1.F.2 The Octameric Exocyst (Exocyst) Family

The exocyst is a conserved eight-subunit complex involved in the docking of exocytic vesicles. The exocyst has been identified as an effector for five small GTPases, including Sec4, Rho1, Rho3, Cdc42 and RalA (Lipschutz and Mostov 2002).  TRAPP and the exocyst are the structurally best characterized tethering complexes, but their comparison fails to reveal any similarity. Interactions with regulatory Rab GTPases vary, with TRAPP acting as a nucleotide exchange factor and the exocyst being an effector. Kümmel and Heinemann 2008 suggested that tethering complexes do not mediate a strictly conserved process in vesicular transport but are diverse regulators acting after vesicle budding but prior to membrane fusion. The exocyst tethers secretory vesicles at the plasma membrane to provide quality control of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion. Exocyst interactions with membrane partners may involve conformational changes accompanying the tethering reactions (Lepore et al. 2018).

The exocyst complex plays a critical role for the complex as a spatiotemporal regulator through the numerous protein and lipid interactions of its subunits. The exocyst is also important for additional trafficking steps and cellular processes beyond exocytosis, with links to development and disease (Heider and Munson 2012). The eight subunits of the exocyst do not show appreciable sequence similarity with each other, but most of them are rich in acidic amino acids, glutamate and aspartate, and shows low sequence similarity with Vegetative Insecticidal protein-3 family members (TC# 1.A.105), especiallly in subfamily 2. Sec3p and Exo70p can function on the plasma membrane while the other subunits are brought to them on secretory vesicles (Liu et al. 2018).

Plant cells and tissues rely on targeted exocytosis, and the exocyst regulates cell polarity and morphogenesis, including cytokinesis, plasma membrane protein recycling (including PINs, the auxin efflux carriers), cell wall biogenesis, fertilization, stress and biotic interactions including defence against pathogens. The dramatic expansion of the EXO70 subunit gene family, of which individual members are likely responsible for exocyst complex targeting, implies that there are specialized functions of different exocysts with different EXO70s in plants (Zárský et al. 2013). One of these functions comprises a role in autophagy-related Golgi independent membrane trafficking into the vacuole or apoplast. The exocyst has the potential to function as a regulatory hub to coordinate endomembrane dynamics.