Control of polar plant cell growth: functional characterization of regulators and effectors of the tobacco pollen tube Rac/Rop GTPase Nit-Rac5
Zusammenfassung der Projektergebnisse
The work 1) resulted in a much improved understanding of the molecular mechanisms responsible for the polarization of Rac/Rop signaling and cell expansion at the tip of pollen tubes, and 2) enhanced our knowledge concerning Rac/Rop dependent downstream signaling in these cells. The functional characterization of Nt-RhoGAP1, an important upstream regulator of the tobacco pollen tube Rac/Rop GTPase Nt-Rac5, was completed and published. Together with the functional characterization of another Nt-Rac5 regulator called Nt-RhoGDI2, which we performed independently of the support by DFG, this allowed us to develop a compelling model of how upstream regulatory factors act together to maintain polarized Nt-Rac5 activity at the pollen tube apex. Nt-RhoGAP1 accumulates laterally at the plasma membrane at the pollen tube tip, where it inactivates the signaling function of Nt-Rac5 by promoting the low intrinsic GTPase activity of this protein, and by thereby converting it from the GTP to the GDP bound conformation. GDP bound inactive Nt-Rac5 is then transferred from the plasma membrane to the cytoplasm by Nt-RhoGDI2. Subsequently, Nt-RhoGDI2 escorts inactive Nt- Rac5 back to the pollen tube apex, where Nt-Rac5 re-associates with the plasma membrane and is reactivated. In essence, we propose that constant Nt-RhoGAP1 and Nt-RhoGDI2 mediated Nt-Rac5 recycling from the flanks of the pollen tube tip to the apex is required for the maintenance of polarized apical N-Rac5 activity, which is essential for directional cell expansion. One of the effectors with which active Nt-Rac5 at the pollen tube apex interacts to stimulate downstream signaling is Nt-Rak1, as our partial functional characterization of this protein has established. The identification of two potential downstream targets of N-Rak1 (Nt-PubPT and Nt-Rik) has opened up new avenues of future research, which we are very interested in pursuing. Interestingly, Nt-Rak1 appears to function not only as an effector, but also as an upstream regulator, of Nt- Rac5. Experiments are underway to determine, whether positive feed-back regulation of Nt-Rac5 by Nt-Rak1 acts alongside the Nt-RhoGAP1 and Nt-RhoGDI2 recycling mechanism outlined above to enhance and maintain polarization of Nt-Rac5 activity in pollen tubes. Alternatively, these experiments may show that Nt-Rak1 negatively feed-back regulates Nt-Rac5 and has a function in pathway attenuation. In any case, we find the outcome of our work on the project extremely exciting and we are looking forward to further extend these investigations.
Projektbezogene Publikationen (Auswahl)
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(2006) NtRhoGAP1 spatially restricts signalling of RAC/ROP to the apex of tobacco pollen tubes. Plant Cell 18: 3033
U. Klahre and B. Kost
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(2008) Polarization of Rho (Rac/Rop) signalling in tip-growing plant cells. Trends in Cell Biology 18:119
B. Kost
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(2008) Regulation of membrane trafficking, cytoskeleton dynamics and cell polarity by Rop/Rac GTPases. Plant Physiology 147:1527
S. Yalovsky, D. Bloch, N. Sorek and B. Kost
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(2010) Regulatory and Cellular Functions of Plant RhoGAPs and RhoGDIs. In: Integrated G Proteins Signaling in Plants; eds: S. Yalovsky, F. Baluska and A. Jones. Springer Verlag, Heidelberg, Germany, pp 27-48
B. Kost
