Endoplasmic Reticulum Export of Glycosyltransferases Depends on Interaction of a Cytoplasmic Dibasic Motif with Sar1

Abstract

Membrane proteins exit the endoplasmic reticulum (ER) in COPII-transport vesicles. ER export is a selective process in which transport signals present in the cytoplasmic tail (CT) of cargo membrane proteins must be recognized by coatomer proteins for incorporation in COPII vesicles. Two classes of ER export signals have been described for type I membrane proteins, the diacidic and the dihydrophobic motifs. Both motifs participate in the Sar1-dependent binding of Sec23p-Sec24p complex to the CTs during early steps of cargo selection. However, information concerning the amino acids in the CTs that interact with Sar1 is lacking. Herein, we describe a third class of ER export motif, [RK](X)[RK], at the CT of Golgi resident glycosyltransferases that is required for these type II membrane proteins to exit the ER. The dibasic motif is located proximal to the transmembrane border, and experiments of cross-linking in microsomal membranes and of binding to immobilized peptides showed that it directly interacts with the COPII component Sar1. Sar1GTP-bound to immobilized peptides binds Sec23p. Collectively, the present data suggest that interaction of the dibasic motif with Sar1 participates in early steps of selection of Golgi resident glycosyltransferases for transport in COPII vesicles.

Specific Enzyme Complex of β-1,4-Galactosyltransferase-II and Glucuronyltransferase-P Facilitates Biosynthesis of N-linked Human Natural Killer-1 (HNK-1) Carbohydrate

“…The distribution of PST-AAA is slightly different from GlcAT-P-AAA, i.e. PST-AAA was detected not only in ER but in Golgi, suggesting that the effect of the mutation of dibasic motif on ER distribution varies by an individual glycosyltransferase as described previously (32). However, the altered distribution of PST-AAA did not perturb the Golgibased localization of ␤4GalT-I and -II-myc (Fig.…”

“…3A). Some glycosyltransferases have this motif in their cytoplasmic tail, which is required for transport from the ER to the Golgi apparatus (32). Actually, GlcAT-P lacking this motif (GlcAT-P-AAA) was retained in the ER (Fig.…”

An antibody produced in tobacco expressing a hybrid β-1,4-galactosyltransferase is essentially devoid of plant carbohydrate epitopes

“…For now, it is unclear what causes the obvious similarities between N-glycan profiles from tobacco BY2 cells expressing a truncated form of human GalT and leaves from tobacco plants expressing the xylGalT gene. The truncation removes a cytoplasmic dibasic motif that has been found to be important for COPII (coat protein complex II)-mediated ER-toGolgi export of membrane proteins, but this fact does not by itself explain the apparently medial-Golgi localization of the truncated enzyme (30,31). Although subcellular localization differences exist between the naturally occurring truncated and full-length GalT in mammalian cells, available reports on sub-Golgi compartmentalization do not support the notion of the truncated enzyme residing in the medial Golgi (32).…”

Missense mutation in theN-acetylglucosamine-1-phosphotransferase gene (GNPTA) in a patient with mucolipidosis II induces changes in the size and cellular distribution of GNPTG

“…However, the molecular mechanism determining the exit of integral proteins and retention in the Golgi apparatus is still unclear. It has been reported that a basic ER export motif, [RK](X) [RK], located proximal to the transmembrane border of type II Golgi resident glycosyltransferases is required for proper interaction with the COPII component Sar1 [Giraudo and Maccioni, 2003]. At present, however, no antibodies are available to examine the effect of the mutation on the GNPTA protein level.…”