SPIKE1 Signals Originate from and Assemble Specialized Domains of the Endoplasmic Reticulum

Department of Agronomy, Purdue University, West Lafayette, IN 47907-2054, USA.
Current biology: CB (Impact Factor: 9.57). 12/2010; 20(23):2144-9. DOI: 10.1016/j.cub.2010.11.016
Source: PubMed


In the leaf epidermis, intricately lobed pavement cells use Rho of plants (ROP) small GTPases to integrate actin and microtubule organization with trafficking through the secretory pathway. Cell signaling occurs because guanine nucleotide exchange factors (GEFs) promote ROP activation and their interactions with effector proteins that direct the cell growth machineries. In Arabidopsis, SPIKE1 (SPK1) is the lone DOCK family GEF. SPK1 promotes polarized growth and cell-cell adhesion in the leaf epidermis; however, its mode of action in cells is not known. Vertebrate DOCK proteins are deployed at the plasma membrane. Likewise, current models place SPK1 activity and/or active ROP at the plant plasma membrane and invoke the localized patterning of the cortical cytoskeleton as the mechanism for shape control. In this paper, we find that SPK1 is a peripheral membrane protein that accumulates at, and promotes the formation of, a specialized domain of the endoplasmic reticulum (ER) termed the ER exit site (ERES). SPK1 signals are generated from a distributed network of ERES point sources and maintain the homeostasis of the early secretory pathway. The ERES is the location for cargo export from the ER. Our findings open up unexpected areas of plant G protein biology and redefine the ERES as a subcellular location for signal integration during morphogenesis.

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    • "The Arabidopsis DOCK family ROP guanine nucleotide exchange factor (GEF) protein SPIKE1 is localized to ER exit sites (ERES) and participates in the maintenance of ER homeostasis by promoting the formation of ERES (Zhang et al., 2013b "
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    • "SPK1 confers ROP from the inactive guanine diphosphate (GDP) bound state to its active form that is coupled to guanine triphosphate (GTP) (Basu et al. 2008). The endoplasmic reticulum may be the central point of ROP2/4 signalling since large quantities of the SCAR/WAVE-complex reside at the ER (Zhang et al. 2013a) and also SPIKE signalling is linked to the ER (Zhang et al. 2010). Furthermore, it has been shown that MT patterning may be influenced indirectly by the protein ANGUSTIFOLIA (AN) (Kim et al. 2002), a C-terminal binding protein/brefeldin A-ADP ribosylated substrate (CtBP/BARS) transcriptional regulator that influences the expression pattern of e.g. "
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    • "We also consistently detected an intranuclear pool of ARP2/3 in pavement cells (Figure 4). Although reports of endogenous nuclear ARP2/3 are rare, our results are consistent with previous reports that other plant ARP2/3 pathway components and actin have been detected in the nucleus (Cruz and Moreno Diaz de la Espina, 2008; Zhang et al., 2010; Dyachok et al., 2011). Historically in the actin field, the topic of F-actin in the nucleus in general has been controversial, and the precise function of nuclear ARP2/3 is not known (Goley et al., 2006; Yoo et al., 2007). "
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