Relationship of a putative receptor protein kinase from maize to the S-locus glycoproteins of Brassica

Abstract

THE protein kinase family of enzymes mediates the responses of eukaryotic cells to both inter- and intracellular signals^1,2. These enzymes are either serine/threonine-specific or tyrosine-specific. Many of the latter are transmembrane receptors and are important in transduction of extracellular signals across the plasma membrane³, whereas few examples of receptor serine kinases have been reported⁴. We have now identified a complementary DNA clone from Zea mays (L.) encoding a putative serine/threonine-specific protein kinase structurally related to the receptor tyrosine kinases. This structural similarity is evidence for a previously undescribed class of transmembrane receptor in higher plants likely to be involved in signal reception and transduction. Furthermore, the catalytic domain of this protein kinase is linked through a transmembrane domain to an extracellular domain similar to that of glycoproteins encoded in the self-incompatibility locus of Brassica which are involved in the self-recognition system between pollen and stigma^5,6.