SCD1 and SCD2 Form a Complex That Functions with the Exocyst and RabE1 in Exocytosis and Cytokinesis

Abstract

Although exocytosis is critical for the proper trafficking of materials to the plasma membrane, relatively little is known about the mechanistic details of post-Golgi trafficking in plants. Here, we demonstrate that the DENN (Differentially Expressed in Normal and Neoplastic cells) domain protein STOMATAL CYTOKINESIS DEFECTIVE1 (SCD1) and SCD2 form a previously unknown protein complex, the SCD complex, that functionally interacts with subunits of the exocyst complex and the RabE1 family of GTPases in Consistent with a role in post-Golgi trafficking, and mutants display defects in exocytosis and recycling of PIN2-GFP. Perturbation of exocytosis using the small molecule Endosidin2 results in growth inhibition and PIN2-GFP trafficking defects in and mutants. In addition to the exocyst, the SCD complex binds in a nucleotide state-specific manner with Sec4p/Rab8-related RabE1 GTPases and overexpression of wild-type RabE1 rescues temperature-sensitive mutants. Furthermore, SCD1 colocalizes with the exocyst subunit, SEC15B, and RabE1 at the cell plate and in distinct punctae at or near the plasma membrane. Our findings reveal a mechanism for plant exocytosis, through the identification and characterization of a protein interaction network that includes the SCD complex, RabE1, and the exocyst.

The Plant Trans-Golgi Network: Not Just a Matter of Distinction

“…In addition to RABAs, RABE1s also localize to the cell plate, where they interact with the Stomatal CytokinesisDefective (SCD) complex. Inhibition of RABE1 causes cytokinesis defects, emphasizing its role in trafficking to the cell plate (Speth et al, 2009;Ahn et al, 2013;Mayers et al, 2017).…”

“…Additionally, SCD1 and SCD2 act as a complex to mediate post-Golgi trafficking to the plasma membrane and the cell plate. Multiple lines of evidence point to the interaction between the SCD complex, the exocyst, and RABE1 GTPases in the trafficking of material to the cell plate (Mayers et al, 2017).…”

Phosphorylation of the exocyst subunit Exo70B2 contributes to the regulation of its function

“…At early stages of post-Golgi trafficking STOMATAL CYTOKINESIS DEFECTIVE1 (SCD1) and SCD2 proteins are though to recruit RabE1 and the exocyst to vesicles (Mayers et al, 2017). During the delivery of secretory vesicles, Sec3 and Exo70 subunits of the exocyst mediate binding to the PM (Ahmed et al, 2018).…”

“…Localization of Exo70 in humans and yeast to the PM is mediated by its ability to bind phosphatidylinositol 4,5-bisphosphate (PI(4,5)P 2 ) (He et al, 2007a; Liu et al, 2007). Although direct membrane binding of an exocyst component has not been shown in plants, the small molecule endosidin2 (ES2) binds and inhibits Exo70A1 exocytosis and recycling (Zhang et al, 2016; Mayers et al, 2017). Residues contributing to ES2 binding are located in the C-domain of Exo70A1 at its C-terminus, suggesting that the membrane-binding function of this region is conserved in plants.…”

Arabidopsis Myosins XI Are Involved in Exocytosis of Cellulose Synthase Complexes

“…Mutations of STOMATAL CYTOKINESIS DEFECTIVE 1 (SCD1) exhibit secretory vesicle accumulation in cytokinesis-defective cells as well as perturbed cell plate formation. SCD1 and SCD2 have been shown to regulate post-Golgi vesicle exocytosis to the PM through interaction with RabE1 and the exocyst complex (Falbel et al, 2003;Mayers et al, 2017). Interestingly, a recent study also shows concentration of myosin XIK at the forming cell plate in root and shoot meristem understanding of how myosin activity is correlated with plant cell growth and could help explain the previously described phenotypes in myosin xi mutants, such as reduced cell expansion/elongation rate, polar auxin transport and plant size (Peremyslov et al, 2010;Ojangu et al, 2012;Madison et al, 2015;Ueda et al, 2015;Abu-Abied et al, 2018;Ryan and Nebenführ, 2018).…”