Definition of individual components within the cytoskeleton of Trypanosoma brucei by a library of monoclonal antibodies.

The detergent-insoluble T. brucei cytoskeleton consists of several morphologically distinct regions and organelles, many of which are detectable only by electron microscopy. We have produced a set of monoclonal antibodies that define each structural component of this highly ordered cytoskeleton. The monoclonal antibodies were selected by cloning of hybridomas produced from mice injected with complex mixtures of proteins of either the cytoskeleton itself or salt extracts thereof. Four antibodies define particular tubulin isotypes and locate the microtubules of the axoneme and sub-pellicular array; two antibodies recognize the flagellum attachment zone; one recognizes the paraflagellar rod and another the basal bodies. Finally, one antibody defines a detergent-insoluble component of the nucleus. The antigens detected by each monoclonal antibody have been analysed by immunofluorescence microscopy, immunogold electron microscopy and Western blotting.

Tubulin post-translational modifications and the construction of microtubular organelles in Trypanosoma brucei.

We have used specific monoclonal antibodies to facilitate a study of acetylated and tyrosinated alpha-tubulin in the microtubule (MT) arrays in the Trypanosoma brucei cell. Acetylated alpha-tubulin is not solely located in the stable microtubular arrays but is present even in the ephemeral microtubules of the mitotic spindle. Moreover, there is a uniform distribution of this isoform in all arrays. Studies of flagella complexes show that acetylation is concomitant with assembly of MTs. There is no subsequent major modulation in the content of acetylated alpha-tubulin in MTs. Conversely, polymerizing flagellar MTs have a high tyrosinated alpha-tubulin content, which is subsequently reduced to a basal level at a discrete point in the cell cycle. The MTs of the intranuclear mitotic spindle appear never to contain tyrosinated alpha-tubulin, suggesting that they are actually constructed of detyrosinated alpha-tubulin or that detyrosination is extremely rapid at this time in the cell cycle. T. brucei therefore, represents a cell type with extremely active mechanisms for the post-translational modification of alpha-tubulin. Our analyses of the timing of acquisition and modulation in relation to MT construction in T. brucei, suggest that acetylation and detyrosination of alpha-tubulin are two independently regulated post-translational modifications, that are not uniquely associated with particular subsets of MTs of defined lability, position or function. Post-assembly detyrosination of alpha-tubulin may provide a mechanism whereby the cell could discriminate between new and old MTs, during construction of the cytoskeleton through the cell cycle. However, we also suggest that continuation of detyrosination, allows the cell, at cell division, to partition into daughter cells two equivalent sets of cytoskeletal MTs.

Distinct localization and cell cycle dependence of COOH terminally tyrosinolated alpha-tubulin in the microtubules of Trypanosoma brucei brucei.

alpha-Tubulin can be posttranslationally modified in that its COOH-terminal amino acid residue, tyrosine, can be selectively removed and replaced again. This reaction cycle involves two enzymes, tubulin carboxypeptidase and tubulin tyrosine ligase. The functional significance of this unusual modification is unclear. The present study demonstrates that posttranslational tyrosinolation of alpha-tubulin does occur in the parasitic hemoflagellate Trypanosoma brucei brucei and that posttranslational tyrosinolation can be detected in both alpha-tubulin isoforms found in this organism. Trypanosomes contain a number of microtubular structures: the flagellar axoneme; the subpellicular layer of singlet microtubules which are closely associated with the cell membrane; the basal bodies; and a cytoplasmic pool of soluble tubulin. Tyrosinolated alpha-tubulin is present in all these populations. However, immunofluorescence studies demonstrate a distinct localization of tyrosinolated alpha-tubulin within individual microtubules and organelles. This localization is subject to a temporal modulation that correlates strongly with progress of a cell through the cell cycle. Our results indicate that the presence of tyrosinolated alpha-tubulin is a marker for newly formed microtubules.

Subpellicular and flagellar microtubules of Trypanosoma brucei brucei contain the same alpha-tubulin isoforms.

The cytoskeleton of the parasitic hemoflagellate Trypanosoma brucei brucei essentially consists of two microtubule-based structures: a subpellicular layer of singlet microtubules, which are in close contact with the cell membrane, and the flagellar axoneme. In addition, the cells contain a small pool of soluble tubulin. Two-dimensional gel electrophoretic analysis of the tubulins present in these subcellular compartments revealed two distinct electrophoretic isoforms of alpha-tubulin, termed alpha 1 and alpha 3. alpha 1-Tubulin most likely represents the primary translation product, while alpha 3-tubulin is a posttranslationally acetylated derivative of alpha 1-tubulin. In the pool of soluble cytoplasmic tubulin, alpha 1 is the predominant species, while the very stable flagellar microtubules contain almost exclusively the alpha 3-tubulin isoform. The subpellicular microtubules contain both isoforms. Neither of the two alpha-tubulin isoforms is organelle specific, but the alpha 3 isoform is predominantly located in stable microtubules.

Acetylated alpha-tubulin in Physarum: immunological characterization of the isotype and its usage in particular microtubular organelles.

We have used monoclonal antibodies specific for acetylated and unacetylated alpha-tubulin to characterize the acetylated alpha-tubulin isotype of Physarum polycephalum, its expression in the life cycle, and its localization in particular microtubular organelles. We have used the monoclonal antibody 6-11B-1 (Piperno, G., and M. T. Fuller, 1985, J. Cell Biol., 101:2085-2094) as the probe for acetylated alpha-tubulin and have provided a biochemical characterization of the monoclonal antibody KMP-1 as a probe for unacetylated tubulin in Physarum. Concomitant use of these two probes has allowed us to characterize the acetylated alpha-tubulin of Physarum as the alpha 3 isotype. We have detected this acetylated alpha 3 tubulin isotype in both the flagellate and in the myxameba, but not in the plasmodium. In the flagellate, acetylated tubulin is present in both the flagellar axonemes and in an extensive array of cytoplasmic microtubules. The extensive arrangement of acetylated cytoplasmic microtubules and the flagellar axonemes are elaborated during the myxameba-flagellate transformation. In the myxameba, acetylated tubulin is not present in the cytoplasmic microtubules nor in the mitotic spindle microtubules, but is associated with the two centrioles of this cell. These findings, taken together with the apparent absence of acetylated alpha-tubulin in the ephemeral microtubules of the plasmodium suggest a natural correspondence between the presence of acetylated alpha-tubulin and microtubule organelles that are intrinsically stable or cross-linked.

Tubulin isotypes: generation of diversity in cells and microtubular organelles.

Diversity of tubulin isotypes is illustrated by consideration of the beta-tubulin isotypes of higher plants and the eukaryotic microbe, Physarum polycephalum, and by the alpha-tubulin isotypes of the protozoan, Trypanosoma brucei. The carrot plant expresses six, well-defined beta-tubulin isotypes that possess characteristic two-dimensional gel coordinates. These six beta-tubulin isotypes are differentially expressed during development of the flowering plant. In a similar manner, Physarum expresses three separate beta-tubuli isotypes during its life cycle; of the two beta 1 isotypes, one is expressed solely in the myxamoeba whilst the other is expressed both in the myxamoeba and in the plasmodium. A further beta-tubulin isotype, beta 2, is expressed only in the plasmodium. In carrot and in Physarum the generation of beta-tubulin diversity appears, in the main, to be generated by the differential expression of a beta-tubulin multi-gene family. However, tubulin isotypes can also be generated by post-translational modifications and T. brucei utilizes two different modifications within one cell. First, the primary translation product, the alpha 1-tubulin isotype, can be acetylated to produce the alpha 3 isotype. Second, both the alpha 1 and alpha 3 isotypes appear to exist in both tyrosinated and detyrosinated forms. The generation of these alpha-tubulin isotypes within the same cell and their presence in particular cellular domains, modulated throughout the cell cycle, reveals a complex relationship between alpha-tubulin isotypes produced by post-translational modifications and the dynamics of microtubule construction.