XBP-1-deficient plasmablasts show normal protein folding but altered glycosylation and lipid synthesis.
The accumulation of misfolded secreted IgM in the endoplasmic reticulum (ER) of X-box binding protein 1 (XBP-1)-deficient B cells has been held responsible for the inability of such cells to yield plasma cells, through the failure to mount a proper unfolded protein response. LPS-stimulated B cells incapable of secreting IgM still activate the XBP-1 axis normally, as follows: XBP-1 is turned on by cues that trigger differentiation and not in response to accumulation of unfolded IgM, but the impact of XBP-1 deficiency on glycoprotein folding and assembly has not been explored. The lack of XBP-1 compromised neither the formation of functional hen egg lysozyme-specific IgM nor the secretion of free kappa-chains. Although XBP-1 deficiency affects the synthesis of some ER chaperones, including protein disulfide isomerase, their steady state levels do not drop below the threshold required for proper assembly and maturation of the Igalpha/Igbeta heterodimer and MHC molecules. Intracellular transport and surface display of integral membrane proteins are unaffected by XBP-1 deficiency. Given the fact that we failed to observe any defects in folding of a variety of glycoproteins, we looked for other means to explain the requirement for XBP-1 in plasma cell development. We observed significantly reduced levels of phosphatidylcholine, sphingomyelin, and phosphatidylinositol in total membranes of XBP-1-deficient B cells, and reduced ER content. Terminal N-linked glycosylation of IgM and class I MHC was altered in these cells. XBP-1 hence has important roles beyond folding proteins in the ER.
McGehee
Annette M
AM
Whitehead Institute for Biomedical Research, Cambridge, MA 02142, USA.
Dougan
Stephanie K
SK
Klemm
Elizabeth J
EJ
Shui
Guanghou
G
Park
Boyoun
B
Kim
You-Me
YM
Watson
Nicki
N
Wenk
Markus R
MR
Ploegh
Hidde L
HL
Hu
Chih-Chi Andrew
CC
D000818:Animals; D001402:B-Lymphocytes / Q000166:cytology; D004268:DNA-Binding Proteins / Q000172:deficiency; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism; D006031:Glycosylation; D015395:Histocompatibility Antigens Class I / Q000737:chemistry / Q000378:metabolism; D007075:Immunoglobulin M / Q000737:chemistry / Q000378:metabolism; D008055:Lipids / Q000096:biosynthesis* / Q000502:physiology*; D008563:Membrane Lipids / Q000032:analysis; D051379:Mice; D008817:Mice, Mutant Strains; D010950:Plasma Cells / Q000166:cytology*; D017510:Protein Folding; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors / Q000172:deficiency; D000071717:X-Box Binding Protein 1
D004268:DNA-Binding Proteins; D015395:Histocompatibility Antigens Class I; D007075:Immunoglobulin M; D008055:Lipids; D008563:Membrane Lipids; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors; D000071717:X-Box Binding Protein 1; C000605544:Xbp1 protein, mouse