Golgi Bypass: Skirting Around the Heart of Classical Secretion

109

Background: GLUT4 glucose transporters are trapped and sequestered intracellularly in adipocytes by TUG. Results: Insulin stimulates TUG cleavage, which separates regions of TUG that bind GLUT4 and Golgi matrix proteins. Cleavage is required for highly insulin-responsive GLUT4 translocation. Conclusion: TUG proteolysis liberates GLUT4 trapped at the Golgi matrix. Significance: Endoproteolytic cleavage is a novel biochemical mechanism for insulin action to regulate glucose uptake.

Section: Discussionsupporting

confidence: 90%

Section: Discussionmentioning

confidence: 99%

Endoproteolytic Cleavage of TUG Protein Regulates GLUT4 Glucose Transporter Translocation

Bogan

Rubin

et al. 2012

109

“…It is now well demonstrated that the secretory pathway in higher eukaryotic cells is broadly classified into a classical conventional pathway and an unconventional pathway (51). In the conventional pathway, newly synthesized proteins are transported from the ER to their final destination via Golgi, and it is usually a COPII-mediated and Rab1-dependent process, whereas in the unconventional secretory pathway, newly synthesized proteins either exit ER in a COPII-independent process or bypass Golgi to reach their final destination (52). However, the secretory pathway in the parasitic protozoa is not well characterized.…”

Section: Discussionmentioning

confidence: 99%

Functional Characterization of Monomeric GTPase Rab1 in the Secretory Pathway of Leishmania

Bahl

Parashar

Malhotra

et al. 2015

Leishmania secretes a large number of its effectors to the extracellular milieu. However, regulation of the secretory pathway in Leishmania is not well characterized. Here, we report the cloning, expression, and characterization of the Rab1 homologue from Leishmania. We have found that LdRab1 localizes in Golgi in Leishmania. To understand the role of LdRab1 in the secretory pathway of Leishmania, we have generated transgenic parasites overexpressing GFP-LdRab1:WT, GFP-LdRab1:Q67L (a GTPase-deficient dominant positive mutant of Rab1), and GFP-LdRab1:S22N (a GDP-locked dominant negative mutant of Rab1). Surprisingly, our results have shown that overexpression of GFP-LdRab1:Q67L or GFP-LdRab1:S22N does not disrupt the trafficking and localization of hemoglobin receptor in Leishmania. To determine whether the Rab1-dependent secretory pathway is conserved in parasites, we have analyzed the role of LdRab1 in the secretion of secretory acid phosphatase and Ldgp63 in Leishmania. Our results have shown that overexpression of GFP-LdRab1:Q67L or GFP-LdRab1:S22N significantly inhibits the secretion of secretory acid phosphatase by Leishmania. We have also found that overexpression of GFP-LdRab1: Q67L or GFP-LdRab1:S22N retains RFP-Ldgp63 in Golgi and blocks the secretion of Ldgp63, whereas the trafficking of RFPLdgp63 in GFP-LdRab1:WT-expressing cells is unaltered in comparison with control cells. Taken together, our results have shown that the Rab1-regulated secretory pathway is well conserved, and hemoglobin receptor trafficking follows an Rab1-independent secretory pathway in Leishmania.Small GTPases of the Rab family are master regulators of intracellular trafficking (1). These proteins are localized on specific compartments and regulate the transport through fusion between two compartments in a nucleotide-dependent process (2, 3). About 70 Rab proteins are identified in mammalian cells. In the endocytic pathway, Rab5 is present in the sorting endosome, whereas Rab4 and Rab11 are localized in recycling endosomes (4, 5). Rab7, Rab9, and Rab24 are associated with the late endosomal compartment. Rab7 mediates transport from the late endosomes to lysosomes, whereas Rab9 regulates the trafficking of lysosomal enzymes from the trans-Golgi network to lysosomes (6 -9). In the secretory pathway, Rab1 regulates the anterograde transport from the endoplasmic reticulum (ER) 3 to the Golgi (10), whereas retrograde transport from Golgi to ER is mediated by Rab2 (11). Rab6 localizes in the Golgi and is involved in intra-Golgi trafficking (12). Moreover, Rab18, Rab30, and Rab43 are found to be important for maintaining Golgi structure (13,14). In addition, transport of cargo from the Golgi to the cell surface is regulated by different Rabs. For example, Rab3 plays a role in release of neurotransmitter, Rab27 regulates the trafficking of lytic granules and melanosomes toward the plasma membrane, and Rab8 is involved in regulating the traffic from the trans-Golgi network to the plasma membrane (15). However, the secretory pathway is n...

“…Furthermore, the lack of complex glycosylation observed also suggests that these channels do not transit through the Golgi apparatus. Alternative routes to direct proteins to the plasma membrane bypassing the Golgi have been described (68). In fact, de Groot et al (51) have shown that the nonfunctional ⌬1-38 TRPV5 escaped the endoplasmic reticulum quality control, appearing at the plasma membrane without being processed by the Golgi apparatus.…”

Section: Discussionmentioning

confidence: 99%

Bidirectional Modulation of Thermal and Chemical Sensitivity of TRPM8 Channels by the Initial Region of the N-terminal Domain

Pertusa

González

Hardy

et al. 2014

Background:The functional role of the TRPM8 N terminus remains poorly understood. Results: Truncation of the first 40 residues increases TRPM8 responses to cold and menthol, whereas deletions within the 40 -60-residue region yield nonfunctional channels retained in the ER. Conclusion: Initial N terminus of TRPM8 is important for proper biogenesis and function. Significance: Variations within the N terminus can modulate thermal and chemical sensitivity of TRPM8.