Protein C-Mannosylation and C-Mannosyl Tryptophan in Chemical Biology and Medicine

Abstract

C-Mannosylation is a post-translational modification of proteins in the endoplasmic reticulum. Monomeric α-mannose is attached to specific Trp residues at the first Trp in the Trp-x-x-Trp/Cys (W-x-x-W/C) motif of substrate proteins, by the action of C-mannosyltransferases, DPY19-related gene products. The acceptor substrate proteins are included in the thrombospondin type I repeat (TSR) superfamily, cytokine receptor type I family, and others. Previous studies demonstrated that C-mannosylation plays critical roles in the folding, sorting, and/or secretion of substrate proteins. A C-mannosylation-defective gene mutation was identified in humans as the disease-associated variant affecting a C-mannosylation motif of W-x-x-W of ADAMTSL1, which suggests the involvement of defects in protein C-mannosylation in human diseases such as developmental glaucoma, myopia, and/or retinal defects. On the other hand, monomeric C-mannosyl Trp (C-Man-Trp), a deduced degradation product of C-mannosylated proteins, occurs in cells and extracellular fluids. Several studies showed that the level of C-Man-Trp is upregulated in blood of patients with renal dysfunction, suggesting that the metabolism of C-Man-Trp may be involved in human kidney diseases. Together, protein C-mannosylation is considered to play important roles in the biosynthesis and functions of substrate proteins, and the altered regulation of protein C-manosylation may be involved in the pathophysiology of human diseases. In this review, we consider the biochemical and biomedical knowledge of protein C-mannosylation and C-Man-Trp, and introduce recent studies concerning their significance in biology and medicine.

Keywords: C-mannosyl tryptophan; C-mannosylation; DPY19; cytokine receptor type I; thrombospondin type I repeat.

Figure 1

The structures of C-mannosyl tryptophan (C-Man-Trp), C-glucosyl tryptophan (C-GlC-Trp), and N-mannosyl tryptophan (N-Man-Trp).

Figure 2

Schematic structure and sequence of thrombospondin type I repeat 2 (TSR2) in human thrombospondin-1 (TSP-1). TSR2 consists of an antiparallel three-stranded domain, in which the A strand assumes a unique rippled conformation and B and C strands form β structures [9]. The A strand has a conserved sequence motif of W-x-x-W-x-x-W-x-x-C, and the first two Trp residues were confirmed to be C-mannosylated [27]. The disulfide bonds are marked by green dashed lines connecting paired Cys residues. The TSR2 domain is stabilized by the “Trp-Arg ladder” motif, in which three Trp residues from the A strand are stacked with Arg residues from the B strand. Man, mannose.

Scheme 1

Synthesis of C-Man-Trp by lithiated Trp to 1,2-anhydro mannose 3.

Scheme 2

Synthesis of C-Man-Trp from mannose lactone.

Scheme 3

C-Man-Trp synthesis by organometallic reaction.

Scheme 4

C-Man-Trp synthesis through quinoline-assisted Pd-mediated reaction.

Scheme 5

Photoreductive cross-coupling C-Man-Trp synthesis.

Scheme 6

Chemical “post-translational” glycopeptide synthesis.

Scheme 7

Anomerization of protected C-Man-Trp under acidic conditions.

Figure 3

Cremer–Pople with 38 canonical states.

Figure 4

Importance of mannose phosphate isomerase in glucose-mannose transformation.

Figure 5

Biosynthetic pathway of C-mannosylated proteins in the endoplasmic reticulum (ER). The dolichyl-P-mannose (Dol-P-Man) synthase complex consists of DPM1, 2, and 3. Dol-P-Man is synthesized on the cytosolic face of the ER from Dol-P and GDP-Man, and flips into the ER lumen. The C-mannosyltransferase (i.e., DPY19L1 and DPY19L3) transfers an α-mannose to Trp in the W-x-x-W motif of the target proteins. C-Mannosylation promotes the formation of appropriate disulfide bonds and secretion of the substrate proteins. Man, mannose.