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 [1-5]. 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 [7, 8]. 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 [9, 10]. 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 [1, 4, 6, 11]. 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.
All Science Journal Classification (ASJC) codes
- Biochemistry, Genetics and Molecular Biology(all)
- Agricultural and Biological Sciences(all)