The Sar1 Gtpase Coordinates Biosynthetic Cargo Selection with Endoplasmic Reticulum Export Site Assembly

Abstract

Cargo selection and export from the endoplasmic reticulum is mediated by the COPII coat machinery that includes the small GTPase Sar1 and the Sec23/24 and Sec13/31 complexes. We have analyzed the sequential events regulated by purified Sar1 and COPII coat complexes during synchronized export of cargo from the ER in vitro. We find that activation of Sar1 alone, in the absence of other cytosolic components, leads to the formation of ER-derived tubular domains that resemble ER transitional elements that initiate cargo selection. These Sar1-generated tubular domains were shown to be transient, functional intermediates in ER to Golgi transport in vitro. By following cargo export in live cells, we show that ER export in vivo is also characterized by the formation of dynamic tubular structures. Our results demonstrate an unanticipated and novel role for Sar1 in linking cargo selection with ER morphogenesis through the generation of transitional tubular ER export sites.

Biosynthesis and secretion of mouse cysteine‐rich with EGF‐like domains 2

“…Overexpression of Sar1 (H79G), which is the GTP-bound mutant and constitutive active form, dampens the export of cargo from ER [18,23]. In this study, co-transfection of Sar1 (H79G) with wtCRELD2 or CRELD2-MH markedly reduced the CRELD2 secretion and increased the accumulation of intracellular CRELD2 (Fig.…”

“…Each type of Myc/His-tagged mouse CRELD2 gene was amplified by PCR and then cloned into the pcDNA3.1/myc-His vector (FL-, D1 (1-287 aa)-and D2 GRP78 (wtGRP78) and GRP78 lacking four C-terminal amino acids (KDEL) (DGRP78) genes were also amplified by PCR using mouse GRP78 cDNA and cloned into the pcDNA3.1 vector. HA-tagged Sar1 construct (H79G) were kindly provided by Dr. Wei Liu and Dr. Jennifer Lippincott-Schwartz [18].…”

Sterols block binding of COPII proteins to SCAP, thereby controlling SCAP sorting in ER

“…Extensive characterization of ER export signals in transmembrane secretory proteins has failed to identify a general consensus sequence necessary for COPII binding and efficient ER export. The two best-characterized motifs for COPII binding contain a diacidic (DxE) or diphenylalanine sequence located in short cytoplasmic domains at the COOH terminus of membrane proteins, such as VSVG, ERGIC-53, and the p24 family of proteins (21)(22)(23). In contrast, the large cytosolic COOH terminus of SCAP forms a complex with SREBP and bears little structural similarity to these proteins.…”

H-Ras Does Not Need COP I- or COP II-dependent Vesicular Transport to Reach the Plasma Membrane

“…Formation of vesicles on the ER begins with assembly of a COP II complex of proteins around the site, termed an ER exit site (ERES), at which the newly cargo-loaded vesicle will form (23)(24)(25). The protein that initiates this event is the GTPase, Sar1.…”