Rab proteins and the secretory pathway: the case of rab18 in neuroendocrine cells

Abstract

The secretory pathway is a process characteristic of cells specialized in secretion such as endocrine cells and neurons. It consists of different stages that are dependent on specific transport of proteins in vesicular-tubular carriers. Biochemical analyses have unveiled a number of protein families that confer identity to carrier vesicles and specificity to their transport. Among them is the family of Rab proteins, Ras-like small GTPases that anchor to the surface of transport vesicles and participate in vesicle formation from the donor compartment, transport along cytoskeletal tracks, and docking and fusion with the acceptor compartment. All of these functions are accomplished through the recruitment of effector proteins, such as sorting adaptors, tethering factors, kinases, phosphatases, and motors. The numerous Rab proteins have distinct subcellular distributions throughout the endomembrane system, which ensures efficient cargo transfer. Rab proteins act as molecular switches that alternate between a cytosolic GDP-bound, inactive form and a membrane-associated GTP-bound, active conformation. Cycling between inactive and active states is a highly regulated process that enables Rabs to confer spatio-temporal precision to the different stages through which a vesicle passes during its lifespan. This review focuses on our current knowledge on Rab functioning, from their structural features to the multiple regulatory proteins and effectors that control Rab activity and translate Rab function. Furthermore, we also summarize the information available on a particular Rab protein, Rab18, which has been linked to the control of secretory granule traffic in neuroendocrine cells.

Keywords: Rab proteins; Rab18; neuroendocrine cells; secretory pathway; vesicle traffic.

Figure 1

Different stages of vesicle budding from the donor compartment and fusion with the acceptor compartment. (1) Vesicle formation and initiation of coat and adaptor proteins assembly. Specific Rab proteins interact with tethering factors anchored to the donor membrane to form multi-subunit Rab tethers that address coat proteins such as COPI, COPII and clathrin, and adaptor proteins that determine the specificity of cargo to the surface of the budding vesicle. (2) Uncoating and specific transport of carrier vesicles along cytoskeletal tracks. Newly formed vesicles lose their coat by inactivation of particular Rab GTPases and activation of uncoating enzymes. Motor protein complexes recognize and recruit uncoated vesicles to cytoskeletal tracks to transport the cargo to the corresponding acceptor compartment. These motor protein complexes include Rab proteins that modulate the processivity and direction of the motor protein movement. During transport, specific v-SNARE complexes are added to the surface of vesicles, thus conferring specificity to their fusion with the corresponding target compartment. (3) Tethering of vesicles to the acceptor compartment. Vesicles in the proximity of the target compartment become tethered to its membrane in a process driven by a Rab protein/tether factor complex. (4) Docking of vesicles to the acceptor compartment. v-SNARE and t-SNAREs assemble into a four-helix bundle and vesicles are approached to the target membrane. (5) Membrane fusion and release of cargo into the target compartment. Trans-SNARE complexes promote fusion of the vesicle and acceptor lipid bilayers. Cargo is transferred to the acceptor compartment, and the SNAREs are recycled. Rab proteins are inactivated and released into the cytosol, wherein they remain until a new cycle of activation is initiated.

Figure 2

Theoretical model of the contribution of Rab18 to the hypersecretory state of somatotropinoma cells in acromegalic patients. In physiological conditions (left panel), somatotrope cells express normal levels of Rab18, which retains a number of GH-containing secretory granules from being released upon stimulatory inputs, thus ensuring the maintenance of the secretory response of cells within non-pathological limits. In acromegalic patients (right panel), somatotropinoma cells express lower levels of Rab18, which would cause uncontrolled secretory granule transport toward the plasma membrane and, consequently, an abnormally enhanced secretory response.