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Differential Palmitoylation Regulates Intracellular Patterning of SNAP25

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Differential Palmitoylation Regulates Intracellular Patterning of SNAP25 Research Article 1351 Differential palmitoylation regulates intracellular patterning of SNAP25 Jennifer Greaves and Luke H. Chamberlain* Centre for Integrative Physiology, School of Biomedical Sciences, Hugh Robson Building, University of Edinburgh, Edinburgh EH8 9XD, UK *Author for correspondence ([email protected]) Accepted 24 November 2010 Journal of Cell Science 124, 1351-1360 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jcs.079095 Summary SNAP25 regulates membrane fusion events at the plasma membrane and in the endosomal system, and a functional pool of the protein is delivered to recycling endosomes (REs) and the trans Golgi network (TGN) through an ARF6-dependent cycling pathway. SNAP25 is a peripheral membrane protein, and palmitoylation of a cluster of four cysteine residues mediates its stable association with the membrane. Here, we report that palmitoylation also determines the precise intracellular distribution of SNAP25, and that mutating single palmitoylation sites enhances the amount of SNAP25 at the RE and TGN. The farnesylated CAAX motif from Hras was ligated onto a SNAP25 mutant truncated immediately distal to the cysteine-rich domain. This construct displayed the same intracellular distribution as full-length SNAP25, and decreasing the number of cysteine residues in this construct increased its association with the RE and TGN, confirming the dominant role of the cysteine-rich domain in directing the intracellular distribution of SNAP25. Marked differences in the localisations of SNAP25-CAAX and Hras constructs, each with two palmitoylation sites, were observed, showing that subtle differences in palmitoylated sequences can have a major impact upon intracellular targeting. We propose that the cysteine- rich domain of SNAP25 is designed to facilitate the dual function of this SNARE protein at the plasma membrane and endosomes, and that dynamic palmitoylation acts as a mechanism to regulate the precise intracellular patterning of SNAP25. Key words: SNAP25, Palmitoylation, Acylation, Intracellular trafficking Introduction cargo to REs (Aikawa et al., 2006a). Although this cycling pathway Intracellular-membrane-fusion events in eukaryotes are regulated is central to the intracellular functions of SNAP25, it is not clear by members of the soluble N-ethylmaleimide-sensitive fusion how entry into this pathway is achieved. protein-attachment protein receptor (SNARE) protein family (Jahn The majority of SNARE proteins are anchored to membranes by Journal of Cell Science and Scheller, 2006; Sollner et al., 1993), and these proteins are the transmembrane domains. SNAP25, however, is an interesting minimal membrane fusion machinery in vitro (Weber et al., 1998). exception and the membrane binding of this protein is regulated by One of the most extensively characterised membrane fusion palmitoylation, a common post-translational modification, most pathways is regulated exocytosis, which involves the controlled often involving the attachment of palmitic acid groups (C16:0) to fusion of intracellular secretory vesicles with the plasma membrane. cysteine residues (Resh, 2006). Previous work defined the minimal Regulated exocytosis mediates the secretion of a wide variety of palmitoylation domain of SNAP25b, the major SNAP25 isoform essential molecules, including neurotransmitters, peptides and expressed in adult brain (Bark et al., 1995); this region comprises hormones, in response to specific intracellular signals, most often residues 85–120 and includes four palmitoylated cysteine residues a rise in cytosolic Ca2+ levels. In neurons and neuroendocrine present at amino acid positions 85, 88, 90 and 92 (Gonzalo et al., cells, regulated exocytosis is dependent upon the interaction of the 1999). It is important to note that palmitoylation is not simply a plasma membrane SNAREs syntaxin 1 and SNAP25 with the hydrophobic membrane anchor. Recent work, in different systems, vesicle SNARE VAMP2 (Blasi et al., 1993; Gary et al., 1994; has highlighted the diverse array of effects that palmitoylation can Sadoul et al., 1995; Schiavo et al., 1993; Sollner et al., 1993). have on proteins, including regulating intracellular sorting, Although SNAP25 has been intensively studied as an exocytotic mediating association with membrane microdomains, regulating SNARE protein, more recent work has shown that SNAP25 protein–protein interactions and modulating protein stability depletion also inhibits the trafficking of cargo from sorting (Greaves and Chamberlain, 2007; Greaves et al., 2009b; Linder endosomes to recycling endosomes (REs) in PC12 cells (Aikawa and Deschenes, 2007; Resh, 2006). Furthermore, the versatility et al., 2006a). Consistent with this, previous work has also of palmitoylation as a protein regulator is enhanced by its highlighted an important role for SNAP23, a ubiquitous homologue reversibility, with many proteins undergoing continuous cycles of of SNAP25, in endosomal recycling of internalised transferrin to depalmitoylation and repalmitoylation (Rocks et al., 2005). the basolateral plasma membrane in Madin–Darby canine kidney Intracellular palmitoylation reactions are mediated by a large (MDCK) cells (Leung et al., 1998). The endosomal function of family of more than 23 DHHC (for aspartate-histidine-histidine- SNAP25 is supported by an ARF6-dependent cycling pathway, cysteine) palmitoyl transferases; these proteins are defined by the operating between the plasma membrane and the RE and trans presence of an ~50-amino-acid cysteine-rich domain containing a Golgi network (TGN) compartments, which ensures a sufficient DHHC motif (Fukata et al., 2004; Mitchell et al., 2006; Putilina et pool of SNAP25 to support endosomal membrane fusion (Aikawa al., 1999). All DHHC proteins are predicted polytopic membrane et al., 2006b); disruption of this pathway inhibits trafficking of proteins, associating with distinct intracellular compartments, 1352 Journal of Cell Science 124 (8) including the endoplasmic reticulum (ER), Golgi and plasma Studies analysing whether SNAP25 palmitoylation is constitutively membrane (Ohno et al., 2006). As DHHC proteins are membrane- dynamic have produced conflicting results, probably owing to the associated, peripheral proteins that undergo palmitoylation require limitations of the techniques employed (Heindel et al., 2003; Kang additional primary membrane-targeting signals that mediate et al., 2004; Lane and Liu, 1997). However, there is good evidence membrane interactions before palmitoylation. Primary membrane- that SNAP25 palmitoylation is subject to dynamic regulation under targeting signals include N-myristoylation (e.g. certain G subunits some conditions, and palmitate turnover is notably enhanced and SRC family kinases) and prenylation of C-terminal CAAX following inhibition of synaptic activity in cortical neurons (Kang motifs (e.g. Hras and Nras). SNAP25 is not modified by either N- et al., 2004). myristoylation or prenylation, and we have recently shown that the Although palmitoylation plays a dominant role in defining the cysteine-rich domain of this protein is important for initial intracellular localisation of proteins such as Ras, far less is membrane interactions before palmitoylation. In particular, cysteine understood about how multiple palmitoylation of cysteine-rich hydrophobicity and the hydrophobic character of the surrounding domains coordinates the trafficking of soluble proteins such as amino acids are required for efficient membrane association SNAP25. Similarly, the potential for dynamic interplay between (Greaves et al., 2009a). Single cysteine-to-alanine mutations lead palmitoylated cysteine residues in the regulation of protein sorting to a marked reduction in SNAP25 membrane association, whereas is poorly understood (Roy et al., 2005). As regulation of SNAP25 replacement of single cysteine residues with the more hydrophobic intracellular targeting is central to its dual function at the plasma leucine residue preserves membrane binding (Greaves et al., 2009b). membrane and endosomes, we have investigated how multiple However, double cysteine replacement mutants dramatically inhibit palmitoylation impacts upon endosomal targeting and whether the membrane association of SNAP25, even when leucine is the changes in SNAP25 palmitoylation might function as a switch to replacement amino acid. In addition to cysteine residues and the regulate this sorting pathway. surrounding hydrophobic amino acids, membrane binding of SNAP25 also requires residues at the C-terminus of the minimal- Results membrane-targeting domain (in particular, Q116 and P117) The minimal palmitoylated domain regulates plasma (Gonzalo et al., 1999; Greaves et al., 2009a). The residue P117 membrane and RE and TGN targeting of SNAP25 appears to play an important role in determining the specificity of A previous study, in PC12 cells, described an intracellular pool of the interaction with DHHC proteins: in human embryonic kidney SNAP25 on REs and the TGN that cycles between these (HEK)-293 cells, mutation of this residue prevented palmitoylation compartments and the plasma membrane (Aikawa et al., 2006b). by DHHC17 but not by DHHC3. There are several ways that P117 This intracellular pool is readily observed for both endogenous might regulate the interaction with DHHC17, including by directly SNAP25 and eGFP-tagged SNAP25b (Fig. 1A). As a first step participating in protein–protein interaction, by modulating the towards defining the role of palmitoylation in regulating SNAP25 strength
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