Gamma-Interferon-inducibleLysosomal Thiol Reductase (GILT)

Abstract

We recently identified a gamma-interferon-inducible lysosomal thiol reductase (GILT), constitutively expressed in antigen-presenting cells, that catalyzes disulfide bond reduction both in vitro and in vivo and is optimally active at acidic pH. GILT is synthesized as a 35-kDa precursor, and following delivery to major histocompatibility complex (MHC) class II-containing compartments (MIICs), is processed to the mature 30-kDa form via cleavage of N-and C-terminal propeptides. The generation of MHC class II epitopes requires both protein denaturation and reduction of intra-and interchain disulfide bonds prior to proteolysis. GILT may be important in disulfide bond reduction of proteins delivered to MIICs and consequently in antigen processing. In this report we show that, like its mature form, precursor GILT reduces disulfide bonds with an acidic pH optimum, suggesting that it may also be involved in disulfide bond reduction in the endocytic pathway. We also show that processing of precursor GILT can be mediated by multiple lysosomal proteases and provide evidence that the mechanism of action of GILT resembles that of other thiol oxidoreductases.

Functional Requirements for the Lysosomal Thiol Reductase GILT in MHC Class II-Restricted Antigen Processing

“…The mature form is found in multivesicular late endosomes and multilamellar lysosomes (6,8). Additionally, a small portion of precursor GILT is secreted as a disulfide-linked dimer and has reductase activity (9). A thioredoxin-like CXXC motif involving Cys 46 and Cys 49 constitutes GILT's reductase active site in a cell-free assay using 125 I-F(abЈ) 2 as a substrate (6).…”

“…GILT is synthesized as a 35-kDa precursor and targeted to the endocytic pathway via the mannose-6 phosphate receptor (6,8). In the endocytic pathway, N-and C-terminal propeptides are cleaved to generate a 28-kDa mature form by multiple cathepsins (9). The mature form is found in multivesicular late endosomes and multilamellar lysosomes (6,8).…”

An Unexpected Functional Link between Lysosomal Thiol Reductase and Mitochondrial Manganese Superoxide Dismutase

“…GILT is synthesized as a 35-kDa soluble glycoprotein precursor and is transported to the endosomal compartment via the mannose-6-P receptor pathway (9). It is processed into the mature form (30 kDa) by proteolytic removal of N-and C-terminal peptides.…”

“…In addition to endosomal/lysosomal localization, GILT is secreted in the tissue culture medium of the GILT-expressing cell lines (7,10), and is present in mouse sera. 3 GILT is constitutively expressed in professional antigen-presenting cells (APCs), but it is also inducible by pro-inflammatory cytokines such as interferon ␥, tumor necrosis factor ␣, and interleukin 1␤ (9).…”

Roles of Cellular Redox Factors in Pathogen and Toxin Entry in the Endocytic Pathways

“…Known redox enzymes, such as thioredoxin reductase and glutathione reductase, normally function in neutral pH environments, so they are obviously not the likely candidates. The enzymes that catalyze the reduction within acidic compartments had been elusive until recently when GILT was identified as the first thiol reductase optimally active at low pH (4.0 -5.5) [76,77]. GILT is expressed constitutively in antigen-presenting cells, in which it is synthesized as a 35-kDa glycoprotein precursor containing a mannose-6-phosphate signal sequence and is colocalized with early endosomes.…”