An understanding of the basic defect in the inherited disorder cystic fibrosis requires cloning of the cystic fibrosis gene and definition of its protein product. In the absence of direct functional information, chromosomal map position is a guide for locating the gene. Chromosome walking and jumping and complementary DNA hybridization were used to isolate DNA sequences, encompassing more than 500,000 base pairs, from the cystic fibrosis region on the long arm of human chromosome 7. Several transcribed sequences and conserved segments were identified in this cloned region. One of these corresponds to the cystic fibrosis gene and spans approximately 250,000 base pairs of genomic DNA.

Approximately 70 percent of the mutations in cystic fibrosis patients correspond to a specific deletion of three base pairs, which results in the loss of a phenylalanine residue at amino acid position 508 of the putative product of the cystic fibrosis gene. Extended haplotype data based on DNA markers closely linked to the putative disease gene locus suggest that the remainder of the cystic fibrosis mutant gene pool consists of multiple, different mutations. A small set of these latter mutant alleles (about 8 percent) may confer residual pancreatic exocrine function in a subgroup of patients who are pancreatic sufficient. The ability to detect mutations in the cystic fibrosis gene at the DNA level has important implications for genetic diagnosis.

BMP7 and activin are members of the transforming growth factor ␤ superfamily. Here we characterize endogenous activin and BMP7 signaling pathways in P19 embryonic carcinoma cells. We show that BMP7 and activin bind to the same type II receptors, ActRII and IIB, but recruit distinct type I receptors into heteromeric receptor complexes. The major BMP7 type I receptor observed was ALK2, while activin bound exclusively to ALK4 (ActRIB). BMP7 and activin elicited distinct biological responses and activated different Smad pathways. BMP7 stimulated phosphorylation of endogenous Smad1 and 5, formation of complexes with Smad4 and induced the promoter for the homeobox gene, Tlx2. In contrast, activin induced phosphorylation of Smad2, association with Smad4, and induction of the activin response element from the Xenopus Mix.2 gene. Biochemical analysis revealed that constitutively active ALK2 associated with and phosphorylated Smad1 on the COOH-terminal SSXS motif, and also regulated Smad5 and Smad8 phosphorylation. Activated ALK2 also induced the Tlx2 promoter in the absence of BMP7. Furthermore, we show that ALK1 (TSRI), an orphan receptor that is closely related to ALK2 also mediates Smad1 signaling. Thus, ALK1 and ALK2 induce Smad1-dependent pathways and ALK2 functions to mediate BMP7 but not activin signaling.Recent studies have advanced significantly our understanding of how TGF␤ 1 superfamily members mediate their biological effects. The discovery of TGF␤ receptors and Smad proteins along with recent insights into the mechanism of their activation have allowed us to trace a TGF␤ signal transduction pathway from the cell membrane to the nucleus (reviewed in Refs. 1 and 2). TGF␤ family members initiate signaling at the cell surface by binding and bringing together two different but related serine/threonine kinase receptors, type I and type II.First, the ligand binds to the type II receptor, which recruits and transphosphorylates the type I receptor on the GS domain, a region in the juxtamembrane domain that is rich in serine and glycine residues (3). A mutation in the GS domain can lead to constitutive activation of the receptor (4) and such an activated receptor mimics the effects of the entire receptor-ligand complex in the absence of growth factor and the type II receptor. Thus, the type I receptor is considered the primary transducer of signals to downstream components of the pathway. So far, seven type I receptors or activin receptor-like kinases (ALK1-7) have been identified in vertebrates (reviewed in Refs. 2 and 5).Once the type I receptor is activated, it associates with specific receptor-regulated Smad proteins and phosphorylates them on the last two serine residues on the carboxyl-terminal domain (6 -9). Smad proteins are essential components of TGF␤ signaling that link ligand/receptor signals to transcriptional control (10 -16). All members possess two highly conserved MAD homology domains in the amino (MH1) and carboxyl (MH2) terminus that are connected by a proline-rich nonconserved region (reviewed in R...