Successive action of DnaK, DnaJ and GroEL along the pathway of chaperone-mediated protein folding.

The main stress proteins of Escherichia coli function in an ordered protein-folding reaction. DnaK (heat-shock protein 70) recognizes the folding polypeptide as an extended chain and cooperates with DnaJ in stabilizing an intermediate conformational state lacking ordered tertiary structure. Dependent on GrpE and ATP hydrolysis, the protein is then transferred to GroEL (heat-shock protein 60) which acts catalytically in the production of the native state. This sequential mechanism of chaperone action may represent an important pathway for the folding of newly synthesized polypeptides.     

Human aminopeptidase N is a receptor for human coronavirus 229E.

Human coronaviruses (HCV) in two serogroups represented by HCV-229E and HCV-OC43 are an important cause of upper respiratory tract infections. Here we report that human aminopeptidase N, a cell-surface metalloprotease on intestinal, lung and kidney epithelial cells, is a receptor for human coronavirus strain HCV-229E, but not for HCV-OC43. A monoclonal antibody, RBS, blocked HCV-229E virus infection of human lung fibroblasts, immunoprecipitated aminopeptidase N and inhibited its enzymatic activity. HCV-229E-resistant murine fibroblasts became susceptible after transfection with complementary DNA encoding human aminopeptidase N. By contrast, infection of human cells with HCV-OC43 was not inhibited by antibody RBS and expression of aminopeptidase N did not enhance HCV-OC43 replication in mouse cells. A mutant aminopeptidase lacking the catalytic site of the enzyme did not bind HCV-229E or RBS and did not render murine cells susceptible to HCV-229E infection, suggesting that the virus-binding site may lie at or near the active site of the human aminopeptidase molecule.     

Retinoic acid alters hindbrain Hox code and induces transformation of rhombomeres 2/3 into a 4/5 identity.

It has been suggested that Hox genes play an important part in the patterning of limbs, vertebrae and craniofacial structures by providing an ordered molecular system of positional values, termed the Hox code. Little is known about the nature of the signals that govern the establishment and regulation of Hox genes, but retinoic acid can affect the expression of these genes in cell lines and in embryonic tissues. On the basis of experimental and clinical evidence, the hindbrain and branchial region of the head are particularly sensitive to the effects of retinoic acid but the phenotypes are complex and hard to interpret, and how and if they relate to Hox expression has not been clear. Here we follow the changes induced by retinoic acid to hindbrain segmentation and the branchial arches using transgenic mice which contain lacZ reporter genes that reveal the endogenous segment-restricted expression of the Hox-B1 (Hox-2.9), Hox-B2(Hox-2.8) and Krox-20 genes. Our results show that these genes rapidly respond to exposure to retinoic acid at preheadfold stages and undergo a progressive series of changes in segmental expression that are associated with specific phenotypes in hindbrain of first branchial arch. Together the molecular and anatomical alterations indicate that retinoic acid has induced changes in the hindbrain Hox code which result in the homeotic transformation of rhombomeres (r) 2/3 to an r4/5 identity. A main feature of this rhombomeric phenotype is that the trigeminal motor nerve is transformed to a facial identity. Furthermore, in support of this change in rhombomeric identity, neural crest cells derived from r2/3 also express posterior Hox markers suggesting that the retinoic acid-induced transformation extends to multiple components of the first branchial arch.     

Effects of an Rb mutation in the mouse.

The retinoblastoma gene is mutated in several types of human cancer and is the best characterized of the tumour-suppressor genes. A mouse strain has been constructed in which one allele of Rb is disrupted. These heterozygous animals are not predisposed to retinoblastoma, but some display pituitary tumours arising from cells in which the wild-type Rb allele is absent. Embryos homozygous for the mutation die between days 14 and 15 of gestation, exhibiting neuronal cell death and defective erythropoiesis.     

A functional barrier to movement of lipids in polarized neurons.

In polarized neurons, axons and dendrites perform different functions, which are reflected in their different molecular organization. Studies on the sorting of viral and endogenous glycoproteins in epithelial cells and hippocampal neurons suggest that there may be similarities in the mechanism of sorting in these two cell types. The mechanisms that maintain the distinct composition of the two plasma membrane domains in these two cell types must, however, be different. We have proposed the existence of a functional barrier at the axonal hillock/initial segment which prevents the intermixing of membrane constituents. Here we test this hypothesis by fusing liposomes containing fluorescent phospholipids into the plasma membrane of polarized hippocampal cells in culture. Fusion was induced by lowering the pH and mediated by influenza virus haemagglutinin expressed on the axonal surface of neurons infected with fowl plague virus. Labelling was found exclusively on axons after fusion. Although the fused lipids were mobile on the axonal membrane, no labelling was detected on the cell body and dendritic surfaces. These results suggest that there is a diffusion barrier at the axonal hillock/initial segment which maintains the compositional differences between the axonal and somatodendritic domains.     

Communal nesting patterns in mice implicate MHC genes in kin recognition.

House mice (Mus musculus domesticus) form communal nests and appear to nurse each other's pups indiscriminately. Communal nesting probably functions to reduce infanticide, but it also makes females vulnerable to exploitation if nursing partners fail to provide their fair share of care. Kinship theory predicts that females will preferentially form communal nests with relatives to minimize exploitation and further increase inclusive fitness. Here we provide evidence from seminatural populations that females prefer communal nesting partners that share allelic forms of major histocompatibility complex genes. Such behaviour would lead to the selection of close relatives as communal nesting partners. Although criteria for the demonstration of kin recognition are currently embroiled in controversy, this is the first vertebrate study to meet Grafen's restrictive requirements: discrimination is based on genetic similarity at highly polymorphic loci, incidental correlations due to relatedness are experimentally controlled, and strong reasons exist for expecting the assayed behaviour to be kin-selected.     

Mutation of the beta-amyloid precursor protein in familial Alzheimer's disease increases beta-protein production.

Progressive cerebral deposition of the 39-43-amino-acid amyloid beta-protein (A beta) is an invariant feature of Alzheimer's disease which precedes symptoms of dementia by years or decades. The only specific molecular defects that cause Alzheimer's disease which have been identified so far are missense mutations in the gene encoding the beta-amyloid precursor protein (beta-APP) in certain families with an autosomal dominant form of the disease (familial Alzheimer's disease, or FAD). These mutations are located within or immediately flanking the A beta region of beta-APP, but the mechanism by which they cause the pathological phenotype of early and accelerated A beta deposition is unknown. Here we report that cultured cells which express a beta-APP complementary DNA bearing a double mutation (Lys to Asn at residue 595 plus Met to Leu at position 596) found in a Swedish FAD family produce approximately 6-8-fold more A beta than cells expressing normal beta-APP. The Met 596 to Leu mutation is principally responsible for the increase. These data establish a direct link between a FAD genotype and the clinicopathological phenotype. Further, they confirm the relevance of the continuous A beta production by cultured cells for elucidating the fundamental mechanism of Alzheimer's disease.     

A cell line that can induce thymocyte positive selection.

The thymus positively selects thymocytes that bear T-cell receptors which recognize antigen presented by self major histocompatibility complex (MHC) proteins. Positive selection is usually driven by MHC products on radiation-resistant cortical epithelial cells. It is unknown whether positive selection is mediated by all thymic epithelial cells or by some specialized subsets. Here we introduce an H-2b-expressing thymic epithelial cell line into the thymuses of lethally irradiated H-2k animals reconstituted with H-2b/k F1 BM or fetal liver cells. I-Ab-restricted T cells are found in these animals, demonstrating that selection occurs on the introduced epithelial cells.     

Otavipithecus namibiensis, first Miocene hominoid from southern Africa.

We report here the discovery of a Miocene hominoid from Berg Aukas, Namibia, the first known from the African continent south of equatorial East Africa. This represents a major range extension of Miocene Hominoidea in Africa to latitude 20 degrees S. The holotype, a right mandibular corpus preserving the crowns of the P4-M3, partial crown and root of the P3, partial root of the canine, alveoli for all four incisors, and partial alveolus for the left canine, was found during paleontological explorations of karst-fill breccias in the Otavi region of northern Namibia. The mandible has unique characteristics that differentiate it from other middle Miocene hominoids of Africa and Eurasia and represents the only fossil evidence documenting a pre-australopithecine stage of hominoid evolution in southern Africa. Faunal analyses indicate that the breccia block containing the specimen accumulated during the latter part of the middle Miocene, about 13 +/- 1 Myr. Fauna from other breccia blocks at Berg Aukas are of diverse ages, including the earlier part of the middle Miocene, the upper Miocene, Plio-Pleistocene and Holocene.