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 fr om an Arabidopsis thaliana inflorescence cDNA library. The deduced ATN 1 protein sequence of 356 amino acid residues contains all the invaria nt hallmarks of the protein kinase superfamily. The ATN1 protein is ho wever unusual in that it contains not only amino acid motifs character istic 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 p lants or other organisms, A database search revealed only 6 characteri sed protein kinases which shared greater than 30% identities with ATN1 (ranging from 33.5-41.4%) in their catalytic domains; each of the rel ated sequences has mixed serine/threonine and tyrosine kinase affiniti es. The N-terminal region of ATN1 displays an N-myristoylation motif s imilar to those found in a number of other protein kinases. Although A TN1 does not belong to an established plant protein kinase subfamily, it is evolutionarily closest to a group which includes kinases involve d in transmembrane signalling (the 'receptor-like kinases'). The ATN1 gene forms part of a small multigene family and is expressed in all or gans of the plant studied.