Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Rab27

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101791

Synonyms

 Ash;  c25KG;  Rab27a;  Rab27A;  Rab27b;  Rab27B;  Ram

Historical Background

Rab27 is a member of the Rab family small GTPases, which constitute the largest family of membrane trafficking proteins in all eukaryotes. Rab27 is widely conserved in metazoans, including Caenorhabditis elegans (nematode), Drosophila melanogaster (fruit fly), and Loligo pealei (long-finned squid), and is conserved in all species of vertebrates, but it is not found in yeasts or plants (Diekmann et al. 2011). Like other small GTPases, Rab27 functions as a switch molecule by cycling between a GTP-bound active state and a GDP-bound inactive state (Fig. 1). In its GTP-bound active state, Rab27 has been shown to regulate a variety of secretory pathways by recruiting specific effector molecules (Fukuda 2013). Although invertebrates appear to contain a single Rab27 isoform, vertebrates contain two Rab27 isoforms, Rab27A and Rab27B, which exhibit more than 70% identity at the amino acid level. RAB27A is the first RAB gene whose mutations have been shown to cause a human disease. Functional disruption of RAB27A is known to cause type 2 Griscelli syndrome (GS2) and its corresponding mouse model, ashen, both of which are characterized by pigment dilution in the hair and skin, due to a defect in actin-based melanosome transport, and immunodeficiency, due to a defect in cytotoxic T lymphocyte (CTL) lytic granule exocytosis (Van Gele et al. 2009). Because Rab27A and Rab27B (Rab27A/B) are often expressed in the same secretory cells (and on the same secretory vesicles), Rab27A/B were thought to function redundantly in the same secretory pathways; however, recent evidence indicates that they regulate different steps of certain secretory pathways through interactions with different Rab27 effectors in some cell types (Fukuda 2013).
Rab27, Fig. 1

A schematic model of spatiotemporal regulation of Rab27 GTPase activity. Prior to membrane recruitment, Rab27 undergoes geranylgeranylation at its two C-terminal cysteine residues by Rab-GGT together with REP. The geranylgeranylated Rab27 functions as a switch molecule by cycling between two nucleotide-bound states, a GTP-bound active state and a GDP-bound inactive state. The Rab27 GTP/GDP cycling is regulated by two regulatory enzymes, a Rab27-GEF DENN (also called MADD or Rab3GEP) and Rab27-GAPs EPI64A/B (also called TBC1D10A/B) (Figueiredo et al. 2008; Itoh and Fukuda 2006). The active GTP-Rab27 localizes to specific vesicles or organelles by largely unknown mechanisms, and by recruiting its specific effector, it promotes their intracellular trafficking (Fukuda 2013). After the vesicle/organelle trafficking, Rab27 is inactivated by Rab27-GAPs, and the resulting inactive GDP-Rab27 is released from the membrane and is present in the cytosol with the assistance of GDI (Imai et al. 2011)

Regulators of Rab27

Rab27 is intrinsically soluble and requires geranylgeranylation at two C-terminal cysteine residues for membrane association. Rab geranylgeranyl transferase (Rab-GGT) together with Rab escort protein (REP) mediates the geranylgeranylation of Rab27, and REP presumably then facilitates delivery of the geranylated Rab27 to the cytosolic surface of specific organelles or vesicles (Fig. 1). The geranylated Rab27 cycles between two nucleotide-bound states, a GTP-bound active state and a GDP-bound inactive state, and the cycling are controlled by two regulatory enzymes, a guanine nucleotide exchange factor for Rab27 (Rab27-GEF) and a GTPase-activating protein for Rab27 (Rab27-GAP). One Rab27-GEF, named DENN (also called MADD or Rab3GEP), and two Rab27-GAPs, named EPI64A and EPI64B (also called TBC1D10A and TBC1D10B, respectively), have been reported thus far. DENN was initially identified as a Rab3-GEF in rat brain that plays an important function in neurotransmitter release, and it was subsequently shown to function as a Rab27-GEF in epidermal melanocytes (Figueiredo et al. 2008). Similarly, the Caenorhabditis elegans DENN orthologue AEX-3 regulates both RAB-3 and RAB-27 in neurotransmitter release (Mahoney et al. 2006). EPI64A/B contain a Tre-2/Bub2/Cdc16 (TBC) domain, which is known to be a key domain responsible for Rab-GAP activity (Fukuda 2011). Both EPI64A and EPI64B exhibit Rab27-GAP activity in vitro, and their forced expression in epidermal melanocytes causes perinuclear melanosome aggregation (Itoh and Fukuda 2006). However, EPI64A/B are unlikely to be Rab27-specific GAPs, because they have been shown to exhibit broader substrate specificity in vitro (e.g., they also exhibit Rab35-GAP activity). In its GTP-bound active state, Rab27 recruits specific effectors that regulate various secretory pathways (summarized in Table 1) (Fukuda 2013). After the vesicle/organelle trafficking, Rab27 is inactivated by Rab27-GAPs, and the resulting inactivated GDP-Rab27 is released from the membrane and retained in the cytosol with the help of GDP dissociation inhibitor (GDI) until the next round of trafficking (Fig. 1) (Imai et al. 2011). Before extraction of GDP-Rab27 from the membrane by GDI, GDP-Rab27 has been shown to promote endocytosis of the secretory membrane in pancreatic β-cells through interaction with coronin 3 and IQGAP1 (Yamaoka et al. 2015).
Rab27, Table 1

Rab27 effectors

Name

Synonyms

Other binding partners

Representative functions

Slp family

   

Slp1

JFC1 Exophilin-7

PIP3, GMIP (Rho-GAP), Rap1GAP2, CRMP-2, TrkB

Vesicle exocytosis by certain secretory cells (e.g., neutrophils)

Slp2-a

Exophilin-4

PS, PIP2, PP1β, Rap1GAP2

Melanosome anchoring, vesicle exocytosis by certain secretory cells (e.g., gastric surface mucous cells), polarized trafficking of apical proteins in epithelial cells, and epithelial cell size control

Slp3-a

Exophilin-6

PS/Ca2+, kinesin-1

Lytic granule exocytosis by CTLs

Slp4-a/b

Granuphilin-a/b

Exophilin-2

Munc18–1/2, syntaxin-1a/2/3, myosin-Va, Rab3/8

Vesicle exocytosis by certain secretory cells (e.g., pancreatic β-cells) and exosome secretion by nonsecretory cells

Slp5

Exophilin-9

PS/Ca2+

Chemokine-induced cell migration

Rabphilin

Rabphilin-3A

Exophilin-1

PS/Ca2+, PIP2, SNAP25, myosin-Va, Rab3/8

Vesicle exocytosis by certain secretory cells (e.g., neurons and some endocrine cells)

Slac2 family

   

Slac2-a

Melanophilin

Exophilin-3

Myosin-Va, actin, EB1

Melanosome transport in epidermal melanocytes and granule exocytosis by BMMCs

Slac2-b

Exophilin-5

 

Exosome secretion by nonsecretory cells

Slac2-c

MyRIP exophilin-8

Myosin-Va/Vlla, actin, PKA, Sec6/8

Melanosome transport in RPEs and vesicle exocytosis by certain secretory cells (e.g., pancreatic β-cells)

Noc2

Rabphilin 3A–like

Munc13–1, Rab3/8

Vesicle exocytosis by certain secretory cells (e.g., pancreatic β-cells)

Others

   

Munc13–4

Unc13d

PS/Ca2+, Doc2α, SNAREs, Rab11/15/37

Vesicle exocytosis by certain secretory cells (e.g., CTLs, neutrophils, and platelets)

Coronin 3

Coronin 1C

Actin

Endocytosis of the secretory membrane in pancreatic β-cells

IQGAP1

p195

Cdc42

Endocytosis of the secretory membrane in pancreatic β-cells

Binding partners, other than Rab27, and representative functions of all Rab27 effectors identified thus far are listed. The functions and features of each Rab27 effector are described in greater detail in Fukuda (2013). PS/Ca2+ refers to calcium-dependent PS binding. PIP3, phosphatidylinositol 3,4,5-trisphosphate; PIP2, phosphatidylinositol 4,5-bisphosphate; PKA, protein kinase A

Roles of Rab27 in Secretory Pathways

Rab27 is now known to regulate various secretory pathways in both secretory cells and nonsecretory cells. Historically, studies on Rab27 have mainly been conducted in epidermal melanocytes and several secretory cells, because mutations in the RAB27A gene cause pigment dilution in the hair and skin (i.e., a defect in melanosome transport in epidermal melanocytes) and immunodeficiency (i.e., a defect in secretion by CTLs) in GS2 patients and its corresponding mouse model, ashen, one of the well-known mouse coat-color mutants (see next section for details) (Van Gele et al. 2009). Rab27A regulates two different steps of melanosome transport through its interaction with two different Rab27A effectors (Fig. 2a) (Kuroda and Fukuda 2004). Rab27A firstly mediates actin-based melanosome transport together with Slac2-a (also called melanophilin) and an actin-based motor myosin-Va, and it then promotes docking of melanosomes to the plasma membrane together with Slp2-a, which interacts with phosphatidylserine (PS) via its C2A domain. Interestingly, mutations in the genes encoding Slac2-a and myosin-Va also cause type 3 GS (GS3; mouse model, leaden) and type 1 GS (GS1; mouse model, dilute), respectively. A similar protein complex composed of Rab27A, Slac2-c (also called MyRIP), and myosin-VIIa regulates melanosome transport in retinal pigment epithelial (RPE) cells. Moreover, Rab27A regulates the transport of other lysosome-related organelles (LROs) besides melanosome, e.g., lytic granule secretion by CTLs. During lytic granule exocytosis, Rab27A mediates terminal transport through interaction with Slp3-a and a microtubule-based motor kinesin-1 (Kurowska et al. 2012), and it then promotes tethering of granules to the plasma membrane through interaction with another Rab27A effector Munc13-4 (Elstak et al. 2011). An association between Rab27B and a motor protein has also been reported: Rab27B together with Slp1 and kinesin-1 regulates anterograde transport of TrkB in axons (Arimura et al. 2009).
Rab27, Fig. 2

Two representative examples of Rab27-mediated membrane trafficking. (a) A schematic model of Rab27A-mediated melanosome transport in epidermal melanocytes. Rab27A regulates two different steps of melanosome transport through interaction with two different Rab27A effectors (Kuroda and Fukuda 2004). Rab27A firstly mediates actin-based melanosome transport together with Slac2-a and an actin-based motor myosin-Va, and it then promotes docking of melanosomes to the plasma membrane together with Slp2-a, which interacts with PS via the C2A domain. (b) A schematic model of Rab27-dependent exosome secretion by nonsecretory cells. Rab27A and Rab27B play different roles in the exosome secretion pathway through interactions with different effector molecules (Ostrowski et al. 2010). Through its interaction with Slac2-b, Rab27B regulates the long-range movements of MVEs from the perinuclear region to the cell periphery, whereas Rab27A regulates docking of MVEs to the plasma membrane through its interaction with Slp4 downstream of Rab27B–Slac2-b

Because Rab27A and Rab27B exhibit more than 70% identity at the amino acid level and are often expressed in the same secretory cells, Rab27A/B were thought to function redundantly in the same secretory pathways; however, recent studies indicated that Rab27A/B play different roles in certain secretory pathways. For example, Rab27A deficiency and Rab27B deficiency in bone marrow-derived mast cells (BMMCs) cause different phenotypes, i.e., hypersecretion and secretory impairment, respectively, during granule exocytosis (Singh et al. 2013). Further analysis of the secretory phenotype of BMMCs from mice in which one of several Rab27 effector proteins has been knocked out has revealed that knockout of either Slac2-a or myosin-Va phenocopies Rab27A deficiency, whereas Munc13-4 knockout phenocopies Rab27B deficiency. Thus, Rab27A and Rab27B are likely to regulate different steps in BMMC granule exocytosis through interactions with different effector molecules: the Rab27A–Slac2-a–myosin-Va complex regulates F-actin stability and the distribution of granules near the plasma membrane upstream of the Rab27B–Munc13-4-dependent granule docking step. Similar different phenotypes in Rab27A deficiency and Rab27B deficiency have also been observed in regard to neutrophil vesicle exocytosis (Johnson et al. 2010).

In addition to the different roles of the two Rab27 isoforms in secretory cells, considerable attention has recently been directed toward the role of Rab27 in “nonsecretory” cells. A typical secretion event in nonsecretory cells is exosome secretion. The same as in secretion by neutrophils and by BMMCs described above, Rab27A and Rab27B play different roles in the exosome secretion pathway through interactions with different effectors (Fig. 2b) (Ostrowski et al. 2010). Through its interaction with Slac2-b, Rab27B regulates the long-range movements of multivesicular endosomes (MVEs) from the perinuclear region to the cell periphery, whereas Rab27A regulates docking of MVEs to the plasma membrane through its interaction with Slp4 downstream of Rab27B–Slac2-b. Another important example of Rab27-mediated membrane trafficking in nonsecretory cells is polarized trafficking in epithelial cells. Epithelial cells have two morphologically and functionally distinct membrane domains, an apical domain facing the lumen and a basolateral domain facing the extracellular matrix or neighboring cells, and proteins and lipids that function in each domain are differently transported by specialized membrane trafficking, called polarized trafficking. In polarized Madin-Darby canine kidney (MDCK) II cells (a model epithelial cell line from canine kidney), Rab27A and its effector Slp2-a regulate apical trafficking of vesicles containing an apical signaling molecule, podocalyxin (Gálvez-Santisteban et al. 2012). In addition to functioning as a Rab27 effector during apical trafficking of podocalyxin, Slp2-a has Rab27-independent functions in MDCK II cells: it recruits Rap1GAP2 via the C2A domain and protein phosphatase 1β (PP1β) via the linker domain to the plasma membrane, and it controls cell size through regulation of ezrin activity (Yasuda and Fukuda 2014).

Associations with Human Diseases and Their Animal Models

RAB27A is the first RAB gene whose mutations have been shown to cause a human disease. Functional disruption of RAB27A is known to cause GS2 and its corresponding mouse model, ashen, both of which are characterized by pigment dilution in the hair and skin, due to a defect in actin-based melanosome transport, and immunodeficiency, due to a defect in CTL lytic granule exocytosis. The same pigment dilution phenotype is also seen in GS1 patients (myosin-Va deficiency; mouse model: dilute) and GS3 patients (Slac2-a deficiency; mouse model: leaden) as described above, although neither group of patients exhibits immunodeficiency (Van Gele et al. 2009), and GS1 patients instead exhibit neurological disorders in addition to pigment dilution. Since C. elegans and D. melanogaster do not contain melanocytes or CTLs, mutants lacking Rab27 exhibit neither pigment dilution nor immunodeficiency. Interestingly, invertebrate Rab27 is enriched on synaptic vesicles in certain types of neurons and has been shown to regulate neurotransmitter release, and absence of Rab27 in C. elegans and D. melanogaster causes defects in neurotransmitter release that result in a defecation defect and a sleep phenotype, respectively.

Dysfunctions of several Rab27 effectors are also known to cause secretory abnormalities in certain secretory cells (Fukuda 2013). For example, Slp2-a knockout mice are characterized by decreased mucus secretion by gastric surface mucous cells (Saegusa et al. 2006), and Slp4 knockout mice are characterized by increased insulin secretion by pancreatic β-cells (Gomi et al. 2005). In addition, mutations in the SLAC2-B gene cause inherited skin fragility (McGrath et al. 2012), although it is unclear whether Rab27 dysfunction is associated with this disease. Since Rab27 is involved in exosome secretion, increased Rab27 activity in cancer cells may be related to their malignancy.

Summary

Rab27 is a member of the Rab family small GTPases, which constitute the largest family of known membrane trafficking proteins in all eukaryotes. Rab27 is an important regulator of secretory pathways in various cell types, including classical secretory cells and nonsecretory cells. In vertebrates, there are two Rab27 isoforms, Rab27A and Rab27B, which exhibit more than 70% identity at the amino acid level. Although Rab27A and Rab27B were previously thought to regulate the same secretory pathway redundantly, recent evidence indicates that Rab27A/B play different roles in certain secretory pathways through interactions with different Rab27 effectors. Because of the importance of Rab27 in secretory pathways, functional disruption of Rab27 in humans, mice, and certain invertebrates causes abnormalities that are associated with secretion (or transport) defects. A representative example is mutations in RAB27A gene, which cause GS2 and its corresponding mouse model ashen, both of which are characterized by pigment dilution in the hair and skin (i.e., defect in melanosome transport), and immunodeficiency (i.e., defect in lytic granule exocytosis by CTLs).

References

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Laboratory of Membrane Trafficking Mechanisms, Department of Developmental Biology and Neurosciences, Graduate School of Life SciencesTohoku UniversitySendaiJapan