In vivo Roles of Rab27 and Its Effectors in Exocytosis

Abstract

The monomeric GTPase Rab27 regulates exocytosis of a broad range of vesicles in multicellular organisms. Several effectors bind GTP-bound Rab27a and/or Rab27b on secretory vesicles to execute a series of exocytic steps, such as vesicle maturation, movement along microtubules, anchoring within the peripheral F-actin network, and tethering to the plasma membrane, via interactions with specific proteins and membrane lipids in a local milieu. Although Rab27 effectors generally promote exocytosis, they can also temporarily restrict it when they are involved in the rate-limiting step. Genetic alterations in Rab27-related molecules cause discrete diseases manifesting pigment dilution and immunodeficiency, and can also affect common diseases such as diabetes and cancer in complex ways. Although the function and mechanism of action of these 2 effectors have been explored, it is unclear how multiple effectors act in coordination within a cell to regulate the secretory process as a whole. It seems that Rab27 and various effectors constitutively reside on individual vesicles to perform consecutive exocytic steps. The present review describes the unique properties and in vivo roles of the Rab27 system, and the functional relationship among different effectors coexpressed in single cells, with pancreatic beta cells used as an example.

Functional hierarchy among different Rab27 effectors involved in secretory granule exocytosis

“…We further show that exophilin-8 and melanophilin associates via different subcomplex components, SEC8 and EXO70, respectively, and that disruption of one subcomplex results in dissociation between the two effectors. To our knowledge, this is the first example showing that different effectors toward the same Rab form a complex in cells, which corroborates the previous suggestion that the continuous presence of GTP-bound Rab27 and multiple effectors on the same granules may smooth the transition between consecutive intermediate exocytic processes (Izumi, 2021). In yeast, the exocyst interacts with Sec4, a member of the Rab protein on secretory vesicles, via exocyst component Sec15 (Guo et al, 1999).…”

“…In monolayer mouse pancreatic beta cells, insulin granules were unevenly distributed with accumulation in the actin cortex (Figure 1—figure supplement 1A). Because the Rab27 effectors, melanophilin and exophilin-8, are known to show affinities to actin motors, myosin-Va and/or - VIIa (Hammer and Sellers, 2012; Izumi, 2021), these effectors are expected to function on granules within this peripheral actin network. In fact, they showed a similar uneven distribution and were colocalized especially at the cell periphery (Figure 1—figure supplement 1B).…”

“…However, this scenario may be too simplified, considering that there is a gating system to prevent spontaneous or unlimited vesicle fusion in regulated exocytosis. In fact, multiple Rab27 effectors that are involved in intracellular granule trafficking show complex and differential effects on exocytosis (Izumi, 2021). For example, granuphilin (also known as exophilin-2 and Slp4) mediates stable granule docking to the plasma membrane but simultaneously prevents their spontaneous fusion by interacting with a fusion-incompetent, closed form of syntaxins (Gomi et al, 2005; Torii et al, 2002).…”

Functional hierarchy among different Rab27 effectors involved in secretory granule exocytosis

“…However, this may be an oversimplification, considering that there is a gating system to prevent spontaneous or unlimited vesicle fusion in regulated exocytosis. In fact, multiple Rab27 effectors that are involved in intracellular granule trafficking show complex and differential effects on exocytosis ( Izumi, 2021 ). For example, granuphilin (also known as exophilin-2 and Slp4) mediates stable granule docking to the plasma membrane but simultaneously prevents their spontaneous fusion by interacting with a fusion-incompetent, closed form of syntaxins ( Gomi et al, 2005 ; Torii et al, 2002 ).…”

Munc13b stimulus-dependently accumulates on granuphilin-mediated, docked granules prior to fusion

“…This is because it specifically interacts with and stabilizes the fusion-incompetent, closed form of syntaxins 1~3 (Fukuda et al, 2005;Torii et al, 2002;Wang et al, 2011), which Cell Structure and Function 47: 31-41 (2022) http://doi.org/10.1247/csf.22005 alone cannot form a complex with other SNARE proteins to mediate fusion reactions. Thus, granuphilin likely serves a gating function by preventing spontaneous fusion in a resting state and unlimited fusion in an evoked state by stalling incoming granules in the docking state (Izumi, 2021). This control is essential for regulated exocytosis, in which bioactive substances are released only in the presence of secretagogues.…”