Receptors for the transforming growth factor (TGF)-beta receptor superfamily are transmembrane protein serine/threonine kinases. By using a polymerase chain reaction (PCR)-based strategy to screen for the protein kinase sequences, we isolated a novel complementary DNA (cDNA) clone from a human breast carcinoma and a human brain cDNA libraries. The PCR primers were designed based upon the published sequence of the kinase domain of human activin receptor-like kinase (ALK)-4. The clone has unique nucleotide sequences with 82% identical to the corresponding region of rat ALK-7 and shows 96% identity at the amino acid level; however, was incomplete at the 5' end. Despite the 5' untranslated and ligand-binding regions, these results indicate the heterogeneity of a transmembrane kinase domain between the current ALK and the previously cloned human TGF-beta receptor families. The expression of messenger RNA in human tissues varied for the different human ALKs might elicit specific TGF-beta receptors functions.

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