EGF receptor transactivation by G-protein-coupled receptors requires metalloproteinase cleavage of proHB-EGF

Cross-communication between different signalling systems allows the integration of the great diversity of stimuli that a cell receives under varying physiological situations. The transactivation of epidermal growth factor receptor (EGFR)-dependent signalling pathways upon stimulation of G-protein-coupled receptors (GPCRs), which are critical for the mitogenic activity of ligands such as lysophosphatidic acid, endothelin, thrombin, bombesin and carbachol, provides evidence for such an interconnected communication network. Here we show that EGFR transactivation upon GPCR stimulation involves proHB-EGF and a metalloproteinase activity that is rapidly induced upon GPCR-ligand interaction. We show that inhibition of proHB-EGF processing blocks GPCR-induced EGFR transactivation and downstream signals. The pathophysiological significance of this mechanism is demonstrated by inhibition of constitutive EGFR activity upon treatment of PC3 prostate carcinoma cells with the metalloproteinase inhibitor batimastat. Together, our results establish a new mechanistic concept for cross-communication among different signalling systems.     

bicoid RNA localization requires specific binding of an endosomal sorting complex

bicoid messenger RNA localizes to the anterior of the Drosophila egg, where it is translated to form a morphogen gradient of Bicoid protein that patterns the head and thorax of the embryo. Although bicoid was the first localized cytoplasmic determinant to be identified, little is known about how the mRNA is coupled to the microtubule-dependent transport pathway that targets it to the anterior, and it has been proposed that the mRNA is recognized by a complex of many redundant proteins, each of which binds to the localization element in the 3' untranslated region (UTR) with little or no specificity. Indeed, the only known RNA-binding protein that co-localizes with bicoid mRNA is Staufen, which binds non-specifically to double-stranded RNA in vitro. Here we show that mutants in all subunits of the ESCRT-II complex (VPS22, VPS25 and VPS36) abolish the final Staufen-dependent step in bicoid mRNA localization. ESCRT-II is a highly conserved component of the pathway that sorts ubiquitinated endosomal proteins into internal vesicles, and functions as a tumour-suppressor by removing activated receptors from the cytoplasm. However, the role of ESCRT-II in bicoid localization seems to be independent of endosomal sorting, because mutations in ESCRT-I and III components do not affect the targeting of bicoid mRNA. Instead, VPS36 functions by binding directly and specifically to stem-loop V of the bicoid 3' UTR through its amino-terminal GLUE domain, making it the first example of a sequence-specific RNA-binding protein that recognizes the bicoid localization signal. Furthermore, VPS36 localizes to the anterior of the oocyte in a bicoid-mRNA-dependent manner, and is required for the subsequent recruitment of Staufen to the bicoid complex. This function of ESCRT-II as an RNA-binding complex is conserved in vertebrates and may clarify some of its roles that are independent of endosomal sorting.     

High-affinity NGF binding requires coexpression of the trk proto-oncogene and the low-affinity NGF receptor.

Nerve growth factor (NGF) interacts with two different low-affinity receptors that can be distinguished by affinity crosslinking. Reconstitution experiments by membrane fusion and transient transfection into heterologous cells indicate that high-affinity NGF binding requires coexpression and binding to both the low-affinity NGF receptor and the tyrosine kinase trk gene product. These studies reveal a new growth factor receptor-mediated mechanism of cellular differentiation involving trk and the low-affinity NGF receptor.     

E. coli recA protein-directed cleavage of phage lambda repressor requires polynucleotide

The recA protein mediates both genetic recombination and several cellular responses to DNA damage, including the induction of temperate bacteriophage. Indication of phage lambda results from proteolytic cleavage of lambda repressor directed by recA protein. We show here that this cleavage reaction requires both polynucleotide and ATP. We suggest that a stoichiometric complex of recA protein and DNA is active both to destroy repressors by proteolytic cleavage and to initiate pairing of this DNA to its homologous sequence in a DNA duplex ('strand invasion').     

Tetanus and botulinum-B neurotoxins block neurotransmitter release by proteolytic cleavage of synaptobrevin.

Clostridial neurotoxins, including tetanus toxin and the seven serotypes of botulinum toxin (A-G), are produced as single chains and cleaved to generate toxins with two chains joined by a single disulphide bond (Fig. 1). The heavy chain (M(r) 100,000 (100K)) is responsible for specific binding to neuronal cells and cell penetration of the light chain (50K), which blocks neurotransmitter release. Several lines of evidence have recently suggested that clostridial neurotoxins could be zinc endopeptidases. Here we show that tetanus and botulinum toxins serotype B are zinc endopeptidases, the activation of which requires reduction of the interchain disulphide bond. The protease activity is localized on the light chain and is specific for synaptobrevin, an integral membrane protein of small synaptic vesicles. The rat synaptobrevin-2 isoform is cleaved by both neurotoxins at the same single site, the peptide bond Gln 76-Phe 77, but the isoform synaptobrevin-1, which has a valine at the corresponding position, is not cleaved. The blocking of neurotransmitter release of Aplysia neurons injected with tetanus toxin or botulinum toxins serotype B is substantially delayed by peptides containing the synaptobrevin-2 cleavage site. These results indicate that tetanus and botulinum B neurotoxins block neurotransmitter release by cleaving synaptobrevin-2, a protein that, on the basis of our results, seems to play a key part in neurotransmitter release.     

In vivo binding pattern of a trans-regulator of homoeotic genes in Drosophila melanogaster

The specification and maintenance of the metameric pattern in Drosophila melanogaster is regulated by complicated gene interactions. The differential expression of the homoeotic genes of the Antennapedia complex (ANT-C) and bithorax complex (BX-C), which determine segmental identities, is partly controlled by cross-regulatory interactions of loci within the two clusters and partly by trans-acting factors located outside the two complexes. One of the trans-regulatory genes, Polycomb (Pc), acts as a repressor of the ANT-C and BX-C. Mutations of Polycomb result in a complete depression of the homoeotic genes, leading to abdominal transformations of all body segments. Polycomb is part of a large class of trans-regulatory genes (Pc-group), estimated to comprise up to 40 loci. We have raised antibodies against the Polycomb protein, and, using an improved immunostaining technique, showed that the Polycomb protein binds to 60 discrete sites along the polytene chromosomes of salivary glands. These sites comprise the ANT-C and the BX-C as well as several locations of Pc-group genes. This is the first clear evidence for a direct interaction of Polycomb with homoeotic loci and other Pc-group genes.     

Computational redesign of endonuclease DNA binding and cleavage specificity

The reprogramming of DNA-binding specificity is an important challenge for computational protein design that tests current understanding of protein-DNA recognition, and has considerable practical relevance for biotechnology and medicine. Here we describe the computational redesign of the cleavage specificity of the intron-encoded homing endonuclease I-MsoI using a physically realistic atomic-level forcefield. Using an in silico screen, we identified single base-pair substitutions predicted to disrupt binding by the wild-type enzyme, and then optimized the identities and conformations of clusters of amino acids around each of these unfavourable substitutions using Monte Carlo sampling. A redesigned enzyme that was predicted to display altered target site specificity, while maintaining wild-type binding affinity, was experimentally characterized. The redesigned enzyme binds and cleaves the redesigned recognition site approximately 10,000 times more effectively than does the wild-type enzyme, with a level of target discrimination comparable to the original endonuclease. Determination of the structure of the redesigned nuclease-recognition site complex by X-ray crystallography confirms the accuracy of the computationally predicted interface. These results suggest that computational protein design methods can have an important role in the creation of novel highly specific endonucleases for gene therapy and other applications.     

Post-translational insertion of a fragment of the glucose transporter into microsomes requires phosphoanhydride bond cleavage

Most eukaryotic secretory and membrane proteins insert co-translationally into the membrane of the rough endoplasmic reticulum (RER), and are targeted there by one or more NH2-terminal or internal signal sequences. However, little is known about the actual translocation and membrane integration processes. In particular, any energy requirements for targeting and integration have remained obscure because of the inability to uncouple the processes from concomitant protein synthesis. We recently showed that the human glucose transporter (GT), an integral membrane glycoprotein, can insert post-translationally into dog pancreatic microsomes with low but demonstrable efficiency in vitro, and that a fragment corresponding to the NH2-terminal 340 amino acids and 8 of the 12 membrane-spanning alpha-helixes of GT (GT-N) can insert with significantly greater efficiency. We report here that post-translational insertion of GT-N into pancreatic microsomes requires energy in the form of a phosphodiester bond, and suggest that co-translational insertion of proteins into the RER may also require energy independent of that used for polypeptide synthesis.     

Structural basis for specific cleavage of Lys 63-linked polyubiquitin chains

Deubiquitinating enzymes (DUBs) remove ubiquitin from conjugated substrates to regulate various cellular processes. The Zn(2+)-dependent DUBs AMSH and AMSH-LP regulate receptor trafficking by specifically cleaving Lys 63-linked polyubiquitin chains from internalized receptors. Here we report the crystal structures of the human AMSH-LP DUB domain alone and in complex with a Lys 63-linked di-ubiquitin at 1.2 A and 1.6 A resolutions, respectively. The AMSH-LP DUB domain consists of a Zn(2+)-coordinating catalytic core and two characteristic insertions, Ins-1 and Ins-2. The distal ubiquitin interacts with Ins-1 and the core, whereas the proximal ubiquitin interacts with Ins-2 and the core. The core and Ins-1 form a catalytic groove that accommodates the Lys 63 side chain of the proximal ubiquitin and the isopeptide-linked carboxy-terminal tail of the distal ubiquitin. This is the first reported structure of a DUB in complex with an isopeptide-linked ubiquitin chain, which reveals the mechanism for Lys 63-linkage-specific deubiquitination by AMSH family members.     

In vitro selection of RNA molecules that bind specific ligands.

Subpopulations of RNA molecules that bind specifically to a variety of organic dyes have been isolated from a population of random sequence RNA molecules. Roughly one in 10(10) random sequence RNA molecules folds in such a way as to create a specific binding site for small ligands.     

Enterotoxin residues determining T-cell receptor V beta binding specificity.

Superantigens such as the staphylococcal enterotoxins bind to major histocompatibility complex (MHC) class II molecules and activate T cells through a specific interaction between the V beta region of the T-cell antigen receptor (TCR) and the toxin. The TCR beta-chain alone is sufficient to produce the interaction with the enterotoxin-class II complex. Identification of the regions of enterotoxins that interact with TCR has so far proved equivocal because of difficulties in distinguishing between direct effects on T-cell recognition and indirect effects resulting from alteration of binding to class II. For example, amino-terminal truncations of SEB abrogated T-cell stimulation whereas carboxy-terminal truncation of SEA stopped its mitogenic activity. The most comprehensive study to date, accounting for both enterotoxin binding to class II and enterotoxin interactions with the TCR, identified two functionally important regions for SEB binding to TCR. Although the amino-acid sequences of staphylococcal enterotoxins A and E are 82% identical, they activate T cells bearing different V beta elements. We have assayed the binding of cells coated with these enterotoxins to soluble secreted TCR beta-chain protein and find that V beta 3 binds enterotoxin A but not E, whereas V beta 11 binds enterotoxin but not A. To map the amino-acid residues responsible for these different binding specificities, we prepared a series of hybrids between the two staphylococcal enterotoxins. We report that just two amino-acid residues near the carboxy terminus of the enterotoxins are responsible for the discrimination between these molecules by V beta 3 and V beta 11.(ABSTRACT TRUNCATED AT 250 WORDS)     

X-ray analysis of glucagon and its relationship to receptor binding.

X-ray analysis of the pancreatic hormone glucagon shows that in crystals the polypeptide adopts a mainly helical conformation, which is stabilised by hydrophobic interactions between molecules related by threefold symmetry. A model is presented in which the glucagon molecule exists in dilute solutions as an equilibrium population of conformers with little retention of conformers with little retention of structure, and in which the helical conformation is stablised by hydrophobic interactions either as an oligomer or as a complex with the receptor.     

Recognition of SUMO-modified PCNA requires tandem receptor motifs in Srs2

Ubiquitin (Ub) and ubiquitin-like (Ubl) modifiers such as SUMO (also known as Smt3 in Saccharomyces cerevisiae) mediate signal transduction through post-translational modification of substrate proteins in pathways that control differentiation, apoptosis and the cell cycle, and responses to stress such as the DNA damage response. In yeast, the proliferating cell nuclear antigen PCNA (also known as Pol30) is modified by ubiquitin in response to DNA damage and by SUMO during S phase. Whereas Ub-PCNA can signal for recruitment of translesion DNA polymerases, SUMO-PCNA signals for recruitment of the anti-recombinogenic DNA helicase Srs2. It remains unclear how receptors such as Srs2 specifically recognize substrates after conjugation to Ub and Ubls. Here we show, through structural, biochemical and functional studies, that the Srs2 carboxy-terminal domain harbours tandem receptor motifs that interact independently with PCNA and SUMO and that both motifs are required to recognize SUMO-PCNA specifically. The mechanism presented is pertinent to understanding how other receptors specifically recognize Ub- and Ubl-modified substrates to facilitate signal transduction.