Control of polar plant cell growth: functional characterization of regulators and effectors of the tobacco pollen tube Rac/Rop GTPase Nit-Rac5
Final Report Abstract
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.
Publications
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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
