Role of the Exocyst Complex Component Sec6/8 in Genomic Stability

Abstract

The exocyst is a heterooctomeric complex well appreciated for its role in the dynamic assembly of specialized membrane domains. Accumulating evidence indicates that this macromolecular machine also serves as a physical platform that coordinates regulatory cascades supporting biological systems such as host defense signaling, cell fate, and energy homeostasis. The isolation of multiple components of the DNA damage response (DDR) as exocyst-interacting proteins, together with the identification of Sec8 as a suppressor of the p53 response, suggested functional interactions between the exocyst and the DDR. We found that exocyst perturbation resulted in resistance to ionizing radiation (IR) and accelerated resolution of DNA damage. This occurred at the expense of genomic integrity, as enhanced recombination frequencies correlated with the accumulation of aberrant chromatid exchanges. Sec8 perturbation resulted in the accumulation of ATF2 and RNF20 and the promiscuous accumulation of DDR-associated chromatin marks and Rad51 repairosomes. Thus, the exocyst supports DNA repair fidelity by limiting the formation of repair chromatin in the absence of DNA damage. The faithful repair of DNA damage is integral to the maintenance of the genome and suppression of oncogenesis (1). This relationship has motivated intense efforts to elaborate the composition and mechanism of action of core DNA repair machinery as well as peripheral molecular systems that modulate this machinery to suppress genomic instability (2-5). With respect to the latter, emerging evidence implicates multiple regulatory layers that link activation of the DNA damage response (DDR), repair pathway choice, and resolution of the DDR to chromatin organization (6-9), RNA metabolism (10, 11), and autophagy (12)(13)(14). By extrapolation, the coordinated response of cellular processes to DNA damage is necessary for efficient DNA repair, and perturbations of these pathways can lead to genomic instability and development of neoplastic disease.The exocyst (also known as the Sec6/8 complex) is a conserved heterooctomeric protein complex, which includes Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo84, and Exo70. The holocomplex is well appreciated for its role in the dynamic trafficking of secretory vesicles to specialized membrane domains such as the basolateral membrane of polarized epithelial cells (15) and abscission planes in dividing cells (16) and to lamellipodia and growth cones of migrating cells and differentiating neurons (17, 18). Accumulating evidence indicates that exocyst subcomplexes, and their regulation by Ras and Rho family GTPases, also selectively participate in the assembly and activation of signal transduction events that mediate host defense, autophagy, cell growth, and oncogene signaling (19-23). An overarching implication is that the exocyst and its subcomplexes serve as physical platforms that coordinate organellar assembly with the activation of attendant regulatory cascades required for the execution of distinct cell biological programs.Here, we ...

Exocyst Dynamics During Vesicle Tethering and Fusion

“…Low but significant GFP fluorescence above the autofluorescence background was detectable by confocal microscopy of live cells for each of the 5 single knock-in lines ( Figure 1D). Sec5, Sec6, Sec8 and Exo70 were enriched at intercellular junctions, consistent with data on fixed and immunostained epithelial cells, but the knock-in cells also have substantial diffuse cytoplasmic pools, and no detectable nuclear signal, contrary to some other reports (Torres et al, 2015). Sec3 was anomalous in showing a predominantly cytoplasmic distribution with no significant accumulation at junctions.…”

The Exocyst Complex in Health and Disease

“…Sec8 has been shown to interact with 33 DDR and chromatin modifying proteins and to modulate the response to DNA damage. Ablation of Sec8 leads to an increase in the frequency of DNA repair but generates chromosomal aberrations (Torres et al, 2015 ). Therefore, paradoxically, the absence of Sec8 induces genomic instability.…”

“…Sec8 may participate in the cellular response to DNA damage by restraining the spacio-temporal recruitment of chromatin remodeling proteins in the absence of appropriate DDR signaling, thus promoting a faithful repair. Depletion of Sec8 also confers radioresistance, which has implications for the treatment by radiotherapy of colorectal cancers, in which Sec8 is deleted (Ashktorab et al, 2010 ; Torres et al, 2015 ). The G 1 /S checkpoint is another mechanism by which propagation of damaged DNA can be prevented.…”

Diverse Functions and Signal Transduction of the Exocyst Complex in Tumor Cells

“…Depletion of Sec8 sensitizes U2OS cells to DNA break formation or detection in a manner that also alters the persistence of the DNA damage response or resolution of the breaks themselves. Thus, disruption of Sec8 results in the accumulation of ATF2 and RNF20 and the anomalous accumulation of DDR‐associated chromatin marks and Rad51 repairosomes (Torres et al, ).…”