Melanoma originates in the epidermis and becomes metastatic after invasion into the dermis. Prior interactions between melanoma cells and dermis are poorly studied. Here, we show that melanoma cells directly affect the formation of the dermal tumour niche by microRNA trafficking before invasion. Melanocytes, cells of melanoma origin, are specialized in releasing pigment vesicles, termed melanosomes. In melanoma in situ, we found melanosome markers in distal fibroblasts before melanoma invasion. The melanosomes carry microRNAs into primary fibroblasts triggering changes, including increased proliferation, migration and pro-inflammatory gene expression, all known features of cancer-associated fibroblasts (CAFs). Specifically, melanosomal microRNA-211 directly targets IGF2R and leads to MAPK signalling activation, which reciprocally encourages melanoma growth. Melanosome release inhibitor prevented CAF formation. Since the first interaction of melanoma cells with blood vessels occurs in the dermis, our data suggest an opportunity to block melanoma invasion by preventing the formation of the dermal tumour niche.

The MHC class I antigen presentation pathway allows the immune system to distinguish between self and nonself. Despite extensive research on the processing of antigenic peptides, little is known about their origin. Here, we show that mRNAs carrying premature stop codons that prevent the production of full-length proteins via the nonsense-mediated decay pathway still produce a majority of peptide substrates for the MHC class I pathway by a noncanonical mRNA translation process. Blocking the interaction of the translation initiation factor eIF4E with the cap structure suppresses the synthesis of full-length proteins but has only a limited effect on the production of antigenic peptides. These results reveal an essential cell biological function for a class of translation products derived during the pioneer round of mRNA translation and will have important implications for understanding how the immune system detects cells harboring pathogens and generates tolerance.MHC class I-restricted antigen presentation | pioneer round of translation

P53 controls the growth and survival of cells by acting in response to a multitude of cellular stresses. It is, however, not yet fully understood how different p53 activation pathways result in either cell cycle arrest or apoptosis. We and others have described an N-terminally truncated p53 protein (p53/47) originating from a second translation initiation site in the p53 messenger RNA (mRNA), which can interact with p53 and impose altered stability and transactivation properties to p53 complexes. Here we show that cap-dependent and cap-independent mechanisms of initiation govern the translation of the p53 mRNA. Changes in synthesis of full-length p53 or p53/47 are regulated through distinct cell stress-induced pathways acting through separate regions of the p53 mRNA. We also show that some cytotoxic drugs require the presence of full-length p53 to induce apoptosis, whereas for others p53/47 is sufficient. This indicates that by harbouring alternative translation initiation sites, the p53 mRNA gives rise to different levels of the p53 isoforms which help to orchestrate the cell biological outcome of p53 activation in response to different types of cell stress. This sheds new light into the way p53 can integrate and differentiate a large multiplicity of changes in the cellular environment.

A yeast two-hybrid screen with the human S6 (TBP7, RPT3) ATPase of the 26 S proteasome has identified gankyrin, a liver oncoprotein, as an interacting protein.Gankyrin interacts with both free and regulatory complex-associated S6 ATPase and is not stably associated with the 26 S particle. Deletional mutagenesis shows that the C-terminal 78 amino acids of the S6 ATPase are necessary and sufficient to mediate the interaction with gankyrin. Deletion of an orthologous gene in Saccharomyces cerevisiae suggests that it is dispensable for cell growth and viability. Overexpression and precipitation of tagged gankyrin from cultured cells detects a complex containing co-transfected tagged S6 ATPase (or endogenous S6) and endogenous cyclin D-dependent kinase CDK4. The proteasomal ATPases are part of the AAA (ATPases associated with diverse cellular activities) family, members of which are molecular chaperones; gankyrin complexes may therefore influence CDK4 function during oncogenesis.The 26 S proteasome is an exquisitely regulated protease responsible for most of the non-lysosomal degradation of intracellular proteins (1). The particle is responsible for the degradation of regulatory proteins including tumor suppressors (2, 3), transcription factors (4, 5), and proteins that regulate the cell cycle (6, 7).The 26 S proteasome consists of a cylindrical catalytic "core" containing 28 subunits (␣ 7 ␤ 7 ␤ 7 ␣ 7 ) with a regulatory complex (RC) 1 attached to each end of the proteolytic core containing at least 15 subunits (1). The 20 S cylinder contains three chambers: two distal antechambers and a central chamber containing the catalytic threonine residues (8, 9). The ends of the cylindrical core appear closed in the yeast 20 S proteasome (8); the ends of these antechambers need to be opened for substrate proteins to enter into the catalytic core for proteolysis. The RC can be subdivided into a "base" and a "lid" complex (10). The base contains six ATPases, which belong to the AAA (ATPases associated with diverse cellular activities) superfamily of ATPases together with the non-ATPase RPN1 and RPN2 subunits. The lid subcomplex contains the remaining non-ATPase subunits of the RC. The AAA ATPase superfamily controls events as diverse as 26 S proteasomal functions, peroxisomal biogenesis (11), membrane docking and fusion (12), protein egress from the endoplasmic reticulum (13), nuclear transport (14), and transcription factor regulation (15). The ATPases of the RC may be involved in substrate unfolding for entry into the catalytic core of the 26 S proteasome. It has recently been demonstrated that the base subcomplex of the RC exhibits chaperone-like activity (16); and both the ClpX and ClpA ATPases of Escherichia coli, multisubunit complexes similar to the base subcomplex of the 26 S proteasome, can mediate protein unfolding (17, 18) and refolding events (19,20).The proteasomal regulatory ATPases have in addition to the Walker A and B motifs involved in ATP binding and hydrolysis, sequence patterns shared with DNA/RNA helicases...

The scanning of maturing mRNAs by ribosomes plays a key role in the mRNA quality control process. When ribosomes first engage with the newly synthesized mRNA, and if peptides are produced, is unclear, however. Here we show that ribosomal scanning of prespliced mRNAs occurs in the nuclear compartment, and that this event produces peptide substrates for the MHC class I pathway. Inserting antigenic peptide sequences in introns that are spliced out before the mRNAs exit the nuclear compartment results in an equal amount of antigenic peptide products as when the peptides are encoded from the main open reading frame (ORF). Taken together with the detection of intron-encoded nascent peptides and RPS6/RPL7-carrying complexes in the perinucleolar compartment, these results show that peptides are produced by a translation event occurring before mRNA splicing. This suggests that ribosomes occupy and scan mRNAs early in the mRNA maturation process, and suggests a physiological role for nuclear mRNA translation, and also helps explain how the immune system tolerates peptides derived from tissue-specific mRNA splice variants.MHC class I restricted antigen presentation | mRNA maturation | nuclear translation R NAs carrying premature termination codons or are recognized as defective in other aspects are prevented from further translation and disposed of by the nonsense-mediated decay (NMD) pathway (1). The detection of premature stop codons is mediated by the scanning ribosome; however, when the ribosomes first engage with the newly synthesized mRNA, and if this event results in the production of peptides, is unclear. Two observations from the field of immunology and the presentation of peptides on MHC class I molecules have highlighted some aspects relevant to the role of the ribosomes in the mRNA maturation process. The first observation is related to the detection of antigenic peptides originating from intron sequences, and the second is related to the observation that the synthesis of antigenic peptide substrates and full-length proteins are two spatiotemporarily distinct events (2-5).The presentation of antigenic peptides on MHC class I molecules allows CD8 + T cells to detect and eliminate cells in which the repertoire of peptide substrates has been altered owing to the presence of viruses or to changes in the presentation of endogenous antigens (6, 7). However, despite the key role of antigen presentation on MHC class I molecules in immune surveillance, the source of peptides for the MHC class I pathway is not yet known. Accumulating evidence indicates that the processing of alternative peptide substrates plays a major role, and that degradation products derived from full-length proteins have limited access to the MHC class I pathway (8-10). We recently demonstrated that pioneer translation products (PTPs) that form antigenic peptide substrates for the MHC class I pathway are produced by a translation event that is distinct from the canonical event giving rise to full-length proteins and does not require the cap-binding ...

Viruses are known to employ different strategies to manipulate the major histocompatibility (MHC) class I antigen presentation pathway to avoid recognition of the infected host cell by the immune system. However, viral control of antigen presentation via the processes that supply and select antigenic peptide precursors is yet relatively unknown. The Epstein-Barr virus (EBV)-encoded EBNA1 is expressed in all EBV-infected cells, but the immune system fails to detect and destroy EBV-carrying host cells. This immune evasion has been attributed to the capacity of a Gly-Ala repeat (GAr) within EBNA1 to inhibit MHC class I restricted antigen presentation. Here we demonstrate that suppression of mRNA translation initiation by the GAr in cis is sufficient and necessary to prevent presentation of antigenic peptides from mRNAs to which it is fused. Furthermore, we demonstrate a direct correlation between the rate of translation initiation and MHC class I antigen presentation from a certain mRNA. These results support the idea that mRNAs, and not the encoded full length proteins, are used for MHC class I restricted immune surveillance. This offers an additional view on the role of virus-mediated control of mRNA translation initiation and of the mechanisms that control MHC class I restricted antigen presentation in general.

The glycine-alanine repeat (GAr) sequence of the Epstein-Barr virus-encoded EBNA-1 prevents presentation of antigenic peptides to major histocompatibility complex class I molecules. This has been attributed to its capacity to suppress mRNA translation in cis. However, the underlying mechanism of this function remains largely unknown. Here, we have further investigated the effect of the GAr as a regulator of mRNA translation. Introduction of silent mutations in each codon of a 30-amino-acid GAr sequence does not significantly affect the translation-inhibitory capacity, whereas minimal alterations in the amino acid composition have strong effects, which underscores the observation that the amino acid sequence and not the mRNA sequence mediates GAr-dependent translation suppression. The capacity of the GAr to repress translation is dose and position dependent and leads to a relative accumulation of preinitiation complexes on the mRNA. Taken together with the surprising observation that fusion of the 5 untranslated region (UTR) of the c-myc mRNA to the 5 UTR of GAr-carrying mRNAs specifically inactivates the effect of the GAr, these results indicate that the GAr targets components of the translation initiation process. We propose a model in which the nascent GAr peptide delays the assembly of the initiation complex on its own mRNA.Epstein-Barr Virus (EBV) nuclear antigen 1 (EBNA-1) and latency-associated nuclear antigen 1 (LANA-1), from Kaposi's sarcoma-associated herpesvirus (KSHV), are major latency proteins of these two gammaherpesviruses that are essential for maintaining viral episomes in infected cells (21,22). Independent studies suggest that both proteins have evolved mechanisms to remain largely invisible to the immune system, which could otherwise eliminate latently infected cells (8,9,19,25). These mechanisms act in cis and are mediated via an internal repeat region. In the case of EBNA-1 this region consists of an N-terminal glycine-alanine repeat (GAr), and for LANA-1 the region consists of a glutamine-glutamate-aspartate central repeat (QED-CR). Although the two domains do not share amino acid homology, both retard their own synthesis to reduce the production of defective ribosomal products that can be processed for the major histocompatibility complex (MHC) class I-restricted antigen presentation pathway (23, 24), highlighting the importance of translation control in regulating MHC class I-restricted antigen presentation. To compensate for their low rates of synthesis, both proteins also have slow turnover rates (4, 8).Regulation of translation for most prokaryotic and eukaryotic mRNAs occurs at the level of initiation, but there are examples where regulation of protein synthesis depends on the elongation stage (17). The two main types of translation initiation are the classic cap-dependent and the less frequent cap-independent translation mechanisms (5,7,11,14,16). In the former, the preinitiation complex is formed around the cap structure in the 5Ј untranslated region (UTR) of the message, whereas i...

Partial degradation or regulated ubiquitin proteasomedependent processing by the 26 S proteasome has been demonstrated, but the underlying molecular mechanisms and the prevalence of this phenomenon remain obscure. Here we show that the Gly-Ala repeat (GAr) sequence of EBNA1 affects processing of substrates via the ubiquitin-dependent degradation pathway in a substrate-and position-specific fashion. GAr-mediated increase in stability of proteins targeted for degradation via the 26 S proteasome was associated with a fraction of the substrates being partially processed and the release of the free GAr. The GAr did not cause a problem for the proteolytic activity of the proteasome, and its fusion to the N terminus of p53 resulted in an increase in the rate of degradation of the entire chimera. Interestingly the GAr had little effect on the stability of EBNA1 protein itself, and targeting EBNA1 for 26 S proteasome-dependent degradation led to its complete degradation. Taken together, our data suggest a model in which the GAr prevents degradation or promotes endoproteolytic processing of substrates targeted for the 26 S proteasome by interfering with the initiation step of substrate unfolding. These results will help to further understand the underlying mechanisms for partial proteasome-dependent degradation.Polyubiquitinated substrates are targeted for 26 S-dependent proteasomal degradation by their ubiquitin moiety that is recognized and bound to one of the 19 S regulatory complexes. Prior to degradation, the substrate has to be unfolded by an ATP-dependent mechanism in the 19 S cap structure. This allows the polypeptide to be translocated and threaded into the central chamber of the 20 S complex where three different types of proteolytic activity catalyze the cleavage of peptide bonds, leading to the complete breakdown of the substrate into small peptide fragments (1, 2).In addition to this classic model of 26 S proteasome-mediated degradation, the proteasome can also partially degrade specific substrates leading instead to the release of larger degradation products with distinct cell biological activity, the socalled regulated ubiquitin proteasome-dependent processing (3-6). For example, the p50 subunit of NF-B is generated by the processing of the larger p105 protein (7,8), and it has been suggested that a glycine-rich region in p105 is responsible and necessary for this partial degradation (9, 10). Partial processing also applies to other transcription factors like the yeast NF-B homologs Spt23 and Mga2 and the Drosophila Ci (3, 4, 11). The direct targeting of proteins carrying polyglutamine repeat sequences, which are characteristic for several neurodegenerative diseases including Huntington disease and spinocerebellar ataxias, for proteasomal degradation also results in partial degradation in vitro and in vivo (12). These different examples highlight the fact that regulated ubiquitin proteasome-dependent processing is an important physiological aspect of the proteasomal activity that might also play a role in t...