The Mammalian Sec6/8 Complex Interacts with Ca2+ Signaling Complexes and Regulates Their Activity

Abstract

The localization of various Ca2+ transport and signaling proteins in secretory cells is highly restricted, resulting in polarized agonist-stimulated Ca2+ waves. In the present work, we examined the possible roles of the Sec6/8 complex or the exocyst in polarized Ca2+ signaling in pancreatic acinar cells. Immunolocalization by confocal microscopy showed that the Sec6/8 complex is excluded from tight junctions and secretory granules in these cells. The Sec6/8 complex was found in at least two cellular compartments, part of the complex showed similar, but not identical, localization with the Golgi apparatus and part of the complex associated with Ca2+ signaling proteins next to the plasma membrane at the apical pole. Accordingly, immunoprecipitation (IP) of Sec8 did not coimmunoprecipitate βCOP, Golgi 58K protein, or mannosidase II, all Golgi-resident proteins. By contrast, IP of Sec8 coimmunoprecipitates Sec6, type 3 inositol 1,4,5-trisphosphate receptors (IP3R3), and the Gβγ subunit of G proteins from pancreatic acinar cell extracts. Furthermore, the anti-Sec8 antibodies coimmunoprecipitate actin, Sec6, the plasma membrane Ca2+ pump, the G protein subunits Gαq and Gβγ, the β1 isoform of phospholipase C, and the ER resident IP3R1 from brain microsomal extracts. Antibodies against the various signaling and Ca2+ transport proteins coimmunoprecipitate Sec8 and the other signaling proteins. Dissociation of actin filaments in the immunoprecipitate had no effect on the interaction between Sec6 and Sec8, but released the actin and dissociated the interaction between the Sec6/8 complex and Ca2+ signaling proteins. Hence, the interaction between the Sec6/8 and Ca2+ signaling complexes is likely mediated by the actin cytoskeleton. The anti-Sec6 and anti-Sec8 antibodies inhibited Ca2+ signaling at a step upstream of Ca2+ release by IP3. Disruption of the actin cytoskeleton with latrunculin B in intact cells resulted in partial translocation of Sec6 and Sec8 from membranes to the cytosol and interfered with propagation of agonist-evoked Ca2+ waves. Our results suggest that the Sec6/8 complex has multiple roles in secretory cells including governing the polarized expression of Ca2+ signaling complexes and regulation of their activity.

The Exocyst Affects Protein Synthesis by Acting on the Translocation Machinery of the Endoplasmic Reticulum

“…3) (possibly, in cells with lower levels of hSec10-GFP, the specific signal of hSec10 in the ER is not detectable above the diffuse cytosolic background, which is characteristic of transfected chimeras of GFP and exocyst subunits (37)). This placed the exocyst complex in an area where it could be involved in the process of protein translocation and/or translation and was similar to the results found by Shin and colleagues (38) who co-localized the endogenous exocyst with IP 3 R3 in pancreatic acinar cells.…”

“…Sec61␤ is extensively phosphorylated, and phosphorylation increases the translocation of preprolactin (4). This may be important given that the exocyst has recently been shown to govern the polarized expression of calcium signaling complexes and regulation of their activity in the ER of pancreatic acinar cells (38).…”

Stabilization of Exocytosis by Dynamic F-actin Coating of Zymogen Granules in Pancreatic Acini

“…6C), with L 3 being significantly reduced by such pretreatment. It was possible, however, that LatA interfered with the agonist-evoked increase in [Ca 2ϩ ] i (29). To exclude this latter possibility, we triggered exocytosis by inducing a rapid and homogeneous increase in [Ca 2ϩ ] i through photolysis of NP-EGTA (Fig.…”

The Mammalian Exocyst, a Complex Required for Exocytosis, Inhibits Tubulin Polymerization

“…In tissue culture studies, inhibition of exocyst function, like in yeast, affected the secretory pathway. Perturbation of exocyst function in MDCK, 1 pancreatic acinar, and NRK cells by antiexocyst subunit sec8 antibodies decreased protein targeting to various plasma membrane domains (13)(14)(15). Likewise, in the neuronal system, disruption of the exocyst function by exocyst subunit sec10 C-terminal deletion mutant overexpression and exocyst subunit sec5 knockout inhibited neurite outgrowth in neuroendocrine PC12 cells and cultured hippocampal neurons, respectively (11,16).…”