Abstract
Here, we report the isolation and characterization of zebrafish activin receptor‐like kinase‐8 (zALK‐8), a novel type I serine/threonine (ser/thr) kinase receptor of the transforming growth factor beta (TGF‐β) family. zALK‐8 is novel, in that it contains an extracellular domain that is quite distinct from that of previously identified ALK receptors 1 through 7. Analysis of the predicted amino acid sequence of the 506 amino acid zALK‐8 receptor reveals an ser/thr kinase domain characteristic of type I TGF‐β family member receptors. zALK‐8, therefore, is a traditional type I ser/thr kinase receptor of the TGF‐β family, but it may exhibit novel ligand‐binding activities. The developmental expression of zALK‐8 mRNA was examined by wholemount in situ hybridization analysis using a probe from the 3′‐untranslated sequence of zALK‐8, which does not cross react with other members of the highly conserved TGF‐β receptor family. zALK‐8 mRNA is present as a maternal message that is expressed ubiquitously before the start of zygotic transcription. By 16 hr postfertilization (hpf), zALK‐8 mRNA is still expressed fairly evenly throughout the embryo. In 24‐hpf embryos, zALK‐8 mRNA is expressed predominantly in the developing eye and neural structures. By 48 hpf, zALK‐8 mRNA is faintly detectable as a diffuse signal throughout the head. zALK‐8 mRNA is not detectable by this method in 72‐hpf or 96‐hpf embryos. Northern analysis of zALK‐8 mRNA in poly(A+) mRNA isolated from 6–9 hpf embryos detects a major transcript of 3.6 kb and a minor transcript of 4.3 kb. zALK‐8 mRNA expression correlates well with known functions of TGF‐β family members as early axial patterning and mesoderm‐inducing growth factors and as potent growth and differentiation factors in craniofacial development. Dev. Dyn. 1998;211:352–361. © 1998 Wiley‐Liss, Inc.