Nuclear targeting of proteins: how many different signals?

The nuclear import of proteins into the cell nucleus involves the recognition of a nuclear localization signal sequence, borne by the protein to be transported, by complex molecules called importins, that will subsequently mediate the crossing over of the nuclear envelope. The most frequently encountered signal sequence is made up of short stretches of basic amino acid residues and is recognized by importins alpha and/or beta. Other signal sequences have been described, and some have been shown to mediate the association with importins other than importin alpha or beta. Recently, approaches have been developed that allow the cloning, on a functional basis, of sequences able to specify the nuclear localization of proteins. A variety of peptidic motifs of limited size which do not contain previously described signal sequences were isolated in such assays. It reveals that the spectrum of sequences that are able to target a protein to the cell nucleus may be wider than currently expected. It will probably also lead to the identification of novel target sequences for importins and will demonstrate the implication of additional members of this family of proteins in nuclear transport.     

Protease-catalysed protein splicing: a new post-translational modification?

In all organisms, proteases catalyse peptide-bond hydrolysis and mediate protein function for a multitude of cellular processes. Mechanistically, nothing prevents proteases from also catalysing peptide-bond ligation; however, this 'reverse' reaction rarely is observed. In eukaryotes its presence has been viewed as an anomaly. Recent studies from plants and animals now challenge this assumption, indicating that protease-catalysed protein splicing is a bona fide post-translational modification. Increasing evidence indicates that the proximity of protein substrates, imposed either by their structure or by the physical constraints of the local environment, dictates when the splicing reaction will occur. This previously under-recognized splicing mechanism could increase intracellular protein diversity, thereby expanding the size of the proteome and sequence diversity beyond the predictions from genomic studies.     

Nuclear targeting sequences--a consensus?

Nuclear targeting sequences are essential for the transport of proteins into the nucleus. The seven-amino-acid nuclear targeting sequence of the SV40 large T antigen has been regarded as the model; however, many nuclear targeting sequences appear to be more complex. We suggest in this review that, despite this diversity, a consensus bipartite motif can be identified.     

Nuclear cGAS: guard or prisoner?

cGAS, an innate immune sensor of cellular stress, recognizes double‐stranded DNA mislocalized in the cytosol upon infection, mitochondrial stress, DNA damage, or malignancy. Early models suggested that cytosolic localization of cGAS prevents autoreactivity to nuclear and mitochondrial self‐DNA, but this paradigm has shifted in light of recent findings of cGAS as a predominantly nuclear protein tightly bound to chromatin. This has raised the question how nuclear cGAS is kept inactive while being surrounded by chromatin, and what function nuclear localization of cGAS may serve in the first place? Cryo‐EM structures have revealed that cGAS interacts with nucleosomes, the minimal units of chromatin, mainly via histones H2A/H2B, and that these protein–protein interactions block cGAS from DNA binding and thus prevent autoreactivity. Here, we discuss the biological implications of nuclear cGAS and its interaction with chromatin, including various mechanisms for nuclear cGAS inhibition, release of chromatin‐bound cGAS, regulation of different cGAS pools in the cell, and chromatin structure/chromatin protein effects on cGAS activation leading to cGAS‐induced autoimmunity.     

Systemic administration of antisense oligonucleotides simultaneously targeting CK2?? and ???? subunits reduces orthotopic xenograft prostate tumors in mice

CK2 is a highly conserved, ubiquitous, signal responsive protein serine/threonine kinase. CK2 promotes cell proliferation and suppresses apoptosis, and increased CK2 expression is observed in all cancers examined. We previously reported that direct injection of antisense (AS) CK2α phosphorothioate oligonucleotides (PTO) into xenograft prostate tumors in mice significantly reduced tumor size. Downregulation of CK2α in tumor cells in vivo appeared to result in overexpression of CK2α' protein. This suggested that in cancer cells downregulation of CK2α might be compensated by CK2α' in vivo, prompting us to design a bispecific (bs) AS PTO (bs-AS-CK2) targeting both catalytic subunits. bs-AS-CK2 reduced CK2α and α' protein expression, decreased cell proliferation, and induced apoptosis in cultured cells. Biodistribution studies of administered bs-AS-CK2 oligonucleotide demonstrated its presence in orthotopic prostate xenograft tumors. High dose injections of bs-AS-CK2 resulted in no damage to normal liver or prostate, but induced extensive cell death in tumor tissue. Intraperitoneal treatment with bs-AS-CK2 PTO decreased orthotopic tumor size and downregulated both CK2 mRNA and protein expression. Tumor reduction was accomplished using remarkably low doses and was improved by dividing the dose using a multi-day schedule. Decreased expression of the key signaling pathway proteins NF-κB p65 and AKT was also observed. We propose that the molecular downregulation of CK2 through bispecific targeting of the two catalytic subunits may be uniquely useful for therapeutic elimination of tumors.     

The formation of the brush-sticks: modification of chimpanzees or the by-product of folding?

Based on field research and experimental treatments of trees, we investigated the formation of the brush-like shape of digging sticks used by chimpanzees (Pan troglodytes troglodytes). Evidence obtained in the field consisted of digging sticks found in Mboete, Equatorial Guinea, which is a newly reported locality for this type of tool, and Campo, Cameroon. Digging sticks used by chimpanzees in these areas had a brush-like shape at one end, which was quite different from the other end that was probably used for digging. In our tree-breaking experiment, 8 out of 17 species acquired a typical brush-like shape without human modification when broken off, and the shapes of the stumps were similar to those found in the field. Other species did not acquire the brush-like shape naturally or even after human modifications, and the stumps had different shapes from those found in the field. Our findings suggest that the brush-like shapes of digging sticks are often naturally formed when broken off from trees, depending on the nature of the fibre structure, and that the brush-like end is not used as the digging tool. We conclude that the vegetation surrounding termite mounds might influence how chimpanzees combine different types of tools, i.e., digging stick, brush-stick and fishing tool, for obtaining termites.     

Morpholino oligos: making sense of antisense?

Since morpholino oligos were first introduced as a means to inhibit gene function in embryos, in the Spring of 2000, they have been tested in a range of model organisms, including sea urchin, ascidian, zebrafish, frog, chick, and mouse. This review surveys the results of these studies and examines the successes and limitations of the approach for targeting maternal and zygotic gene function. The evidence so far suggests that, with careful controls, morpholinos provide a relatively simple and rapid method to study gene function.     

Vaccines targeting drugs of abuse: is the glass half-empty or half-full?

The advent of vaccines targeting drugs of abuse heralded a fundamentally different approach to treating substance-related disorders. In contrast to traditional pharmacotherapies for drug abuse, vaccines act by sequestering circulating drugs and terminating the drug-induced 'high' without inducing unwanted neuromodulatory effects. Drug-targeting vaccines have entered clinical evaluation, and although these vaccines show promise from a biomedical viewpoint, the ethical and socioeconomic implications of vaccinating patients against drugs of abuse merit discussion within the scientific community.     

Zinc inhibition of electron transfer: mechanism of beta receptor inhibition?

Zinc has been shown to inhibit β-receptor activation of adenylate cyclase at a post receptor site. We have postulated that the β-receptor is one of several receptors activated by reduction, followed by transmembrane elector transfer accelerated by GTP. GTP accelerates electron transfer in a model system and this accelerated electron transfer is inhibited by zinc. This could explain the mechanism of the post receptor inhibition by zinc of the adenylate cyclase stimulation which follows β-receptor activation.