On active duty: The available drugs for cardiovascular diseases can promote blood clotting but can also lead to life‐threatening bleeding episodes. A promising target for the development of safer alternatives is the transglutaminase Factor XIII (FXIII), the active structure of which is presented. The binding and coordination of three calcium ions induce major domain movements in the enzyme upon activation.

Caused by a polyglutamine expansion in the huntingtin protein, Huntington's disease leads to striatal degeneration via the transcriptional dysregulation of a number of genes, including those involved in mitochondrial biogenesis. Here we show that transglutaminase 2, which is upregulated in HD, exacerbates transcriptional dysregulation by acting as a selective corepressor of nuclear genes; transglutaminase 2 interacts directly with histone H3 in the nucleus. In a cellular model of HD, transglutaminase inhibition de-repressed two established regulators of mitochondrial function, PGC-1α and cytochrome c and reversed susceptibility of human HD cells to the mitochondrial toxin, 3-nitroproprionic acid; however, protection mediated by transglutaminase inhibition was not associated with improved mitochondrial bioenergetics. A gene microarray analysis indicated that transglutaminase inhibition normalized expression of not only mitochondrial genes but also 40% of genes that are dysregulated in HD striatal neurons, including chaperone and histone genes. Moreover, transglutaminase inhibition attenuated degeneration in a Drosophila model of HD and protected mouse HD striatal neurons from excitotoxicity. Altogether these findings demonstrate that selective TG inhibition broadly corrects transcriptional dysregulation in HD and defines a novel HDAC-independent epigenetic strategy for treating neurodegeneration.

The zymogen of bacterial transglutaminase was found during cultivation of Streptoverticillium mobaraense (DSMZ strain) using rabbit antibodies raised against the active enzyme. Ion‐exchange chromatography at pH 5.0 yielded a highly purified pro‐enzyme. Structure information was obtained by means of Edman degradation and analysis of PCR amplified nucleotide fragments. The data revealed an excess of negatively charged amino acids in the pro‐region resulting in a decreased isoelectric point of the zymogen. Additionally, the new sequence gave rise to some modifications to the previously published hypothetical structure of prepro‐transglutaminase derived from genomic DNA [Washizu, K., Ando, K., Koikeda, S., Hirose, S., Matsuura, A., Takagi, H., Motoki, M. & Takeuchi, K. (1994) Biosci. Biotechnol. Biochem. 58, 82−87]. Inactive transglutaminase, which carries an activation peptide of 45 amino acids, has a calculated molecular mass of 42 445 Da. Its pro‐region provides for both suppression of activity and increased thermostability. Furthermore, it could be shown that the micro‐organism produces a protease which cleaves pro‐transglutaminase at the C‐side of Pro45. Rapid transformation of the mature enzyme also occurs by addition of other proteases. During conversion, 43 and 41 amino acid peptides are released by bovine trypsin and dispase from Bacillus polymyxa, respectively. The detection of endogenous substrates in the murein layer makes discussion of the physiological role of bacterial transglutaminases necessary.

Transglutaminase (TGase) from Streptomyces mobaraensis is secreted as a precursor protein which is completely activated by the endoprotease TAMEP, a member of the M4 protease family [Zotzel, J., Keller, P. & Fuchsbauer, H.‐L. (2003) Eur. J. Biochem. 270, 3214–3222]. In contrast with the mature enzyme, TAMEP‐activated TGase exhibits an additional N‐terminal tetrapeptide (Phe‐Arg‐Ala‐Pro) suggesting truncation, at least, by a second protease. We have now isolated from the culture broth of submerged colonies a tripeptidyl aminopeptidase (SM‐TAP) that is able to remove the remaining tetrapeptide. The 53‐kDa peptidase was purified by ion‐exchange and phenyl‐Sepharose chromatography and subsequently characterized. Its proteolytic activity was highest against chromophoric tripeptides at pH 7 in the presence of 2 mm CaCl2. EDTA and EGTA (10 mm) both diminished the proteolytic activity by half. Complete inhibition was only achieved with 1 mm phenylmethanesulfonyl fluoride, suggesting that SM‐TAP is a serine protease. Alignment of the N‐terminal sequence confirmed its close relation to the Streptomyces TAPs. That removal of Phe‐Arg‐Ala‐Pro from TAMEP‐activated TGase by SM‐TAP occurs in a single step was confirmed by experiments using various TGase fragments and synthetic peptides. SM‐TAP was also capable of generating the mature N‐terminus by cleavage of RAP‐TGase. However, AP‐TGase remained unchanged. As SM‐TAP activity against chromophoric amino acids such as Pro‐pNA or Phe‐pNA could not be detected, the tetrapeptide of TAMEP‐activated TGase must be removed without formation of an intermediate.

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