Ehk-1 and Ehk-2: two novel members of the Eph receptor-like tyrosine kinase family with distinctive structures and neuronal expression.

We have identified two novel members of the Eph RTK family, termed Ehk (eph homology kinase) -1 and -2. Compared to the amino acid sequences of various Eph family members, Ehk-1 and Ehk-2 are closest to the Sek and Cek-4/Mek-4/Hek kinases, and both are more similar to the Elk kinase than they are to the Eck or Eph kinases. Analysis of Ehk-1 cDNAs from various brain libraries reveals alternatively spliced transcripts that can encode five different forms of Ehk-1 transmembrane proteins. By contrast, Ehk-2 cDNAs revealed only a single form of protein coding region. However, the structure of Ehk-2 differs from all known members of the Eph family based on a 42 amino acid insert positioned between homology regions IV and V in the kinase domain. Ehk-1 and Ehk-2 are almost exclusively expressed in the nervous system. RNA in situ hybridization analyses on adult brain show that the Ehks are predominantly expressed in neurons and display overlapping, but distinct patterns of expression in various neuronal populations.