PRP16 is an RNA-dependent ATPase that interacts transiently with the spliceosome.

The assembly of the spliceosome is an ATP-dependent process. The splicing factor PRP16 contains variations of several motifs that define the eIF-4A-like ATP-dependent RNA helicase family. The protein has now been purified and shown to exhibit RNA-dependent ATPase activity. PRP16 is required specifically for the second catalytic step of the splicing reaction in vitro. This function requires ATP binding and/or hydrolysis, which appears to be concomitant with release of the protein from the spliceosome. PRP16 may be the prototype for a set of splicing factors which use ATP to drive a cycle of conformational changes.     

The U1 snRNP protein U1C recognizes the 5' splice site in the absence of base pairing

Splicing of precursor messenger RNA takes place in the spliceosome, a large RNA/protein macromolecular machine. Spliceosome assembly occurs in an ordered pathway in vitro and is conserved between yeast and mammalian systems. The earliest step is commitment complex formation in yeast or E complex formation in mammals, which engages the pre-mRNA in the splicing pathway and involves interactions between U1 small nuclear ribonucleoprotein (snRNP) and the pre-mRNA 5' splice site. Complex formation depends on highly conserved base pairing between the 5' splice site and the 5' end of U1 snRNA, both in vivo and in vitro. U1 snRNP proteins also contribute to U1 snRNP activity. Here we show that U1 snRNP lacking the 5' end of its snRNA retains 5'-splice-site sequence specificity. We also show that recombinant yeast U1C protein, a U1 snRNP protein, selects a 5'-splice-site-like sequence in which the first four nucleotides, GUAU, are identical to the first four nucleotides of the yeast 5'-splice-site consensus sequence. We propose that a U1C 5'-splice-site interaction precedes pre-mRNA/U1 snRNA base pairing and is the earliest step in the splicing pathway.     

RNA helicase activity associated with the human p68 protein

It has been proposed that p68, a nuclear protein of relative molecular mass 68,000, functions in the regulation of cell growth and division. A complementary DNA analysis of the protein has revealed extensive amino-acid sequence homology to the products of a set of genes recently identified in organisms as diverse as Escherichia coli and man, which include the eukaryotic translation initiation factor elF-4A. The protein products of the new gene family have several motifs in common which are thought to be involved in nucleic acid unwinding. As yet, however, only elF-4A, through its effect on RNA, has been shown to possess unwinding activity. Here we report that purified p68 also exhibits RNA-dependent ATPase activity and functions as an RNA helicase in vitro. The protein was first identified by its specific immunological cross reaction with the simian virus 40 large T antigen, the transforming protein of a small DNA tumour virus. Surprisingly, T antigen also has an RNA-unwinding activity: the homology between the two polypeptides, although confined to only a small region resembling the epitope of the cross-reacting antibody (PAb204), should therefore be of functional significance. Furthermore, the RNA-unwinding activity may be involved in the growth-regulating functions of both proteins.     

A role for ADP-ribosylation factor in nuclear vesicle dynamics.

Two distinct steps in nuclear envelope assembly can be assayed in vitro: the protein-mediated binding of nuclear-specific vesicles to chromatin, and the subsequent fusion of these vesicles to enclose the chromatin within a double nuclear membrane. Nuclear vesicle fusion, like fusion in the secretory pathway, requires ATP and cytosol and is inhibited by nonhydrolysable GTP analogues. The sensitivity of nuclear vesicle fusion to GTP-gamma S requires a GTP-dependent soluble factor, the properties of which are strikingly similar to a GTP-dependent Golgi binding factor (GGBF) that inhibits Golgi vesicle fusion in the presence of GTP-gamma S and belongs to the ADP-ribosylation factor (ARF) family of small GTPases. In the presence of GTP-gamma S, ARF proteins and alpha-, beta-, gamma-, delta-COP ('coatomer') subunits are associated with Golgi transport vesicles, but the exact roles of ARF proteins in secretion are not yet understood. We report here that purified ARF1 and GGBF have GTP-dependent soluble factor activity in the nuclear vesicle fusion assay. Our results show that the function of ARF is not limited to the Golgi apparatus, and indicate that there may be a link between the formation of nuclear vesicles during mitosis and proteins involved in secretion.     

Malignant transformation by a eukaryotic initiation factor subunit that binds to mRNA 5' cap

Eukaryotic cellular mRNAs have a 5' cap structure (m7 GpppX) that facilitates binding to ribosomes and is required for efficient translation. A specific initiation factor, eIF-4F, mediates the function of the cap and consists of three subunits, one of which, eIF-4E, binds the cap. This subunit is present in limiting amounts in the cell, and is thought to be regulated by phosphorylation: decreased phosphorylation of eIF-4E following various treatments correlates with a decrease in cellular translation rate. These observations suggest that eIF-4E lies on the mitogenic signal transduction pathway, and we reasoned that overexpression of eIF-4E might profoundly affect cellular growth properties. We report here that overexpression of eIF-4E in NIH 3T3 and Rat 2 fibroblasts causes their tumorigenic transformation as determined by three criteria: formation of transformed foci on a monolayer of cells; anchorage-independent growth; and tumour formation in nude mice.     

T gamma protein is expressed on murine fetal thymocytes as a disulphide-linked heterodimer

During the search for genes coding for the mouse alpha and beta subunits of the antigen-specific receptor of mouse T cells we encountered a third gene, subsequently designated gamma. This gene has many properties in common with the alpha and beta genes, somatic assembly from gene segments that resemble the gene segments for immunoglobulin variable (V), joining (J) and constant (C) regions; rearrangement and expression in T cells and not in B cells; low but distinct sequence homology to immunoglobulin V, J and C regions; other sequences that are reminiscent of the transmembrane and intracytoplasmic regions of integral membrane proteins; and a cysteine residue at the position expected for a disulphide bond linking two subunits of a dimeric membrane protein. Despite these similarities the gamma gene also shows some interesting unique features. These include a relatively limited repertoire of the germ-line gene segments, more pronounced expression at the RNA level in immature T cells such as fetal thymocytes and an apparent absence of in-frame RNA in some functional, alpha beta heterodimer-bearing T cells or cultured T clones and hybridomas. To understand the function of the putative gamma protein it is essential to define the cell population that expresses this protein. To this end we produced a fusion protein composed of Escherichia coli beta-galactosidase and the gamma-chain (hereafter referred to a beta-gal-gamma) using the phage expression vector lambda gt11 and raised rabbit antisera against the gamma determinants. Using the purified anti-gamma antibody we detected a polypeptide chain of relative molecular mass 35,000 (Mr 35K) on the surface of 16-day old fetal thymocytes. The gamma-chain is linked by a disulphide bridge to another component of 45K. No such heterodimer was detected on the surface of a cytotoxic T lymphocyte (CTL) clone 2C from which an in-phase gamma cDNA clone was originally isolated.     

Protein folding in the cell.

In the cell, as in vitro, the final conformation of a protein is determined by its amino-acid sequence. But whereas some isolated proteins can be denatured and refolded in vitro in the absence of other macromolecular cellular components, folding and assembly of polypeptides in vivo involves other proteins, many of which belong to families that have been highly conserved during evolution.     

Inhibition of mRNA binding to ribosomes by localized activation of dsRNA-dependent protein kinase

The initiation of protein synthesis can be regulated in mammalian cells by protein kinases which phosphorylate the alpha subunit of initiation factor eIF-2. This phosphorylation results in a block in the recycling of eIF-2 and in the inhibition of messenger RNA binding to 80S initiation complexes. After eIF-2 alpha is phosphorylated, the mRNA becomes associated with 48S complexes consisting of a 40S ribosomal subunit, eIF-2 (alpha P), GDP and Met-tRNAf. One of the eIF-2 alpha kinases is activated by low concentrations of double-stranded RNA (dsRNA). This kinase (PKds) is present at a basal level in all mammalian cells investigated and its synthesis is induced in cells treated with interferon. The PKds may be involved in the inhibition of translation of viral mRNA in interferon-treated cells infected with RNA viruses, as it is activated by viral replicative complexes. It is not known, however, if the activated PKds preferentially inhibits the translation of viral mRNA when cellular protein synthesis proceeds at a normal rate in infected cells. We now report that mRNA covalently linked to dsRNA is preferentially inhibited from binding to 80S complexes by a localized activation of PKds. This suggests that in interferon-treated cells the binding of some nascent viral mRNAs to functional initiation complexes may be preferentially inhibited by a similar mechanism.     

SEC65 gene product is a subunit of the yeast signal recognition particle required for its integrity.

Protein targeting to the endoplasmic reticulum (ER) in mammalian cells is catalysed by the signal recognition particle (SRP), which consists of six protein subunits and an RNA subunit. Saccharomyces cerevisiae SRP is a 16S particle, of which only two subunits have been identified: a protein subunit, SRP54p, which is homologous to the mammalian SRP54 subunit, and an RNA subunit, scR1 (ref. 3). The sec65-1 mutant yeast cells are temperature-sensitive for growth and defective in the translocation of several secreted and membrane-bound proteins. The DNA sequence of the SEC65 gene suggests that its product is related to mammalian SRP19 subunit and may have a similar function. Here we show that SEC65p is a subunit of the S. cerevisiae SRP and that it is required for the stable association of another subunit, SRP54p, with SRP. Overexpression of SRP54p suppresses both growth and protein translocation defects in sec65-1 mutant cells.     

Dynamics of ATP-dependent chromatin assembly by ACF

The assembly of DNA into chromatin is a critical step in the replication and repair of the eukaryotic genome. It has been known for nearly 20 years that chromatin assembly is an ATP-dependent process. ATP-dependent chromatin-assembly factor (ACF) uses the energy of ATP hydrolysis for the deposition of histones into periodic nucleosome arrays, and the ISWI subunit of ACF is an ATPase that is related to helicases. Here we show that ACF becomes committed to the DNA template upon initiation of chromatin assembly. We also observed that ACF assembles nucleosomes in localized arrays, rather than randomly distributing them. By using a purified ACF-dependent system for chromatin assembly, we found that ACF hydrolyses about 2#150;4 molecules of ATP per base pair in the assembly of nucleosomes. This level of ATP hydrolysis is similar to that used by DNA helicases for the unwinding of DNA. These results suggest that a tracking mechanism exists in which ACF assembles chromatin as an ATP-driven DNA-translocating motor. Moreover, this proposed mechanism for ACF may be relevant to the function of other chromatin-remodelling factors that contain ISWI subunits.     

(Mg-ATP)-dependent self-assembly of molecular chaperone GroEL

The important Escherichia coli heat-shock protein GroEL of relative molecular mass 57,259 is a typical molecular chaperone. It possesses ATPase activity and interacts in ATP-driven reactions with non-folded proteins to stimulate their correct folding and/or assembly by preventing the formation of improper protein structures or aggregates. As GroEL is isolated and functions as a 20-25S tetradecameric particle (GroELp), the question arises--what is the mechanism of its own assembly? Here we show the (Mg-ATP)-dependent self-stimulation ('self-chaperoning') in vitro of GroELp reassembly from its monomeric state.     

A ribozyme that lacks cytidine

The RNA-world hypothesis proposes that, before the advent of DNA and protein, life was based on RNA, with RNA serving as both the repository of genetic information and the chief agent of catalytic function. An argument against an RNA world is that the components of RNA lack the chemical diversity necessary to sustain life. Unlike proteins, which contain 20 different amino-acid subunits, nucleic acids are composed of only four subunits which have very similar chemical properties. Yet RNA is capable of a broad range of catalytic functions. Here we show that even three nucleic-acid subunits are sufficient to provide a substantial increase in the catalytic rate. Starting from a molecule that contained roughly equal proportions of all four nucleosides, we used in vitro evolution to obtain an RNA ligase ribozyme that lacks cytidine. This ribozyme folds into a defined structure and has a catalytic rate that is about 10(5)-fold faster than the uncatalysed rate of template-directed RNA ligation.     

Extracellular domains mediating epsilon subunit interactions of muscle acetylcholine receptor.

Ligand-gated ion channels, a major class of cell-surface proteins, have a pseudosymmetric structure with five highly homologous subunits arranged around a central ion pore. The correct assembly of each channel, whose subunit composition varies with cell type and stage of development, requires specific recognition between the subunits. Assembly of the pentameric form of the acetylcholine receptor from adult muscle (AChR; alpha 2 beta epsilon delta) proceeds by a stepwise pathway starting with the formation of the heterodimers, alpha epsilon and alpha delta. The heterodimers than associate with the beta subunit and with each other to form the complete receptor. We have now determined which parts of the subunits mediate the interactions during assembly of the adult form of the receptor from mouse muscle by using a chimaeric subunit in which the N-terminal and C-terminal extracellular domains are derived from the epsilon subunit with the remainder from the beta subunit. The epsilon and beta subunits were chosen because the epsilon subunit forms a heterodimer with the alpha subunit in the pathway for assembly of the receptor, whereas the beta subunit does not. The epsilon beta chimera can substitute for the epsilon but not the beta subunit in the oligomeric receptor, indicating that the alpha subunit specifically recognizes an extracellular domain of the epsilon subunit.     

Site-directed mutation affecting polyomavirus capsid self-assembly in vitro

Nonequivalent bonding of identical protein subunits occurs in the polyomavirus capsid were identical pentameric capsomeres occupy both hexavalent and pentavalent positions in the icosahedral surface lattice. The polyomavirus major capsid protein VP1, purified after expression of the recombinant gene in Escherichia coli, has been isolated as capsomeres that self-assemble into capsid-like structures in vitro. The ability to switch bonding specificity in different symmetry environments therefore must be intrinsic to the VP1 molecule. In vitro self-assembly provides an assay for VP1 mutations affecting capsomere and capsid formation. We report here that a directed mutation in the VP1 expression vector, leading to a protein truncated at the carboxy terminus, results in a mutant VP1 that forms capsomeres, but not capsids, in the in vitro assembly assay. The carboxy terminus of VP1 therefore appears to be involved in the specific bonding responsible for the non-equivalent association of capsomeres.     

TCP1 complex is a molecular chaperone in tubulin biogenesis.

A role in folding of newly translated proteins in the cytosol of eukaryotes has been proposed for t-complex polypeptide-1 (TCP1), although its molecular targets have not yet been identified. Tubulin is a major cytosolic protein whose assembly into microtubules is critical to many cellular processes. Although numerous studies have focused on the expression of tubulin, little is known about the processes whereby newly translated tubulin subunits acquire conformations that enable them to form alpha-beta-heterodimers. We examined the biogenesis of alpha- and beta-tubulin in rabbit reticulocyte lysate, and report here that newly translated tubulin subunits entered a 900K complex in a protease-sensitive conformation. Addition of Mg-ATP, but not nonhydrolysable analogues, released the tubulin subunits as assembly-competent protein with a conformation that was relatively protease-resistant. The 900K complex purified from reticulocyte lysate contained as its major constituent a 58K protein that cross-reacted with a monoclonal antiserum against mouse TCP1. We conclude that TCP1 functions as a cytosolic chaperone in the biogenesis of tubulin.