Dual function of GTPBP6 in biogenesis and recycling of human mitochondrial ribosomes.

Translation and ribosome biogenesis in mitochondria require auxiliary factors that ensure rapid and accurate synthesis of mitochondrial proteins. Defects in translation are associated with oxidative phosphorylation deficiency and cause severe human diseases, but the exact roles of mitochondrial translation-associated factors are not known. Here we identify the functions of GTPBP6, a homolog of the bacterial ribosome-recycling factor HflX, in human mitochondria. Similarly to HflX, GTPBP6 facilitates the dissociation of ribosomes in vitro and in vivo. In contrast to HflX, GTPBP6 is also required for the assembly of mitochondrial ribosomes. GTPBP6 ablation leads to accumulation of late assembly intermediate(s) of the large ribosomal subunit containing ribosome biogenesis factors MTERF4, NSUN4, MALSU1 and the GTPases GTPBP5, GTPBP7 and GTPBP10. Our data show that GTPBP6 has a dual function acting in ribosome recycling and biogenesis. These findings contribute to our understanding of large ribosomal subunit assembly as well as ribosome recycling pathway in mitochondria.

The Sec translocon mediated protein transport in prokaryotes and eukaryotes.

Protein transport via the Sec translocon represents an evolutionary conserved mechanism for delivering cytosolically-synthesized proteins to extra-cytosolic compartments. The Sec translocon has a three-subunit core, termed Sec61 in Eukaryotes and SecYEG in Bacteria. It is located in the endoplasmic reticulum of Eukaryotes and in the cytoplasmic membrane of Bacteria where it constitutes a channel that can be activated by multiple partner proteins. These partner proteins determine the mechanism of polypeptide movement across the channel. During SRP-dependent co-translational targeting, the ribosome threads the nascent protein directly into the Sec channel. This pathway is in Bacteria mainly dedicated for membrane proteins but in Eukaryotes also employed by secretory proteins. The alternative pathway, leading to post-translational translocation across the Sec translocon engages an ATP-dependent pushing mechanism by the motor protein SecA in Bacteria and a ratcheting mechanism by the lumenal chaperone BiP in Eukaryotes. Protein transport and biogenesis is also assisted by additional proteins at the lateral gate of SecY/Sec61α and in the lumen of the endoplasmic reticulum or in the periplasm of bacterial cells. The modular assembly enables the Sec complex to transport a vast array of substrates. In this review we summarize recent biochemical and structural information on the prokaryotic and eukaryotic Sec translocons and we describe the remarkably complex interaction network of the Sec complexes.

Molecular Mimicry of SecA and Signal Recognition Particle Binding to the Bacterial Ribosome.

Bacteria execute a variety of protein transport systems for maintaining the proper composition of their different cellular compartments. The SecYEG translocon serves as primary transport channel and is engaged in transporting two different substrate types. Inner membrane proteins are cotranslationally inserted into the membrane after their targeting by the signal recognition particle (SRP). In contrast, secretory proteins are posttranslationally translocated by the ATPase SecA. Recent data indicate that SecA can also bind to ribosomes close to the tunnel exit. We have mapped the interaction of SecA with translating and nontranslating ribosomes and demonstrate that the N terminus and the helical linker domain of SecA bind to an acidic patch on the surface of the ribosomal protein uL23. Intriguingly, both also insert deeply into the ribosomal tunnel to contact the intratunnel loop of uL23, which serves as a nascent chain sensor. This binding pattern is remarkably similar to that of SRP and indicates an identical interaction mode of the two targeting factors with ribosomes. In the presence of a nascent chain, SecA retracts from the tunnel but maintains contact with the surface of uL23. Our data further demonstrate that ribosome and membrane binding of SecA are mutually exclusive, as both events depend on the N terminus of SecA. Our study highlights the enormous plasticity of bacterial protein transport systems and reveals that the discrimination between SRP and SecA substrates is already initiated at the ribosome.IMPORTANCE Bacterial protein transport via the conserved SecYEG translocon is generally classified as either cotranslational, i.e., when transport is coupled to translation, or posttranslational, when translation and transport are separated. We show here that the ATPase SecA, which is considered to bind its substrates posttranslationally, already scans the ribosomal tunnel for potential substrates. In the presence of a nascent chain, SecA retracts from the tunnel but maintains contact with the ribosomal surface. This is remarkably similar to the ribosome-binding mode of the signal recognition particle, which mediates cotranslational transport. Our data reveal a striking plasticity of protein transport pathways, which likely enable bacteria to efficiently recognize and transport a large number of highly different substrates within their short generation time.

Yet another job for the bacterial ribosome.

The ribosome is a sophisticated cellular machine, composed of RNA and protein, which translates the mRNA-encoded genetic information into protein and thus acts at the center of gene expression. Still, the ribosome not only decodes the genetic information, it also coordinates many ribosome-associated processes like protein folding and targeting. The ribosomal protein uL23 is crucial for this coordination and is located at the ribosomal tunnel exit where it serves as binding platform for targeting factors, chaperones and modifying enzymes. This includes the signal recognition particle (SRP), which facilitates co-translational protein targeting in pro- and eukaryotes, the chaperone Trigger Factor and methionine aminopeptidase, which removes the start methionine in many bacterial proteins. A recent report revealed the intricate interaction of uL23 with yet another essential player in bacteria, the ATPase SecA, which is best known for its role during post-translational secretion of proteins across the bacterial SecYEG translocon.

The signal recognition particle contacts uL23 and scans substrate translation inside the ribosomal tunnel.

The signal recognition particle (SRP) delivers ∼25% of all bacterial proteins to the membrane for cotranslational insertion. However, a comprehensive model on how the low-abundant SRP scans the vast number of translating ribosomes to identify the correct substrates is lacking. Here, we show that the C-terminal helix of the signal-sequence-binding domain of SRP penetrates into the ribosomal tunnel and contacts the intra-tunnel loop of ribosomal protein uL23. This allows SRP to obtain information about the translational status of the ribosome and possibly the character of the approaching nascent chain. Correct substrates reposition the C-terminal helix of SRP, which facilitates stable binding of the signal sequence by the M-domain of SRP. Thus, SRP already surveys translating ribosomes before the signal sequence is surface exposed. This early interaction probably enables the small number of SRP molecules to scan many ribosomes and to initiate efficient targeting of proper substrates.

Protein translocation across the inner membrane of Gram-negative bacteria: the Sec and Tat dependent protein transport pathways.

Gram negative bacteria possess a large variety of protein transport systems, by which proteins that are synthesised in the cytosol are exported to destinations in the cell envelope or entirely secreted into the extracellular environment. The inner membrane (IM) contains three major transport systems for the translocation and insertion of signal sequence containing proteins: the Sec translocon, the YidC insertase, and the Tat system. The heterotrimeric SecYEG translocon forms a narrow channel in the membrane that serves a dual function; it allows the translocation of unfolded proteins across the pore and the integration of α-helical proteins into the IM. The YidC insertase is a multi-spanning membrane protein that cooperates with the SecYEG translocon during the integration of membrane proteins but also functions as an independent insertase. Depending upon the type of protein cargo that needs to be transported, the Signal Recognition Particle (SRP), the SRP receptor, SecA and chaperones are required to coordinate translation with transport and to target and energise the different transport systems. The Tat system consists of three membrane proteins (TatA, TatB and TatC) which in a still unknown manner accomplish the transmembrane passage of completely folded proteins and protein complexes.

Coinfection of Chlamydia trachomatis, Ureaplasma urealyticum and human papillomavirus among patients attending STD clinics in Estonia.

Women visiting Estonian STD clinics were subjected to PCR assay for human papillomavirus (HPV), Chlamydia trachomatis and Ureaplasma urealyticum biovar 2. The overall prevalence of coinfection was 8%. The chlamydial infection was found to be associated with HPV, especially with high-risk HPV (OR=2.5, p<0.005) and most significantly in women over 41 y of age. C. trachomatis infection also occurred more frequently in U. urealyticum-infected than in U. urealyticum-free patients (OR=2.6, p=0.02). U. urealyticum infection did not associate with HPV status. The clinical significance of the association between C. trachomatis and U. urelyticum infection remains to be elucidated.