Structure of the Alk1 extracellular domain and characterization of its bone morphogenetic protein (BMP) binding properties.
Bone morphogenetic proteins (BMPs) are secreted signaling proteins - they transduce their signals by assembling complexes comprised of one of three known type II receptors and one of four known type I receptors. BMP-9 binds and signals through the type I receptor Alk1, but not other Alks, while BMP-2, -4, and -7 bind and signal through Alk3, and the close homologue Alk6, but not Alk1. The present results, which include the determination of the Alk1 structure using NMR and identification of residues important for binding using SPR, show that the β-strand framework of Alk1 is highly similar to Alk3, yet there are significant differences in loops shown previously to be important for binding. The most pronounced difference is in the N-terminal portion of the β4-β5 loop, which is structurally ordered and includes a similarly placed but shorter helix in Alk1 compared to Alk3. The altered conformation of the β4-β5 loop, and to lesser extent β1-β2 loop, cause clashes when Alk1 is positioned onto BMP-9 in the manner that Alk3 is positioned onto BMP-2. This necessitates an alternative manner of binding, which is supported by a model of the BMP-9/Alk1 complex constructed using the program RosettaDock. The model shows that Alk1 is positioned similar to Alk3 but is rotated by 40 deg. The alternate positioning allows Alk1 to bind BMP-9 through a large hydrophobic interface, consistent with mutational analysis that identified several residues in the central portion of the β4-β5 loop that contribute significantly to binding and are nonconservatively substituted relative to the corresponding residues in Alk3.
Department of Biochemistry and Cancer Therapy and Research Center, University of Texas Health Science Center at San Antonio, San Antonio, TX 78229-3900, USA.
Pardeep
P
Mahlawat
Udayar
U
Ilangovan
Tanuka
T
Biswas
Lu-Zhe
LZ
Sun
Andrew P
AP
Hinck
D030301:Activin Receptors, Type II / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D055396:Bone Morphogenetic Protein 2 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D052005:Bone Morphogenetic Protein Receptors, Type I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D019485:Bone Morphogenetic Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D055427:Growth Differentiation Factor 2; D055412:Growth Differentiation Factors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006801:Humans; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D041681:NIH 3T3 Cells; D019906:Nuclear Magnetic Resonance, Biomolecular; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014162:Transfection
C525718:BMP2 protein, human; D055396:Bone Morphogenetic Protein 2; D019485:Bone Morphogenetic Proteins; C525887:GDF2 protein, human; D055427:Growth Differentiation Factor 2; D055412:Growth Differentiation Factors; D011994:Recombinant Proteins; C425364:ACVRL1 protein, human; D030301:Activin Receptors, Type II; C498211:BMPR1A protein, human; D052005:Bone Morphogenetic Protein Receptors, Type I