Receptor Kinases

Our interest in receptor kinases began through our studies of endosperm development when we discovered that the maize crinkly4 (cr4) gene, which is important for aleurone cell fate, encoded a receptor kinase protein (Becraft et al., 1996; Jin et al., 2000). Receptor kinases are proteins that span the cell membrane. They have an extracellular receptor domain that binds signal molecules. Inside the cell is a protein kinase domain. Binding of a signal outside the cell causes activation of the kinase domain inside the cell, leading to a protein phosphorylation cascade that ultimately alters cellular activity in specific ways. For more information, see one of the following reviews (Becraft, 1998; Becraft, 2000).

The picture at the right shows two diagrammatic views of the CR4 receptor kinase. The extracellular receptor domain has 2 parts: one is similar to a protein called b-lactamase inhibitor (yellow) and the other is similar to the human tumor necrosis factor receptor. Inside the cell, the protein kinase domain is blue and another novel domain of unknown function is shown in green. We are currently using a variety of approaches, including genetics, molecular biology and biochemistry, to identify factors that bind the various domains of CR4 and function in the signal transduction pathway.

Arabidopsis contains a small family of genes for receptor kinases related to CR4 (Tanaka et al., 2002; Schafer and Schmulling, 2002; Cao and Becraft, unpublished). We have isolated mutations in each family member and are studying the expression, function and activity of these genes and proteins. More to come soon!

The Arabidopsis genome codes for about 417 receptor kinases (Shiu and Bleecker, 2001a, b). This is about 6 times as many as are in humans. The functions are only known for a handful (maybe 25 or so) and the signal molecules are only known for about 6 in the entire plant kingdom. What are all these receptor kinases doing in plants? My lab has just begun a collaborative project with Marit Nilsen-Hamilton in the Biochemistry, Biophysics and Molecular Biology Dept. to develop a technique to screen for signaling molecules that activate these receptor kinases. We'll keep you informed!

References

Becraft, P. W. (1998). Receptor kinases in plant development. Trends Plant Sci. 3, 384-388.

Becraft, P. W. (2002). Receptor kinase signaling in plant development. Annu. Rev. Cell Devel. Biol. 18, 163-192.

Becraft, P. W., Stinard, P. S., and McCarty, D. R. (1996). CRINKLY4: a receptor kinase with TNFR similarity, involved in maize epidermal differentiation. Science 273, 1406-1409.

Jin, P., Guo, T., and Becraft, P. W. (2000). The maize CR4 receptor-like kinase mediates a growth factor-like differentiation response. Genesis 27, 104-116.

Schafer, S., and Schmulling, T. (2002). The CRK1 receptor-like kinase gene of tobacco is negatively regulated by cytokinin. Plant Mol Biol 50, 155-66.

Shiu, S. H., and Bleecker, A. B. (2001). Plant receptor-like kinase gene family: diversity, function, and signaling. Sci. Sig. Trans. Knowl. Environ. 2001, RE22. 2.

Shiu, S.-H., and Bleecker, A. B. (2001). Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases. Proc. Natl. Acad. Sci. USA. 98, 10763-10768.

Tanaka, H., Watanabe, M., Watanabe, D., Tanaka, T., Machida, C., and Machida, Y. (2002). ACR4, a putative receptor kinase gene of Arabidopsis thaliana, that is expressed in the outer cell layers of embryos and plants, is involved in proper embryogenesis. Plant Cell Physiol 43, 419-28.