An unusual protein kinase displaying characteristics of both the serine/threonine and tyrosine families is encoded by the Arabidopsis thaliana gene ATN1

A cDNA clone, ATN1, coding for a novel protein kinase, was isolated from an Arabidopsis thaliana inflorescence cDNA library. The deduced ATN1 protein sequence of 356 amino acid residues contains all the invariant hallmarks of the protein kinase superfamily. The ATN1 protein is however unusual in that it contains not only amino acid motifs characteristic of the protein serine/threonine kinase family, but also residues typical of protein tyrosine kinases. The ATN1 protein is not closely related to any other protein kinases characterised to date in either plants or other organisms, A database search revealed only 6 characterised protein kinases which shared greater than 30% identities with ATN1 (ranging from 33.5-41.4%) in their catalytic domains; each of the related sequences has mixed serine/threonine and tyrosine kinase affinities. The N-terminal region of ATN1 displays an N-myristoylation motif similar to those found in a number of other protein kinases. Although ATN1 does not belong to an established plant protein kinase subfamily, it is evolutionarily closest to a group which includes kinases involved in transmembrane signalling (the 'receptor-like kinases'). The ATN1 gene forms part of a small multigene family and is expressed in all organs of the plant studied.