Human SND2 mediates ER targeting of GPI‐anchored proteins with low hydrophobic GPI attachment signals

Abstract

Over 100 glycosylphosphatidylinositol‐anchored proteins (GPI‐APs) are encoded in the mammalian genome. It is not well understood how these proteins are targeted and translocated to the endoplasmic reticulum (ER). Here, we reveal that many GPI‐APs, such as CD59, CD55, and CD109, utilize human SND2 (hSND2)‐dependent ER targeting machinery. We also found that signal recognition particle receptors seem to cooperate with hSND2 to target GPI‐APs to the ER. Both the N‐terminal signal sequence and C‐terminal GPI attachment signal of GPI‐APs contribute to ER targeting via the hSND2‐dependent pathway. Particularly, the hydrophobicity of the C‐terminal GPI attachment signal acts as the determinant of hSND2 dependency. Our results explain the route and mechanism of the ER targeting of GPI‐APs in mammalian cells.

Precursors of select GPI-anchored proteins positively regulate GPI biosynthesis under the control of ERAD

“…Considering that more than 150 different human proteins are GPI-APs, we next investigated whether other GPI-AP precursors had the ability to upregulate GPI biosynthesis, similar to CD55. In HEK293 cells, thirteen of 161 GPI-APs were expressed at higher levels than CD55 47 (Fig. 5a).…”

“…Key residues in GPI attachment signal for GPI biosynthesis GPI transamidase recognizes and cleaves the GPI attachment signal between ω and ω+1 sites, forming the thioester bond on the catalytic Cys residue to generate an enzyme-substrate intermediate. GPI is then transferred to the newly exposed C-terminus of the protein 47,49 . Although all proteins modi ed by GPI contain a GPI attachment signal peptide at the C-terminus, the sequences are not conserved.…”

The Molecular Biodiversity of Protein Targeting and Protein Transport Related to the Endoplasmic Reticulum

“…Both variants of targeting signals share a hydrophobic core element that upon its emergence from the ribosomal exit tunnel is usually considered to initiate the process of selective targeting, translocation, and/or membrane insertion [ 42 ]. Other hydrophobic sequences, such as the C-terminal glycosylphosphatidylinositol (GPI)-attachment signal [ 43 , 44 ], may further shape the choice of a specific targeting pathway but do not act independently of an N-terminal SP. Moreover, adhering to the Latin prefix “trans”, we do not discuss the targeting of monotopic membrane proteins that do not fully traverse both halves of the lipid bilayer and instead insert via an intramembrane domain often showing a hairpin or amphipathic helix [ 45 , 46 ].…”

“…Specifically, SRP recognizes cleavable SPs and TMHs at the N-terminus, whereas the GET pathway preferentially binds the C-terminal TMHs of TA proteins. While both groups of clients share signals with a similar hydrophobicity but a different localization within the polypeptide, the SND pathway catches less hydrophobic targeting signals which occur internally or at the C-terminus [ 22 , 23 , 43 ]. These findings have highlighted a previously unanticipated complexity of ER targeting that expands beyond proteins with classical targeting signals and includes the heterogeneous cohorts of TA and GPI-anchored proteins [ 23 , 43 , 131 ].…”

“…While both groups of clients share signals with a similar hydrophobicity but a different localization within the polypeptide, the SND pathway catches less hydrophobic targeting signals which occur internally or at the C-terminus [ 22 , 23 , 43 ]. These findings have highlighted a previously unanticipated complexity of ER targeting that expands beyond proteins with classical targeting signals and includes the heterogeneous cohorts of TA and GPI-anchored proteins [ 23 , 43 , 131 ]. Next, the current view of pathway organization and operation will be discussed for SRP, GET, and SND before we continue with a structural comparison of the central cargo binding components to discuss the partially overlapping client spectra.…”