A B cell-deficient mouse by targeted disruption of the membrane exon of the immunoglobulin mu chain gene

Of the various classes of antibodies that B lymphocytes can produce, class M (IgM) is the first to be expressed on the membrane of the developing cells. Pre-B cells, the precursors of B-lymphocytes, produce the heavy chain of IgM (mu chain), but not light chains. Recent data suggest that pre-B cells express mu chains on the membrane together with the 'surrogate' light chains lambda 5 and V pre B (refs 2-7). This complex could control pre-B-cell differentiation, in particular the rearrangement of the light-chain genes. We have now assessed the importance of the membrane form of the mu chain in B-cell development by generating mice lacking this chain. We disrupted one of the membrane exons of the gene encoding the mu-chain constant region by gene targeting in mouse embryonic stem cells. From these cells we derived mice heterozygous or homozygous for the mutation. B-cell development in the heterozygous mice seemed to be normal, but in homozygous animals B cells were absent, their development already being arrested at the stage of pre-B-cell maturation.     

A coat subunit of Golgi-derived non-clathrin-coated vesicles with homology to the clathrin-coated vesicle coat protein beta-adaptin

Four high-molecular-weight proteins form the main subunits of the coat of Golgi-derived (non-clathrin) coated vesicles. One of these coat proteins, beta-COP, is identical to a Golgi-associated protein of relative mass 110,000 (110K) that shares homology with the adaptin proteins of clathrin-coated vesicles. This connection, and the comparable molecular weights of the coat proteins of Golgi-derived and clathrin-coated vesicles, indicates that they may be structurally related. The identification of beta-COP as the 110K protein explains the blocking of secretion by the drug brefeldin A.     

A homologue of the bacterial heat-shock gene DnaJ that alters protein sorting in yeast

Heat-shock proteins have been implicated in assembly of protein complexes, correct protein folding and uptake of proteins into organelles. In Escherichia coli, the heat-shock protein DnaJ and the Hsp70 homologue, DnaK, act together to disassemble a protein complex involved in bacteriophage lambda replication. We report the identification of SCJ1, a gene in the yeast Saccharomyces cerevisiae that encodes a homologue of the bacterial DnaJ protein. SCJ1 was identified by a genetic screen in which increased expression of candidate genes results in missorting of a nuclear-targeted test protein. The predicted amino-acid sequence of SCJ1 is 37% identical to the entire E. coli DnaJ protein. Hybridization experiments indicate that there is a family of yeast genes related to SCJ1. These findings suggest that the Hsp70 DnaK-DnaJ interaction is general to eukaryotes.     

New use of BCG for recombinant vaccines

BCG, a live attenuated tubercle bacillus, is the most widely used vaccine in the world and is also a useful vaccine vehicle for delivering protective antigens of multiple pathogens. Extrachromosomal and integrative expression vectors carrying the regulatory sequences for major BCG heat-shock proteins have been developed to allow expression of foreign antigens in BCG. These recombinant BCG strains can elicit long-lasting humoral and cellular immune responses to foreign antigens in mice.     

A pseudoknot-like structure required for efficient self-cleavage of hepatitis delta virus RNA

Hepatitis delta virus genomic and antigenomic RNAs contain a self-cleavage site hypothesized to function in processing the viral RNA during replication. Self-cleavage requires only a divalent cation and is mediated at the genomic site by a sequence of less than 85 nucleotides. We propose that the genomic self-cleaving sequence element and a corresponding sequence from the anti-genomic RNA could generate related secondary structures. The region of the antigenomic sequence, predicted from the proposed structure, was synthesized and shown to be sufficient for self-cleavage. Evidence for two stems which form a tertiary interaction was obtained by site-specific mutagenesis of the antigenomic sequence. Efficient self-cleavage in 10 M formamide or 5 M urea, also a property of the genomic sequence, was dependent on base-pairing in both stems. But in the absence of denaturants, the stem distal to the site of cleavage was not required, suggesting that the tertiary interaction stabilizes the structure required for self-cleavage.     

A novel cyclin encoded by a bcl1-linked candidate oncogene

We have previously identified a candidate oncogene (PRAD1 or D11S287E) on chromosome 11q13 which is clonally rearranged with the parathyroid hormone locus in a subset of benign parathyroid tumours. We now report that a cloned human placental PRAD1 complementary DNA encodes a protein of 295 amino acids with sequence similarities to the cyclins. Cyclins can form a complex with and activate p34cdc2 protein kinase, thereby regulating progress through the cell cycle. PRAD 1 messenger RNA levels vary dramatically across the cell cycle in HeLa cells. Addition of the PRAD1 protein to interphase clam embryo lysates containing inactive p34cdc2 kinase and lacking endogenous cyclins allows it to be isolated using beads bearing p13suc1, a yeast protein that binds cdc2 and related kinases with high affinity and coprecipitates kinase-associated proteins. Addition of PRAD1 also induces phosphorylation of histone H1, a preferred substrate of cdc2. These data suggest that PRAD1 encodes a novel cyclin whose overexpression may play an important part in the development of various tumours with abnormalities in 11q13.     

Mutational analysis of a protein-folding pathway

The effects of amino-acid replacements on the disulphide-coupled folding pathway of bovine pancreatic trypsin inhibitor have been examined. Replacements at three sites destabilize the native protein relative to the unfolded state, but have different effects on the relative stabilities of the disulphide-bonded folding intermediates, thus allowing the roles of the altered residues during folding to be distinguished.     

Inhibition of endocytic vesicle fusion in vitro by the cell-cycle control protein kinase cdc2

Membrane transport between the endoplasmic reticulum and the plasma membrane, which involves the budding and fusion of carrier vesicles, is inhibited during mitosis in animal cells. At the same time, the Golgi complex and the nuclear envelope, as well as the endoplasmic reticulum in some cell types, become fragmented. Fragmentation of the Golgi is believed to facilitate its equal partitioning between daughter cells. In fact, it has been postulated that both the inhibition of membrane traffic and Golgi fragmentation during mitosis are due to an inhibition of vesicle fusion, while vesicle budding continues. Although less is known about the endocytic pathway, internalization and receptor recycling are also arrested during mitosis. We have now used a cell-free assay to show that the fusion of endocytic vesicles from baby hamster kidney cells is reduced in Xenopus mitotic cytosol when compared with interphase cytosol. We reconstituted this inhibition in interphase cytosol by adding a preparation enriched in the starfish homologue of the cdc2 protein kinase. Inhibition was greater than or equal to 90% when the added cdc2 activity was in the range estimated for that in mitotic Xenopus eggs, which indicates that during mitosis the cdc2 kinase mediates an inhibition of endocytic vesicle fusion, and possibly other fusion events in membrane traffic.