LETTERS TO NATURE domain of this protein and triggers the activation of the protein kinase moiety residing on the cytoplasmic side of the plasma membrane. The protein kinase then phosphorylates specific substrates, resulting in the production of second messengers, amplification of the signal and, ultimately, alteration or activation of cellular processes. This putative receptor is exceptional because it has sequence homology typical of the serine/ threonine protein kinases. Most transmembrane protein kinases described so far are of the tyrosine class. The only other putative receptor serine kinase that we are aware of is the daf-1 gene from the nematode Caenorhabditis elegans 4 • Although at this time there is no biochemical evidence that either the daf-1 or the ZmPK1 gene products are serine/threonine-specific protein kinases, the amino-acid sequence is strong evidence that these proteins represent a new type of signal-transducing molecule-receptor serine/threonine protein kinases. The identification of ZmPK1 as a putative receptor protein kinase-to our knowledge the first transmembrane receptor identified in higher plants-provides a unique opportunity to gain fresh insights into signal transduction in higher plants. D
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