Eukaryotes have "two-component" signal tranducers
The eukaryotic proteins discussed above share a number of amino acid sequence features with prokaryotic members of the two-component signal transducer family. ETRI and Sln1 each contain putative HPK and receivers, while Skn7 has similarity to receivers only. The other proteins - phytochromes, BCKDH kinase and the dr6 gene product - appear to be distant relatives of the two-component family because they have less-conserved motifs, although even biochemically defined prokaryotic family members can lack conserved motifs (Parkinson and Kofoid, 1992). The eukaryotic and prokaryotic proteins also show similar diversity with respect to the arrangements of the components; transmitters and receivers can lie within the same protein (for example, ETR1 and Sln1 in eukaryotes and BarA and LemA in prokaryotes) or receivers can lie on a separate protein (for example, Skn7 in yeast and CheY in bacteria (Stock et al., 1985)). The new eukaryotic members of the two-component family raise a number of questions, especially since their functions are not yet clearly defined. The identities of the signals transduced are unknown, except for the red light signal for phytochrome. Ethylene is presumably the signal for ETR1, but this has not been demonstrated directly. It also remains to be seen whether eukaryotic two-component modules are functionally homologous to the prokaryotic two-component systems; eukaryotic modules could be utilized in uncharacterized ways, possibly in combination with different signalling components. Among the eukaryotic proteins discussed here, BCKDH kinase has the best-understood function, yet its substrate is not an orthodox response regulator, and moreover, the substrate is phosphorylated at serine residues. The eukaryotic two-component regulators that have been recognized so far are from diverse organisms (yeast, trypanosome, plant and mammal); they serve in apparently unassociated pathways, and they are more closely related to particular prokaryotic sequences than they are to each other. Preliminary data indicates that ETR1 is a member of a gene family in A. thaliana; one member of the family encodes an HPK domain highly similar to that of ETR1, but does not encode a receiver domain (Chang, Sun, Hua and Meyerowitz, unpublished). These results, and those in yeast, indicate that multiple members are encoded by individual species. Thus, two-component signal transducers could well be widespread in eukaryotes, and the genes described here just the first examples of many more to be uncovered.
© 1994 Elsevier Masson SAS.