More MS news articles for September 2002

A common, autoimmunity predisposing signal peptide variant of the cytotoxic T-lymphocyte antigen-4 results in inefficient glycosylation of the susceptibility allele

J Biol Chem 2002 Sep 19
Anjos S, Nguyen A, Ounissi-Benkalha H, Tessier MC, Polychronakos C.
Pediatrics, McGill University Health Center, Montral, Qubec H3H 1P3.

A common Thr17Ala polymorphism in the signal peptide of the cytotoxic T-lymphocyte anti-gen 4 (CTLA-4), a T-cell receptor that negatively regulates immune responses, is associated with risk for autoimmune disease.

Since the polymorphism is absent from the mature protein, we hy-pothesized that its biological effect must involve early stages of protein processing, prior to sig-nal peptide cleavage.

Constructs representing the two alleles were compared by in vitro transla-tion, in the presence of endoplasmic reticulum (ER) membranes.

We studied glycosylation by endoglycosidase-H digestion and glycosylation mutant constructs, cleavage of peptide with in-hibitors, and membrane integration by ultracentrifugation and proteinase-K sensitivity.

A major cleaved and glycosylated product was seen for both alleles of the protein but a band representing incomplete glycosylation was markedly more abundant in the predisposing Ala allele (32.7% 1.0 vs. 10.6% 1.2 for Thr, P<10-9).

In addition, differential intracellular/surface partitioning was studied with co-transfection of the alleles fused to distinct fluorescent proteins.

By quanti-tative confocal microscopy we found a higher ratio of cell-surface/total CTLAThr17 versus CTLAAla17 (P=0.01).

Our findings corroborate observations, in other proteins, that the signal peptide can determine the efficiency of post-translational modifications other than cleavage and suggest inefficient processing of the autoimmunity-predisposing Ala allele as the explanation for the genetic effect.