Previously, we have demonstrated the presence of a protein factor [tubulin polymerization perturbing protein (TPPP)] in brain and neuroblastoma cell but not in muscle extract that uniquely influences the microtubule assembly. Here we describe a procedure for isolation of this protein from the cytosolic fraction of bovine brain and present evidence that this protein is a target of both tubulin and microtubules in vitro. The crucial step of the purification is the cationic exchange chromatography; the bound TPPP is eluted at high salt concentrations, indicating the basic character of the protein. By IDA-nanoLC-MS analysis of the peptides extracted from the gel-digested purified TPPP, we show the presence of a single protein in the purified fraction that corresponds to p25, a brain-specific protein the function of which has not been identified. Circular dichroism data have revealed that, on one hand, the alpha-helix content of p25 is very low (4%) with respect to the predicted values (30-43%), and its binding to tubulin induces remarkable alteration in the secondary structure of the protein(s). As shown by turbidimetry, pelleting experiments, and electron microscopy, p25 binds to paclitaxel-stabilized microtubules and bundles them. p25 induces formation of unusual (mainly double-walled) microtubules from tubulin in the absence of paclitaxel. The amount of aberrant tubules formed depends on the p25 concentration, and the process occurs at substoichiometric concentrations. Our in vitro data suggest that p25 could act as a unique MAP in vivo.

Recently, we isolated from bovine brain a protein, TPPP͞p25 and identified as p25, a brain-specific protein that induced aberrant tubulin assemblies. The primary sequence of this protein differs from that of other proteins identified so far; however, it shows high homology with p25-like hypothetical proteins sought via BLAST. Here, we characterized the binding of TPPP͞p25 to tubulin by means of surface plasmon resonance; the kinetic parameters are as follows: kon, 2.4 ؋ 10 4 M ؊1 ⅐s ؊1 ; koff, 5.4 ؋ 10 ؊3 s ؊1 ; and Kd, 2.3 ؋ 10 ؊7 M. This protein at substoichometric concentration promotes the polymerization of tubulin into double-walled tubules and polymorphic aggregates or bundles paclitaxel-stabilized microtubules as judged by quantitative data of electron and atomic force microscopies. Injection of bovine TPPP͞p25 into cleavage Drosophila embryos expressing tubulin-GFP fusion protein reveals that TPPP͞p25 inhibits mitotic spindle assembly and nuclear envelope breakdown without affecting other cellular events like centrosome replication and separation, microtubule nucleation by the centrosomes, and nuclear growth. GTP counteracts TPPP͞p25 both in vitro and in vivo.T he cytoplasm of eukaryotic cells contains an elaborate network of cytoskeletal elements, consisting of actin and intermediate filaments and microtubules (MTs) engaged in a variety of cell functions, such as the extension and guidance of neurons or the formation of mitotic spindles required for chromosomal segregation. The polymerization dynamics of tubulin to MTs is under strict control (1). Numerous proteins have been reported to interact with the MTs as positive regulators of MT assembly (microtubule-associated proteins), either by promoting the polymerization of tubulin or by stabilizing MTs (1, 2). Only a few proteins are known to act as destabilizers, such as Op18͞stathmin, katanin, and some kinesin-like proteins (1, 2). We have reported recently that the M1 isoform of pyruvate kinase and Dictyostelium discoideum phosphofructokinase inhibit tubulin polymerization or promote disassembly of the MTs to thread-like oligomers in vitro (3-5).Recently, from bovine brain, we isolated and identified a protein that we denoted TPPP͞p25 (6), which corresponded to p25 (NCBI accession no. 2498194). This protein is a heat stable, cationic protein that induces polymerization of tubulin into unusual forms or bundling of paclitaxel-stabilized MT. The TPPP͞p25 protein was partially copurified with a tau kinase (7). The bovine TPPP͞p25-coding gene has been cloned (8), and recently the human homologue of TPPP͞p25 was cloned and mapped to the p15.3 region of chromosome 5. The identity between the bovine and human TPPP͞p25 proteins is 90% (9). This protein was also described as p24, a heat-resistant glycogen synthase kinase-3 inhibitor (10). It is important to note that TPPP͞p25 differs completely from the extensively characterized protein p25, which is a truncated form of p35 that deregulates cyclin-dependent kinase-5 activity by causing prolonged activation and m...