Abstract
Mouse GnT1IP-L, and membrane-bound GnT1IP-S (MGAT4D) expressed in cultured cells
inhibit MGAT1, the N-acetylglucosaminyltransferase that initiates the synthesis of
hybrid and complex N-glycans. However, it is not known where in the secretory pathway
GnT1IP-L inhibits MGAT1, nor whether GnT1IP-L inhibits other N-glycan branching
N-acetylglucosaminyltransferases of the medial Golgi. We show here that the luminal
domain of GnT1IP-L contains its inhibitory activity. Retention of GnT1IP-L in the
endoplasmic reticulum (ER) via the N-terminal region of human invariant chain p33,
with or without C-terminal KDEL, markedly reduced inhibitory activity. Dynamic
fluorescent resonance energy transfer (FRET) and bimolecular fluorescence
complementation (BiFC) assays revealed homomeric interactions for GnT1IP-L in the ER,
and heteromeric interactions with MGAT1 in the Golgi. GnT1IP-L did not generate a
FRET signal with MGAT2, MGAT3, MGAT4B or MGAT5 medial Golgi GlcNAc-tranferases.
GnT1IP/Mgat4d transcripts are expressed predominantly in
spermatocytes and spermatids in mouse, and are reduced in men with impaired
spermatogenesis.
DOI:http://dx.doi.org/10.7554/eLife.08916.001
Proteins are made up of chains of amino acids that fold into three-dimensional shapes
and many are assembled in a cell compartment known as the endoplasmic reticulum. From
here, these new proteins move to another compartment called the Golgi, where they may
be further modified before they are transported to their final destination in the
cell.
One way that proteins may be modified is known as glycosylation, in which sugar
molecules are attached to specific amino acids. Some sugar molecules can act as
labels that ensure the new proteins are transported to the correct destination in the
cell. For proteins that are delivered to the surface of the cell, the sugar molecules
can also play important roles in communication with other cells.
A simple sugar molecule, or a complex arrangement of many sugar molecules, may be
attached to an amino acid by glycosylation. An enzyme called MGAT1 controls the
synthesis of sugars called complex N-glycans in the Golgi. In 2010, researchers
reported that a glycoprotein called GnT1IP-L binds to MGAT1 and inhibits its
activity, thereby blocking the production of complex N-glycans. GnT1IP-L was found in
the endoplasmic reticulum and Golgi, but it was not clear how it inhibits MGAT1.
Huang et al.—including some of the researchers from the 2010 study—have
now investigated the activity of GnT1IP-L in cells grown in the laboratory using
several biochemical techniques. The experiments show that GnT1IP-L only binds to
MGAT1 when both proteins are in the Golgi. There are three sections (or
‘domains’) in GnT1IP-L, but Huang et al. found that only the domain
that is on the inside of the Golgi is involved in this interaction.
Previous work indicated that GnT1IP-L may be involved in the formation of sperm in
mice. Huang et al. have now analyzed previously published data on samples of testis
tissue from human patients and found that the gene that encodes GnT1IP-L is present
in very low amounts in patients whose sperm do not develop properly.
Huang et al.'s findings suggest that GnT1IP-L may inhibit MGAT1 to control the
glycosylation of proteins in the Golgi of developing sperm. The next step is to test
this hypothesis by generating mutant mice that lack GnT1IP-L, or to make GnT1P-L in
other cells in which it is not normally made, to find out if this affects the
production of sperm.
DOI:http://dx.doi.org/10.7554/eLife.08916.002
Collapse