THE LTK GENE FAMILY ENCODES NOVEL RECEPTOR-LIKE KINASES WITH TEMPORALEXPRESSION IN DEVELOPING MAIZE ENDOSPERM

We describe the isolation and characterization of maize cDNAs that are transcribed from a small gene family and encode a novel group of rece ptor-like kinases (RLKs). The distinctive extracellular domain of thes e novel RLKs includes a unique number and arrangement of leucine-rich repeats (LRRs), a proline-rich region (PRR), a putative protein degrad ation target sequence (PEST), and a serine-rich region (SRR). The intr acellular domain contains a putative serine/threonine protein kinase. To distinguish them from other reported RLKs, these novel RLKs were te rmed leucine-rich repeat transmembrane protein kinases (LTKs). Based o n analysis of available deduced protein sequences, LTK1 and LTK2 were predicted to be 92.1% identical, while LTK2 and LTK3 were predicted to be 97.5% identical. Though the three LTK proteins showed high homolog y, the region that most distinguished LTK1 from LTK2 and LTK3 was foun d in the extracellular domain, in the SRR. To differentiate between ex pression of the individual ltk genes, we used the reverse transcriptas e polymerase chain reaction (RT-PCR) in combination with restriction e nzyme analysis. While ltk1 transcripts were constantly present in all tissues tested, ltk2 and ltk3 transcripts were only detected in the en dosperm. Furthermore, transcript levels for both ltk1 and ltk2 showed modulation during endosperm development, peaking at 20 days after poll ination. These results suggest that members of the ltk gene family med iate signals associated with seed development and maturation.