Protein transport in the halophilic archaeon Halobacterium sp. NRC-1: a major role for the twin-arginine translocation pathway?

The importance of the Tat-dependent protein secretion pathway in Streptomyces as revealed by phenotypic changes in tat deletion mutants and genome analysis

Streptomyces are Gram-positive soil bacteria that are used industrially, not only as a source of medically important natural compounds, but also as a host for the secretory production of a number of heterologous proteins. A good understanding of the different secretion processes in this organism is therefore of major importance. The functionality of the recently discovered bacterial twin-arginine translocation (Tat) pathway has already been shown in Streptomyces lividans. Here, the aberrant phenotype of S. lividans DeltatatB and DeltatatC single mutants is described. Both mutants are characterized by a dispersed growth in liquid medium, an impaired morphological differentiation on solid medium and growth retardation. To reveal the extent to which the Tat pathway is used in Streptomyces, putative Tat-dependent precursor proteins of Streptomyces coelicolor, a very close relative of S. lividans, and of Streptomyces avermitilis, of which the genomes have been completely sequenced, were identified by a modified version of the TATFIND computer program designed by Rose and colleagues [Rose, R. W., Brüser, T., Kissinger, J. C. & Pohlschröder, M. (2002). Mol Microbiol 45, 943-950]. A list of 230 precursor proteins was obtained; this is the highest number of putative Tat substrates found in any genome so far. In addition to the Streptomyces antibioticus tyrosinase, it was also demonstrated that the secretion of the S. lividans xylanase C is Tat-dependent. The predicted Tat substrates belong to a variety of protein classes, with a high number of proteins functioning in degradation of macromolecules, in binding and transport, and in secondary metabolism. Only a minor fraction of the proteins seem to bind a cofactor. The aberrant phenotype of the DeltatatB and DeltatatC mutants together with the high number of putative Tat-dependent substrates suggests that the Streptomyces Tat pathway has a distinct and more important role in protein secretion than in most other bacteria.

Archaeal signal peptides--a comparative survey at the genome level

The correct delivery of noncytoplasmic proteins to locations both within and outside the cell depends on the appropriate targeting signals. Protein translocation across the bacterial plasma membrane and the eukaryal endoplasmic reticulum membrane relies on cleavable N-terminal signal peptides. Although the signal peptides of secreted proteins in Bacteria and Eukarya have been extensively studied at the sequence, structure, and functional levels, little is known of the nature of archaeal signal peptides. In this report, genome-based analysis was performed in an attempt to define the amino acid composition, length, and cleavage sites of various signal peptide classes in a wide range of archaeal species. The results serve to present a picture of the archaeal signal peptide, revealing the incorporation of bacterial, eukaryal, and archaeal traits.

The twin-arginine translocation (Tat) system is essential for Rhizobium-legume symbiosis

The Tat (twin-arginine translocation) system mediates export of periplasmic proteins in folded conformation. Proteins transported via Tat contain a characteristic twin-arginine motif in their signal peptide. Genetic determinants (tatABC genes) of the Tat system from Rhizobium leguminosarum bv. viciae were cloned and characterized, and a tatBC deletion mutant was constructed. The mutant lacked the ability for membrane targeting of hydrogenase, a known Tat substrate, and was impaired in hydrogenase activity. Interestingly, in the absence of a functional Tat system, only small, white nodules unable to fix nitrogen were induced in symbiosis with pea plants. Analysis of nodule structure and location of green fluorescent protein (GFP)-tagged bacteria within nodules indicated that the symbiotic process was blocked in the tat mutant at a stage previous to bacteria release into cortical cells. The R. leguminosarum Tat-deficient mutant lacked a functional cytochrome bc1 complex. This was consistent with the fact that R. leguminosarum Rieske protein, a key component of the symbiosis-essential cytochrome bc1 complex, contained a typical twin-arginine signal peptide. However, comparative analyses of nodule structure indicated that nodule development in the tat mutant was arrested at an earlier step than in a cytochrome bc1 mutant. These data indicate that the Tat pathway is also critical for proteins relevant to the initial stages of the symbiotic process.

Post-translational secretion of fusion proteins in the halophilic archaea Haloferax volcanii

Although protein secretion occurs post-translationally in bacteria and is mainly a cotranslational event in Eukarya, the relationship between the translation and translocation of secreted proteins in Archaea is not known. To address this question, the signal peptide-encoding region of the surface layer glycoprotein gene from the Haloarchaea Haloferax volcanii was fused either to the cellulose-binding domain of the Clostridium thermocellum cellulosome or to the cytoplasmic enzyme dihydrofolate reductase from H. volcanii. Signal peptide-cleaved mature versions of both the cellulose-binding domain and dihydrofolate reductase could be detected in the growth medium of transformed H. volcanii cells. Immunoblot analysis revealed, however, the presence of full-length signal peptide-bearing forms of both proteins inside the cytoplasm of the transformed cells. Proteinase accessibility assays confirmed that the presence of cell-associated signal peptide-bearing proteins was not due to medium contamination. Moreover, the pulse-radiolabeled signal peptide cellulose-binding domain chimera could be chased from the cytoplasm into the growth medium even following treatment with anisomycin, an antibiotic inhibitor of haloarchaeal protein translation. Thus, these results provide evidence that, in Archaea, at least some secreted proteins are first synthesized inside the cell and only then translocated across the plasma membrane into the medium.