Regulating the regulators: role of phosphorylation in modulating the function of the GBF1/BIG family of Sec7 ARF‐GEFs

Abstract

Membrane traffic between secretory and endosomal compartments is vesiclemediated and must be tightly balanced to maintain a physiological compartment size. Vesicle formation is initiated by guanine nucleotide exchange factors (GEFs) that activate the ARF family of small GTPases. Regulatory mechanisms, including reversible phosphorylation, allow ARF-GEFs to support vesicle formation only at the right time and place in response to cellular needs. Here, we review current knowledge of how the Golgi-specific brefeldin A-resistance factor 1 (GBF1)/brefeldin A-inhibited guanine nucleotide exchange protein (BIG) family of ARF-GEFs is influenced by phosphorylation and use predictive paradigms to propose new regulatory paradigms. We describe a conserved cluster of phosphorylation sites within the N-terminal domains of the GBF1/BIG ARF-GEFs and suggest that these sites may respond to homeostatic signals related to cell growth and division. In the C-terminal region, GBF1 shows phosphorylation sites clustered differently as compared with the similar configuration found in both BIG1 and BIG2. Despite this similarity, BIG1 and BIG2 phosphorylation patterns are divergent in other domains. The different clustering of phosphorylation sites suggests that the nonconserved sites may represent distinct regulatory nodes and specify the function of GBF1, BIG1, and BIG2.

Biogenesis of specialized lysosomes in differentiated keratinocytes relies on close apposition with the Golgi apparatus

“…The ubiquitous function of Arf1 is mediated by its activation through specific GEFs (Guanine exchange factors). For instance, GBF1 (Golgi-specific brefeldin A-resistance guanine nucleotide exchange factor 1) localizes to the cis -medial Golgi and regulates COPI vesicle assembly through Arf1 (Garcia-Mata et al, 2003; Kawamoto et al, 2002; Manolea et al, 2008); BIG1 and BIG2 (Brefeldin A-inhibited guanine nucleotide exchange proteins) localize to the late Golgi and regulate the membrane traffic between TGN and endosomes (Ishizaki et al, 2008; Manolea et al, 2008; Walton et al, 2020). Hence, Arf1 function can be selectively inhibited by specific inhibition of Arf1-GEFs (Fig.…”

Sec7 regulatory domains scaffold autoinhibited and active conformations

“…In light of our new findings, several observations reconcile the previous interpretations and provide additional support for our structural model of Sec7 autoregulation. Constructs used previously to test the function of the HDS1 domain (13, 54, 55) considered the GEF-HDS1 linker to be part of the HDS1 domain. We now know that these linker residues serve to position the GEF domain relative to the DCB-HUS domain in both the Gea2 cryo-EM structure(19) and the predicted active conformation of Sec7 (SI Appendix, Fig.…”

2,4-Diacetylphloroglucinol Reduces Beta-Amyloid Production and Secretion by Regulating ADAM10 and Intracellular Trafficking in Cellular and Animal Models of Alzheimer’s Disease

“…These differential effects of ZCL-278 and BFA indicated that DAPG-mediated changes of sAPPα and Aβ levels were at least partly independent. BFA inhibits ADP-ribosylation factors–guanine–nucleotide–exchange factor (ARF-GEFs) including Golgi-specific brefeldin-resistance factor a (GBF1) and BFA-inhibited guanine nucleotide exchange protein 1 and 2 (BIG1 and BIG2) [ 39 , 40 , 41 ]. Compared with BFA, GCA is a relatively specific inhibitor of GBF1 but not BIG1/2 [ 33 ].…”

“…This differential localization likely results in the distinct roles of the ARF-GEFs and their inhibitors [ 33 , 40 ]. Furthermore, BIG2 is also associated with recycling endosomes, an organelle other than the Golgi complex [ 39 , 41 ], and BFA also disrupts sorting pathways in the endosomal system [ 39 , 42 ]. These reports suggest that BFA effects in DAPG-treated 293sw cells depended on BIG1, BIG2, or endosomal trafficking, but not on GBF1 or the cis-Golgi compartment.…”