Exocytic Trafficking

Exocytic Trafficking

Two of the most fundamental processes underlying plant development are cell division and cell expansion. Both processes require the transport of membrane and protein cargo in a temporally and spatially regulated manner originating from the central distribution venue of the plant cell, the trans-Golgi network/early endosome (TGN/EE).

SCD complex-mediated exocytic transport

The Bednarek Lab is biochemically and genetically dissecting the mechanisms by which essential plant-specific proteins regulate evolutionarily conserved exocytic trafficking pathways. Proper regulation and function of secretory trafficking pathways depend on numerous membrane transport and fusion proteins, including the largest family of small GTPases, Rab GTPases. Rab GTPases function as molecular switches that alternate between two conformational states: the GTP-bound, “on” form and the GDP-bound, “off” form. In their GTP-bound form, Rabs recruit divergent effectors to coordinate the formation, transport, tethering, and fusion of transport vesicles and organelles (Zhen and Stenmark, J. Cell Sci. 2015). To cycle between GTP- and GDP-bound states, Rabs must interact with GEFs (guanine nucleotide exchange factors) and Rab GAPs (GTPase-activating proteins). The association of Rab GTPases with downstream effector proteins and the integrities of the trafficking pathways regulated by specific Rabs are therefore dependent on the interactions between Rab GTPases with GEFs and GAPs. In the plant, A. thaliana, where root and pollen tube growth rely on exocytic trafficking, making this model system particularly useful for the study of membrane trafficking pathways, there are currently 57 known Rab GTPases (Orr et al. Curr Opin Cell Biol 2020). However, the identification and characterization of GEFs and GAPs and their downstream effectors is still in their infancies.

Model of SCD complex function in exocytic membrane trafficking in plants

Data from the Bednarek Lab have identified the Stomatal Cytokinesis Defective (SCD) comoplex as a plant-specific putative GEF for the evolutionarily conserved, GTPase, RabE1 and as a interactor of the exocytic vesicle tethering complex, exocyst (Mayers et al., Plant Cell 2017). Two hallmark proteins of this complex, SCD1 and SCD2, are essential for multiple aspects of plant development, and scd1 and scd2 mutants are consequently dwarfed and exhibit defects in cell division and expansion manifesting in aberrant stomatal guard cells and root hair morphogenesis (Falbel et al., 2003; Korasick et al., 2010; McMichael et al., 2013).

Ongoing projects in the Bednarek Lab continue to explore the regulation of this essential  of exocytic trafficking player from several perspectives.

  1. The identification of accessory proteins of the SCD complex and their roles in facilitating SCD complex assembly as well as the associations of SCD1 and SCD2 with compartments of the endomembrane trafficking pathway
  2. Comparative functional studies of the SCD complex using the model plant system, Physcomitrium (Physcomitrella) patens to understand the extent by which the SCD complex regulates the evolutionarily conserved, RabE1 GTPase across multiple plant lineages
  3. Dissection of the regulatory mechanisms of SCD1 enzymatic activity through post-translational modification