Exocytosis, mediated by an octameric protein complex called

Exocytosis, an evolutionary conserved
biological event, is possible due to fusion of secretory vesicles with the
targeted membrane to carry out various cellular processes such as cell
polarity, growth, division, cell migration, ciliogenesis and autophagy (Novick
et al. 1980; He and Guo, 2009; Heider and Munson, 2012). The initial
interaction of vesicle and the target membrane before fusion is called vesicle
tethering, which is mediated by an octameric protein complex called exocyst (TerBush
et al. 1996; Guo et al. 1999a; He and Guo, 2009). The exocyst complex consists
of eight subunits: Sec3, Sec 5, Sec 6, Sec8, Sec10, Sec15, Exo70 and Exo84 (TerBush
et al. 1995, 1996; Hsu et al. 1996; Guo et al. 1999a; Lipschutz and Mostov,
2002). These subunits are coiled -coil proteins and share some structural
homology of helical bundles (Haarer et al. 1996; TerBush et al. 1996) that help
them interact for complex formation (TerBush et al. 1996). The exocyst complex
is rod shaped structure with C and N termini at opposite poles (Hamburger et
al. 2006; Croteau et al. 2009; Yamashita et al. 2010) and help in tethering of
vesicles to the plasma membrane and delivering cargos packed in vesicles to the
apoplast (TerBush and Novick, 1995; Guo et al. 1999a; He and Guo, 2009; Picco
et al. 2017). The exocyst connects vesicles with its subunits; Sec 10 and Sec15
(Guo et al. 1999; Roth et al. 1998) and plasma membrane with Sec3 and exo 70 (Finger
et al. 1998; Boyd et al. 2004; He and Guo, 2009). Sec3 and Exo 70 bind to phosphatidylinositol
4, 5-biphosphate (PI (4,5) P2) located in plasma membrane (He et al.
2007; Liu et al. 2007; Zhang et al. 2008). The movement of vesicles is
regulated by Sec4, that encodes small GTP-binding protein (Salminen and Novick,
1987) and are directed to the plasma membrane at targeted site (Bourne, 1988;
Goud et al. 1988; Walworth et al. 1989). The Sec4p regulates the assembly of
exocyst through its interaction with Sec15p (Guo et al. 1999; Mizuno-Yamasaki
et al. 2012).

The vesicles that are at the targeted
sites are fused with the help of SNARE proteins (Jahn and Scheller, 2006). The
exocyst complex located at the target site help prepare SNARE for docking and
subsequent release of vesicular contents after fusion (TerBush and Novick,
1995). The complex connects with sec1p/Munc 18 and t-SNARES Sec9p(SNAP25) for
tethering and fusion of the secretory vesicles (Wiederkehr et al. 2004; Sivaram
et al. 2005). During this process the proteins located at vesicles membranes,
Synaptobrevin (VAMP) (Trimble et al. 1988; Baumert et al. 1989) assembles with
membrane proteins Syntaxin and SNAP 25 (Oyler et al. 1989; Bennett et al. 1992)
and form a ternary complex (Hanson et al. 1997). These proteins are the
receptors for NSF (N-ethylmaleimide-sensitive factor) and SNAPs (Soluble NSF
attachment proteins) and so-called SNAREs (SNAP receptors) (Söllner et al. 1993a, 1993b). These SNARE proteins located at vesicles (v-SNARE) interacts
with proteins at the targeted membrane (t-SNARE) and help in fusion (Söllner et al.
1993a; Rothman and Warren, 1994). SNAREs that are aligned parallel to their
transmembrane anchors during docking (Otto et al. 1997), connects two membranes
thereby zippering of v-SNARE and t-SNARE (Hanson et al. 1997). The ATPase
activity of NSF dissociates the ternary SNARE complex (Söllner et al. 1993a; Hayashi et al. 1995). This leads to the conformational
change of associated proteins and induce fusion of secretory vesicle to the
target membrane (Hanson et al. 1997). This activity of NSF could be to dock new
secretory vesicles thereby recruiting new SNARE complexes (Hanson et al. 1997).

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The importance of exocyst in cellular processes
is inevitable as it plays key role in exocytosis thereby mediating SNARE
mediated membrane fusion (He and Guo, 2009). The exocyst complex acts as a
signal receiver for various signaling pathways, help tether vesicles at the
receptor membrane and mediate fusion by inducing formation of SNARE assembly (He
and Guo, 2009; Žárský et al. 2013).
Various experiments have been done to prove its efficacy for growth, migration,
repair and defense by increasing or decreasing proteins, breaking the
association among the subunits and its associated proteins that are necessary
in this process (Novick et al. 1980; Hala et al. 2002; He et al. 2007; Zhang et
al. 2008). The exocyst mutants show secretion defects and intracellular
accumulation of the secretory vesicles (Novick et al. 1980; Guo et al. 1999; Zhang
et al. 2005, He et al. 2007; Zhang et al. 2008; Heider and Munson, 2012). The
mutation in exocyst subunit exo70 shows defects in secretion of secretory
vesicles (He et al. 2007) that transport endoglucanase Bg12 required for cell
membrane expansion and cell wall remodeling (He and Guo, 2009). Impairing the
connection of sec 3 and exo 70 with PI (4,5) P2 halts the fusion
process leading to cell death (He et al. 2007; Zhang et al. 2008). As exocyst
mediates the SNARE assembly, their mutants failed to form SNARE complex (Grote
et al. 2000).