Abstract
In the human experience SCARs (suppressor of cAMP receptors) are permanent reminders of past events, not always based on bad decisions, but always those in which an interplay of opposing forces leaves behind a clear record in the form of some permanent watery mark. During plant morphogenesis, SCARs are important proteins that reflect an unusual evolutionary outcome, in which the plant kingdom relies heavily on this single class of actin-related protein (ARP) 2/3 complex activator to dictate the time and place of actin filament nucleation. This unusually simple arrangement may serve as a permanent reminder that cell shape control in plants is fundamentally different from that of crawling cells in mammals that use the power of actin polymerization to define and maintain cell shape. In plant cells, actin filaments indirectly affect cell shape by determining the transport properties of organelles and cargo molecules that modulate the mechanical properties of the wall. It is becoming increasingly clear that polarized bundles of actin filaments operate at whole cell spatial scales to organize the cytoplasm and dictate the patterns of long-distance intracellular transport and secretion. The number of actin-binding proteins and actin filament nucleators that are known to participate in the process of actin network formation are rapidly increasing. In plants, formins and ARP2/3 are two important actin filament nucleators. This review will focus on ARP2/3, and the apparent reliance of most plant species on the SCAR/WAVE (WASP family verprolin homologous) regulatory complex as the sole pathway for ARP2/3 activation.
Highlights
Actin filaments are polar, unstable polymers composed of Gactin subunits that reversibly associate with one another through non-covalent interactions
Active ROP/Rac proteins may antagonize trans-inhibition of the suppressor of cAMP receptor (SCAR) subunit that is assembled into /SCAR regulatory complex (W/SRC), and promote conformational changes in the complex that can lead to actin-related protein 2/3 complex (ARP2/3) activation (Figure 2B)
Unlike the central regions of vertebrate WASP family verprolin homologous (WAVE), which is dominated by a proline rich region (Figure 3), that binds to both the globular actin (G-actin)-binding protein profilin and Src homology 3-domain-containing signaling proteins (Miki et al, 1998b), plant SCARs lack a large proline rich region; they do contain a short conserved proline rich motif at the near C-terminus that may have a functional importance
Summary
Unstable polymers composed of Gactin subunits that reversibly associate with one another through non-covalent interactions. The exact nature of the cellular control of the W/SRC–ARP2/3 pathway is complex and poorly understood (see below); a direct physical interaction between active ROP/Rac proteins and the W/SRC subunit SRA1 appears to be evolutionarily conserved and an important aspect of pathway control (Basu et al, 2004; Lebensohn and Kirschner, 2009; Chen et al, 2010).
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