| During cell morphogenesis the actin cytoskeleton controls the polarized distribution of proteins and organelles. ARP2/3 and its activator WAVE are conserved complexes that nucleate actin filaments. In the cell, ARP2/3 functions in plasma membrane recycling, organelle motility and vacuole biogenesis. The Rho-family GTPase, Rac1 activates WAVE complex by directly binding to SRA-1 subunit. Like other eukaryotes, plants have both the complexes. Mutation in the genes encoding subunits of WAVE and ARP2/3 complex in Arabidopsis causes stage-specific swelling and twisting of trichome and thus belongs to the 'distorted (dis)' class of mutants. Arabidopsis encodes 11 Rho-of-Plants (ROPs), and like their mammalian homologs, ROPs regulate endomembrane and cytoskeleton dynamics in a variety of cell types. However, to-date no GEF (Guanine-nucleotide-exchange-factor) has been identified as an upstream regulator of the ROP-WAVE-ARP2/3 pathway. To identify upstream players of ROP-WAVE-ARP2/3, we used trichome swelling as a mutant screening criterion where the SPIKE1 (SPK1) gene was identified. The C-terminal 300 amino acids of SPK1 encodes a conserved domain, DHR-2, that acts as a GEF for multiple ROPs. GEF activity is an essential component of SPK1 function because the spk1-3 protein that lacks an intact DHR2 domain fails to bind ROP and spk1-3 plants display a severe loss-of-function phenotype. This research will provide genetic and biochemical evidence for a SPK1-ROP-WAVE-ARP2/3 pathway. This is the first evidence of a well-defined pathway from GEF to actin filament nucleation in a multi-cellular organism. |
