Abstract
In growing plant cells, the combined activities of the cytoskeleton, endomembrane, and cell wall biosynthetic systems organize the cytoplasm and define the architecture and growth properties of the cell. These biosynthetic machineries efficiently synthesize, deliver, and recycle the raw materials that support cell expansion. The precise roles of the actin cytoskeleton in these processes are unclear. Certainly, bundles of actin filaments position organelles and are a substrate for long-distance intracellular transport, but the functional linkages between dynamic actin filament arrays and the cell growth machinery are poorly understood. The Arabidopsis (Arabidopsis thaliana) "distorted group" mutants have defined protein complexes that appear to generate and convert small GTPase signals into an Actin-Related Protein2/3 (ARP2/3)-dependent actin filament nucleation response. However, direct biochemical knowledge about Arabidopsis ARP2/3 and its cellular distribution is lacking. In this paper, we provide biochemical evidence for a plant ARP2/3. The plant complex utilizes a conserved assembly mechanism. ARPC4 is the most critical core subunit that controls the assembly and steady-state levels of the complex. ARP2/3 in other systems is believed to be mostly a soluble complex that is locally recruited and activated. Unexpectedly, we find that Arabidopsis ARP2/3 interacts strongly with cell membranes. Membrane binding is linked to complex assembly status and not to the extent to which it is activated. Mutant analyses implicate ARP2 as an important subunit for membrane association.
Highlights
In growing plant cells, the combined activities of the cytoskeleton, endomembrane, and cell wall biosynthetic systems organize the cytoplasm and define the architecture and growth properties of the cell
We confirmed the accuracy of this alternative gene model, shown in Figure 1A, by testing an array of oligonucleotides for their ability to amplify the 5# end of ARPC4 mRNA in reverse transcription (RT)-PCR experiments
WAVE-Actin-Related Protein2/3 (ARP2/3) is implicated as one nucleation pathway that has been adopted throughout the plant kingdom to regulate polarized growth (Frank and Smith, 2002; Le et al, 2003; Li et al, 2003, 2004; Mathur et al, 2003a, 2003b; Harries et al, 2005), Suc signaling (Li et al, 2004; Zhang et al, 2008), and root nodulation (Yokota et al, 2009)
Summary
The combined activities of the cytoskeleton, endomembrane, and cell wall biosynthetic systems organize the cytoplasm and define the architecture and growth properties of the cell. In Arabidopsis, loss of either ARP2/3 subunit gene or mutations in WAVE complex genes that positively regulate ARP2/3 cause complicated syndromes, including the loss of polarized diffuse growth throughout the shoot epidermis, defective cell-cell adhesion, and decreased hypocotyl elongation (for review, see Szymanski, 2005). In Arabidopsis, double mutant analyses indicate that WAVE is the sole pathway for ARP2/3 activation and that all subunits positively regulate ARP2/3 (Deeks et al, 2004; Basu et al, 2005; Djakovic et al, 2006). BRICK1 and ARP2/3 mutants have similar phenotypes, suggesting conserved regulatory relationships between WAVE and ARP2/3 in the plant kingdom (Harries et al, 2005; Perroud and Quatrano, 2006, 2008)
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