being developed to target the p53 pathway in humans that could be released into the environment with potential impacts on aquatic animals if they were to become successful pharmaceuticals. In order to understand how p53 drugs might act on fish, the effects of PFT-α on rainbow trout gill epithelial cell line, RTgill-W1, were studied. PFT-α was not cytotoxic to RTgill-W1 in cultures with or without fetal bovine serum (FBS), but at 5.25μg/ml, PFT-α completely arrested proliferation. When FBS was present, PFT-α increased the number of polyploid cells over 12days. Those results suggest that like in mammals, p53 appears to regulate ploidy in fish. However, several effects were seen that have not been observed with mammalian cells. PFT-α caused a transient rise in the mitotic index and a disruption in cytoskeletal microtubules. These results suggest that in fish cells PFT-α affects microtubules either directly through an off-target action on tubulin or indirectly through an on-target action on p53-regulated transcription.

Properties of microtubules assembled from mammalian tubulin synthesized in Escherichia coli

When isolated from tissues, the alpha beta-dimeric protein tubulin consists of multiple isoforms which originate from the expression and subsequent posttranslational modification of multiple polypeptide sequences. Microtubules studied in vitro consist of mixtures of these isoforms. It is therefore not known whether dimers composed of single sequences of alpha- and beta-tubulin can polymerize to form microtubules, or whether posttranslational modifications may be necessary for microtubule assembly. To initiate investigation of these questions, rabbit reticulocyte lysate, which contains the cytoplasmic chaperonin CCT and its cofactors, was employed to prepare substantial quantities (tens of micrograms) of active tubulin by in vitro folding of mouse alpha- and beta-tubulins recombinantly synthesized in E. coli. This recombinant tubulin is composed of only a single alpha-chain and a single beta-chain. When analyzed after folding by isoelectric focusing, each chain yielded only one band, indicating that neither was detectably posttranslationally modified in the course of the folding reaction. When subjected to assembly-promoting conditions, this tubulin formed microtubules without the addition of any exogenous protein. Electron microscopy showed them to be of normal morphology. Analysis of their protein composition showed that they are composed nearly entirely of recombinant tubulin. These results demonstrate that the naturally occurring mixtures of isoforms are not strictly required for the formation of microtubules. They also open a route to other studies, both biomedical and structural, of fully defined tubulin in vitro.

Assembly and colchicine binding characteristics of tubulin with maximally tyrosinated and detyrosinated alpha-tubulins

The posttranslational removal and readdition of tyrosine at the C-terminus of alpha-tubulin is associated with generation of microtubule populations that differ in intracellular distributions, turnover rates, and sensitivities to microtubule-depolymerization agents. Here, we compared the in vitro assembly and colchicine binding characteristics of tubulin dimer preparations composed of alpha-tubulin that had been maximally tyrosinated (approximately 40% tyrosinated) by tubulin-tyrosine ligase and maximally detyrosinated (100% detyrosinated) by carboxypeptidase A. Maximally tyrosinated and detyrosinated tubulins had similar critical concentrations for polymerization and similar association constants for colchicine binding. Microtubules polymerized from the two tubulins also had similar steady-state mean lengths and length distributions. The growing and shortening dynamics (dynamic instability parameters) of individual microtubules made from maximally tyrosinated or detyrosinated alpha-tubulin as determined by video-enhanced dark-field microscopy were similar, but subtle differences in the growing and shortening rates were found. On balance, however, the dynamicity and thus the overall kinetic stability of the two microtubule populations were indistinguishable. The results support the idea that detyrosination of alpha-tubulin does not by itself generate stable microtubules.

Microtubule assembly in cold-adapted organisms: functional properties and structural adaptations of tubulins from antarctic fishes

Fishes native to the coastal waters of the Antarctic have adapted to habitat and body temperatures in the range -1.8 to +2 degrees C. Their cytoplasmic microtubules, unlike those of mammals and temperate poikilotherms, have evolved to assemble efficiently at these low temperatures. To learn about the underlying molecular adaptations, my laboratory is studying microtubule proteins [tubulin alpha beta dimers and microtubule-associated proteins (MAPs)] and tubulin genes from several Antarctic fishes, including the rockcods Notothenia coriiceps and Gobionotothen gibberifrons. We find that the assembly-enhancing adaptations of the fish microtubule proteins are intrinsic to the tubulin subunits themselves. Furthermore, microtubule formation by Antarctic fish tubulins is strongly entropy driven, due in part to an increased reliance, relative to tubulins from other species, on hydrophobic interactions. Based on analyses of tubulin polypeptides and cDNAs, we suggest that the structural adaptations of Antarctic fish tubulins most likely involve alterations in the primary sequences of tubulin isotypes. With respect to neural beta tubulins from other vertebrates, for example, the class II beta-tubulin isotype of N. coriiceps brain contains seven unique amino acid substitutions and one novel insertion in its 446-residue primary sequence. Most of these changes are located in a structural domain that forms contacts between tubulin dimers during microtubule assembly and would be expected to enhance polypeptide flexibility, thereby facilitating addition of tubulin to microtubule ends. The acidic carboxy-terminal tails of the alpha and beta tubulins, by contrast, appear not to be sites of cold adaptation of polymerization. We have also found that brain and egg tubulins from Antarctic fishes differ strikingly in their polymerization efficiencies, which demonstrates, in agreement with the multitubulin hypothesis, that tissue-specific tubulin isoforms can possess distinct functional properties. Thus, study of microtubule proteins from organisms, such as the Antarctic fishes, that have adapted to extreme thermal regimes should contribute significantly to an understanding of the quaternary interactions that control microtubule assembly in all eukaryotes.

Cold-stable and cold-adapted microtubules

Most mammalian microtubules disassemble at low temperature, but some are cold stable. This probably has little to do with a need for cold-stable microtubules, but reflects that certain populations of microtubules must be stabilized for specific functions. There are several routes by which to achieve cold stability. Factors that interact with microtubules, such as microtubule-associated proteins, STOPs (stable tubule only polypeptides), histones, and possibly capping factors, are involved. Specific tubulin isotypes and posttranslational modifications might also be of importance. More permanent stable microtubules can be achieved by bundling factors, associations to membranes, as well as by assembly of microtubule doublets and triplets. This is, however, not the explanation for cold adaptation of microtubules from poikilothermic animals, that is, animals that must have all their microtubules adapted to low temperatures. All evidence so far suggests that cold adaptation is intrinsic to the tubulins, but it is unknown whether it depends on different amino acid sequences or posttranslational modifications.

Comparative study of the colchicine binding site and the assembly of fish and mammalian microtubule proteins

Isolated microtubules from cod (Gadus morhua) are apparently more stable to colchicine than bovine microtubules. In order to further characterize this difference, the effect of the colchicine analogue 2-methoxy-5-(2,3,4-trimethoxyphenyl)-2,4,6-cyclo heptatrien-1-one (MTC) was studied on assembly, as measured by turbidity and sedimentation analysis, and on polymer morphology. MTC has the advantage to bind fast and reversible to the colchicine binding site of tubulin even at low temperatures. It was found to bind to one site in cod brain tubulin, with affinity (6.5 +/- 1.5) x 10(5)M-1 at both low or high temperature, similarly to bovine brain tubulin. However, the effect of the binding differed. At substoichiometric concentrations of MTC bovine brain microtubule assembly was almost completely inhibited, while less effect was seen on the mass of polymerized cod microtubule proteins. A preformed bovine tubulin-colchicine complex inhibited the assembly of both cod and bovine microtubules at substoichiometric concentrations, but the effect on the assembly of cod microtubules was less. At higher concentrations (5 x 10(-5) to 1 x 10(-3) M), MTC induced a large amount of cold-stable spirals of cod proteins, whereas abnormal polymers without any defined structure were formed from bovine proteins. Spirals of cod microtubule proteins were only formed in the presence of microtubule associated proteins (MAPs), indicating that the morphological effect of MTC can be modulated by MAPs. The effects of colchicine and MTC differed. At 10(-5) M colchicine no spirals were formed, while at 10(-4) M and 10(-3) M, a mixture of spirals and aggregates was found. The morphology of the spirals differed both from vinblastine spirals and from the spirals previously found when cod microtubule proteins polymerize in the presence of high Ca2+ concentrations. The present data show that even if the colchicine binding site is conserved between many different species, the bindings have different effects which seem to depend on intrinsic properties of the different tubulins.

Divergent neural beta tubulin from the Antarctic fish Notothenia coriiceps neglecta: potential sequence contributions to cold adaptation of microtubule assembly

The cytoplasmic microtubules of the cold-adapted Antarctic fishes, unlike those of homeotherms and temperate poikilotherms, assemble and function at body temperatures in the range -1.8 to +2 degrees C. To determine whether alterations to the primary sequence of beta tubulin may contribute to enhancement of microtubule assembly at cold temperatures, we have cloned and sequenced a 1.8-kilobase neural beta-chain cDNA, Ncn beta 1, from an Antarctic rockcod, Notothenia coriiceps neglecta. Based on nucleotide sequence homology, Ncn beta 1 probably corresponds to a class-II beta-tubulin gene. The 446-residue beta chain encoded by Ncn beta 1 is closely related (sequence homology approximately 95%) both to the neural class-I/II isotypes and to the neural/testicular class-IV variants of higher vertebrates, but the sequence of its carboxy-terminal isotype-defining region (residues 431-446) has diverged markedly (> or = 25% change relative to the I/II/IV referents). Furthermore, the Ncn beta 1 polypeptide contains six unique amino-acid substitutions (five conservative, one nonconservative) not found in other vertebrate brain isotypes, and the carboxy-terminal region possesses a unique tyrosine inserted at position 442. We conclude that Ncn beta 1 encodes a class-II beta tubulin that contains sequence modifications, located largely in its interdimer contact domain, that may contribute to cold adaptation of microtubule assembly.