To explore the origins and consequences of tetraploidy in the African clawed frog, we sequenced the Xenopus laevis genome and compared it to the related diploid X. tropicalis genome. We demonstrate the allotetraploid origin of X. laevis by partitioning its genome into two homeologous subgenomes, marked by distinct families of “fossil” transposable elements. Based on the activity of these elements and the age of hundreds of unitary pseudogenes, we estimate that the two diploid progenitor species diverged ~34 million years ago (Mya) and combined to form an allotetraploid ~17–18 Mya. 56% of all genes are retained in two homeologous copies. Protein function, gene expression, and the amount of flanking conserved sequence all correlate with retention rates. The subgenomes have evolved asymmetrically, with one chromosome set more often preserving the ancestral state and the other experiencing more gene loss, deletion, rearrangement, and reduced gene expression.

Bone morphogenetic proteins (BMPs) are secreted proteins that interact with cell-surface receptors and are believed to play a variety of important roles during vertebrate embryogenesis. Bmpr, also known as ALK-3 and Brk-1, encodes a type I transforming growth factor-~ (TGF-[3) family receptor for BMP-2 and BMP-4. Bmpr is expressed ubiquitously during early mouse embryogenesis and in most adult mouse tissues. To study the function of Bmpr during mammalian development, we generated Bmpr-mutant mice. After embryonic day 9.5 (E9.5), no homozygous mutants were recovered from heterozygote matings. Homozygous mutants with morphological defects were first detected at E7.0 and were smaller than normal. Morphological and molecular examination demonstrated that no mesoderm had formed in the mutant embryos. The growth characteristics of homozygous mutant blastocysts cultured in vitro were indistinguishable from those of controls; however, embryonic ectoderm (epiblast) cell proliferation was reduced in all homozygous mutants at E6.5 before morphological abnormalities had become prominent. Teratomas arising from E7.0 mutant embryos contained derivatives from all three germ layers but were smaller and gave rise to fewer mesodermal cell types, such as muscle and cartilage, than controls. These results suggest that signaling through this type I BMP-2/4 receptor is not necessary for preimplantation or for initial postimplantation development but may be essential for the inductive events that lead to the formation of mesoderm during gastrulation and later for the differentiation of a subset of mesodermal cell types.

Bone morphogenetic proteins (BMPs), which are members of the trnsming growth factor 13 (TGF-I)

Dystrophin is purified as a complex with several proteins from the digitonin-solubilized muscle cell membrane. Most of dystrophin-associated proteins (DAPs) are assumed to form a large oligomeric transmembranous glycoprotein complex on the sarcolemma and link dystrophin with a basement membrane protein, laminin. In the present study, we found that the purified dystrophin-DAP complex was dissociated into several groups by n-octyl-P-~-glucoside treatment. In particular, we found that the glycoprotein complex stated above was dissociated into two distinct groups: one composed of 156DAG and 43DAG (A3a) and the other composed of SODAG, 35DAG and A3b. We confirmed by crosslinking and immunoaffinity chromatography that these two groups existed in a complexes. We thus concluded that the glycoprotein complex consists of these two subcomplexes. Furthermore, A3b and 43DAG, which had been formerly treated simply as the 43DAG doublets due to their similar electrophoretic mobilities in SDS/PAGE, were shown to be present in two different subcomplexes. Based on the analyses by two-dimensional gel electrophoresis, peptide mapping and immunoblotting, we concluded that A3b is a novel DAP different from 43DAG. Dystrophin, the protein responsible for Duchenne muscular dystrophy [ l , 21, is purified as a complex with several proteins called dystrophin-associated proteins (DAPs) from digitonin-solubilized rabbit skeletal muscle membrane [3, 41. At least four DAPs, given the symbols 156DAG, SODAG, 43DAG doublets (A3a and A3b) and 35DAG, were shown to exist in the transmembranous glycoprotein complex (GPC) [5, 61. In the structural study of dystrophin or the complex of dystrophin and its associated proteins (dystrophin-DAP complex) by limited calpain digestion [7], we showed that the locus of GPC binding on the dystrophin molecule is confined to the cysteine-rich domain and the first half of the Cterminal domain [6]. Since this locus is known to be the region whose loss is responsible for Duchenne muscular dystrophy [8], this finding was the first experimental evidence showing that the interaction of GPC with dystrophin is essential to prevent muscle degeneration.Among the components of GPC, 156DAG [9, 101 and 43DAG [ l l ] were shown to bind directly to laminin and dystrophin, respectively. However, the molecular organization of GPC has not yet been clarified. Thus, its study is indispensable in order to clarify the molecular basis behind muscular dystrophy. We showed immunochemically that the components of GPC (50DAG and 35DAG) are expressed in striated muscles but not in smooth muscles such as uterus and aorta, whereas 43DAG is rather ubiquitously expressed in various tissues [12, 131. On the other hand, it was reported that in the muscles of patients with severe childhood autosoma1 recessive muscular dystrophy (SCARMD), 5ODAG is lost and 35DAG is reduced, while dystrophin and other DAPs are preserved [ 141. Similar observations were made in the skeletal muscle of dystrophic hamster [15, 161. We also observed that in the skeletal...

Direct interaction between the C‐terminal portion of dystrophin and dystrophin‐associated proteins was investigated. The binding of dystrophin to each protein was reconstituted by overlaying bacterially expressed dystrophin fusion proteins onto the blot membranes to which dystrophin‐associated proteins were transferred after separation by SDS/PAGE with the following results. (a) Among the components of the glycoprotein complex which links dystrophin to the sarcolemma, a 43‐kDa dystrophin‐associated glycoprotein binds directly to dystrophin. Although at least one of the binding sites of this protein resides within the cysteine‐rich domain of dystrophin, a contribution of additional amino acid residues within the first half of the C‐terminal domain was also suggested for more secure binding. (b) Two other proteins also directly bind to dystrophin. Their binding sites are suggested to reside within the last half of the C‐terminal domain which is alternatively spliced depending on the tissue type. Previously, based on the enzyme digestion experiments, we showed that the binding site for the glycoprotein complex on dystrophin is present within the cysteine‐rich domain and the first half of the C‐terminal domain [Suzuki, A., Yoshida, M., Yamamoto, H. & Ozawa, E. (1992) FEBS Lett. 308, 154–160]. Here, we have extended this work and found that the region which is involved in interaction with the complex is widely extended to the entire length of this part of the molecule. On the basis of the present results, we propose a model of molecular architecture at the binding site for the complex on dystrophin.