title: Identification of a functional binding site for activin on the type I receptor ALK4.
abstract: Activins, like other members of the transforming growth factor-beta (TGF-beta) superfamily, initiate signaling by assembling a complex of two types of transmembrane serine/threonine receptor kinases classified as type II (ActRII or ActRIIB) and type I (ALK4). A kinase-deleted version of ALK4 can form an inactive complex with activin and ActRII/IIB and thereby acts in a dominant negative manner to block activin signaling. Using the complex structure of bone morphogenetic protein-2 bound to its type I receptor (ALK3) as a guide, we introduced extracellular domain mutations in the context of the truncated ALK4 (ALK4-trunc) construct and assessed the ability of the mutants to inhibit activin function. We have identified five hydrophobic amino acid residues on the ALK4 extracellular domain (Leu40, Ile70, Val73, Leu75, and Pro77) that, when mutated to alanine, have substantial effects on ALK4-trunc dominant negative activity. In addition, eleven mutants partially affected activin binding to ALK4. Together, these residues likely constitute the binding surface for activin on ALK4. Cross-linking studies measuring binding of 125I-activin-A to the ALK4-trunc mutants in the presence of ActRII implicated the same residues. Our results indicate that there is only a partial overlap of the binding sites on ALK4 and ALK3 for activin-A and bone morphogenetic protein-2, respectively. In addition three of the residues required for activin binding to ALK4 are conserved on the type I TGF-beta receptor ALK5, suggesting the corresponding region on ALK5 may be important for TGF-beta binding.
authors.affiliation: Clayton Foundation Laboratories for Peptide Biology, The Salk Institute for Biological Studies, La Jolla, California 92037, USA.
authors.forename: Craig A
authors.initials: CA
authors.lastname: Harrison
authors.forename: Peter C
authors.initials: PC
authors.lastname: Gray
authors.forename: Steven C
authors.initials: SC
authors.lastname: Koerber
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Fischer
authors.forename: Wylie
authors.initials: W
authors.lastname: Vale
mesh_terms: D030201:Activin Receptors, Type I / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D028341:Activins / Q000378:metabolism*; D000596:Amino Acids / Q000235:genetics; D000818:Animals; D001665:Binding Sites; D052005:Bone Morphogenetic Protein Receptors, Type I; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000166:cytology / Q000502:physiology; D015870:Gene Expression; D006801:Humans; D007668:Kidney / Q000166:cytology; D008168:Lung / Q000166:cytology; D008907:Mink; D008969:Molecular Sequence Data; D016296:Mutagenesis; D017346:Protein Serine-Threonine Kinases; D017434:Protein Structure, Tertiary; D011506:Proteins; D000077293:Receptor, Transforming Growth Factor-beta Type I; D017978:Receptors, Growth Factor; D018125:Receptors, Transforming Growth Factor beta / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship
chemical_list: D000596:Amino Acids; D011506:Proteins; D017978:Receptors, Growth Factor; D018125:Receptors, Transforming Growth Factor beta; D028341:Activins; D017346:Protein Serine-Threonine Kinases; C498932:ACVR1B protein, human; D030201:Activin Receptors, Type I; D052005:Bone Morphogenetic Protein Receptors, Type I; D000077293:Receptor, Transforming Growth Factor-beta Type I; C000625841:TGFBR1 protein, human