br Formation of secretory vesicles
Formation of secretory vesicles and trafficking toward plasma membrane Insulin and other components of the secretory granules are sorting and providing certain cues for Gallein off the adjacent TGN membrane, forming immature secretory clathrin coated vesicles (Gehart and Ricci, 2013). Bin/Amphiphysin/Rvs (BAR) domains are crescent or banana shaped positively charged proteins on their concave faces, which interact with the negatively charged phospholipids allowing them to impose or sense curvature on lipid bilayers (Gehart et al., 2012). Certain BAR domain proteins, such as arfaptins and dynamins, are responsible for sensing and/or generating membrane curvature due to localizing and binding to special curved membranes, which are involved in vesicle budding on TGN and vesicle neck scission during endocytosis, respectively. Diacylglycerol (DAG) is a fundamental lipid signal messenger, which plays a key role in the function of β-cells (Kaneko and Ishikawa, 2015) both on plasma membrane and TGN. DAG recruits the serine/threonine protein kinase D (PKD) on TGN, where DAG binds to both PKD and a small G protein termed as ADP-ribosylation factor 1 (ARF1) that belongs to Ras superfamily of small GTPases (Cruz-Garcia et al., 2013; Gehart et al., 2012; Malhotra and Campelo, 2011). The PKD family has three members in mammals, including PKD1, PKD2, and PKD3, which share two highly conserved N-terminal cysteine-rich domains (C1a and C1b). The cysteine-rich C1a domain of PKD binds to DAG, while C1b domain binds to the GTPase ARF1 (via proline 155 and 275, respectively) (Malhotra and Campelo, 2011). An unknown receptor for activated C kinase (RACK) accumulates on the surface of forming granule binds and activates novel protein kinase C (nPKC). nPKC in turn phosphorylates, therefore activates PKD, which was recruited by DAG at the neck of budding secretory granules (Fig. 2, Fig. 3) (Cruz-Garcia et al., 2013; Malhotra and Campelo, 2011). Arfaptin-1 is a BAR domain-containing protein. Dimers of arfaptin-1 associate to ARF1 monomers at the neck of granules to inhibit its activity, and prevent nascent secretory granules fission. PKD phosphorylates arfaptin-1 and consequently disrupts the arfaptin-1-ARF1 complex (Gehart and Ricci, 2013; Gehart et al., 2012), thereby causing scission of the neck of budding granule. PKD stimulates the type III PI4-kinase β (PI4KIIIβ), which promotes the production of phosphatidylinositol-4 phosphate (PI4P) on TGN membrane (Malhotra and Campelo, 2011). Arfaptins are also capable to bind to phosphatidylinositol 4-phosphate (PI4P)-containing liposomes and TGN membrane surface (Cruz-Garcia et al., 2013). PI4P recruits certain pleckstrin homology domain containing proteins/enzymes on the membrane of TGN, including ceramide transfer protein (CERT) and oxysterol binding protein (OSBP). CERT and OSBP separate from TGN surface upon phosphorylation by PKD, which may be necessary for establishment of specific transport carriers (Cruz-Garcia et al., 2013). In the absence of a functional PKD, large tubular vesicles merge rather than the round vesicles (Malhotra and Campelo, 2011). This finding reflects the importance of PKD function for protein export from the Golgi apparatus and membrane fission especially separation of nascent granules that migrate to the plasma membrane (Malhotra and Campelo, 2011). There are several supporting evidences for the regulation of Golgi structure and secretion by sensation of extracellular signals through shuttling of Ras proteins and Gβγ subunits of heterotrimeric G protein resulting from activation of G-protein coupled receptors (GPCRs) cascade on the surface of plasma membrane (Saini et al., 2009; Saini et al., 2010). Moreover, PKD inhibition suppresses Golgi fragmentation, arising from over-activation and localization of the Gβγ subunits of the M3-muscarinic GPCRs on TGN membrane (Saini et al., 2010). COPI coated vesicles are involved in retrograde transport, whereas the clathrin-coated post-Golgi vesicles either are responsible for cargo translocation to the endosomes or cell surface. Vesicles which migrate from the TGN to the cell surface, named CARTS (CARriers of TGN to the cell Surface) (Pfeffer, 2012; Wakana et al., 2012). Certain receptors are emerged at the outer leaflet of TGN membrane that recruit BAR proteins to form bends and also specific clathrin adaptor proteins (adaptins), which mediate the association of clathrin molecules. The adaptor complex are categorised in two main classes as vesicular transport heterotetrameric adaptor proteins (AP1 to AP5) and monomeric GGAs (Golgi-localizing, gamma-adaptin ear homology domain, ARF-binding proteins) adaptors (Boehm and Bonifacino, 2001; Voglmaier and Edwards, 2007). The adaptor complexes play crucial role in the protein targeting/sorting and recruitment of cargo molecules into the growing clathrin-coated pits and specific destination direction of the carries (Cruz-Garcia et al., 2013).