The fibrinogen C-terminal domain is seldom C-mannosylated but its C-mannosylation is important for the secretion of microfibril-associated glycoprotein 4

Microfibril-associated glycoprotein 4 forms octamers that mediate interactions with elastogenic proteins and cells

Microfibril-associated glycoprotein 4 (MFAP4) is a 36-kDa extracellular matrix glycoprotein with critical roles in organ fibrosis, chronic obstructive pulmonary disease, and cardiovascular disorders, including aortic aneurysms. MFAP4 multimerises and interacts with elastogenic proteins, including fibrillin-1 and tropoelastin, and with cells via integrins. Structural details of MFAP4 and its potential interfaces for these interactions are unknown. Here, we present a cryo-electron microscopy structure of human MFAP4. In the presence of calcium, MFAP4 assembles as an octamer, where two sets of homodimers constitute the top and bottom halves of each octamer. Each homodimer is linked together by an intermolecular disulphide bond. A C34S missense mutation prevents disulphide-bond formation between monomers but does not prevent octamer assembly. The atomic model, built into the 3.55 Å cryo-EM map, suggests that salt-bridge interactions mediate homodimer assembly, while non-polar residues form the interface between octamer halves. In the absence of calcium, an MFAP4 octamer dissociates into two tetramers. Binding studies with fibrillin-1, tropoelastin, LTBP4, and small fibulins show that MFAP4 has multiple surfaces for protein-protein interactions, most of which depend upon MFAP4 octamer assembly. The C34S mutation does not affect these protein interactions or cell interactions. MFAP4 assemblies with fibrillin-1 abrogate MFAP4 interactions with cells.

Biogenesis of fibrils requires C-mannosylation of PMEL

Premelanosome protein (PMEL), a melanocyte-specific glycoprotein, has an essential role in melanosome maturation, assembling amyloid fibrils for melanin deposition. PMEL undergoes several post-translational modifications, including N- and O-glycosylations, which are associated with proper melanosome development. C-mannosylation is a rare type of protein glycosylation at a tryptophan residue that might regulate the secretion and localization of proteins. PMEL has one putative C-mannosylation site in its core amyloid fragment (CAF); however, there is no report focusing on C-mannosylation of PMEL. To investigate this, we expressed recombinant PMEL in SK-MEL-28 human melanoma cells and purified the protein. Mass spectrometry analyses demonstrated that human PMEL is C-mannosylated at multiple tryptophan residues in its CAF and N-terminal fragment (NTF). In addition to the W153 or W156 residue (CAF), which lies in the consensus sequence for C-mannosylation, the W104 residue (NTF) was C-mannosylated without the consensus sequence. To determine the effects of the modifications, we deleted the PMEL gene by using CRISPR/Cas9 technology and re-expressed wild-type or C-mannosylation-defective mutants of PMEL, in which the C-mannosylated tryptophan was replaced with a phenylalanine residue (WF mutation), in SK-MEL-28 cells. Importantly, fibril-containing melanosomes were significantly decreased in W104F mutant PMEL-re-expressing cells compared with wild-type PMEL, observed using transmission electron microscopy. Furthermore, western blot and immunofluorescence analysis suggested that the W104F mutation may cause mild endoplasmic reticulumretention, possibly associated with early misfolding, and lysosomal misaggregation, thus reducing functional fibril formation. Our results demonstrate that C-mannosylation of PMEL is required for proper melanosome development by regulating PMEL-derived fibril formation.

Destabilization of vitelline membrane outer layer protein 1 homolog (VMO1) by C-mannosylation

C-mannosylation is a rare type of protein glycosylation whereby a single mannose is added to the first tryptophan in the consensus sequence Trp-Xaa-Xaa-Trp/Cys (in which Xaa represents any amino acid). Its consensus sequence is mainly found in proteins containing a thrombospondin type-1 repeat (TSR1) domain and in type I cytokine receptors. In these proteins, C-mannosylation affects protein secretion, intracellular localization, and protein stability; however, the role of C-mannosylation in proteins that are not type I cytokine receptors and/or do not contain a TSR1 domain is less well explored. In this study, we focused on human vitelline membrane outer layer protein 1 homolog (VMO1). VMO1, which possesses two putative C-mannosylation sites, is a 21-kDa secreted protein that does not contain a TSR1 domain and is not a type I cytokine receptor. Mass spectrometry analyses revealed that VMO1 is C-mannosylated at Trp¹⁰⁵ but not at Trp⁴⁴ . Although C-mannosylation does not affect the extracellular secretion of VMO1, it destabilizes the intracellular VMO1. In addition, a structural comparison between VMO1 and C-mannosylated VMO1 showed that the modification of the mannose changes the conformation of three loops in VMO1. Taken together, our results demonstrate the first example of C-mannosylation for protein destabilization of VMO1.

C-mannosylation regulates stabilization of RAMP1 protein and RAMP1-mediated cell migration

C-mannosylation is a unique type of protein glycosylation via C-C linkage between an α-mannose and a tryptophan residue. This modification has been identified in about 30 proteins and regulates several functions, such as protein secretion and intracellular localization, as well as protein stability. About half of C-mannosylated proteins are categorized as proteins containing thrombospondin type 1 repeat domain or type I cytokine receptors. To evaluate whether C-mannosylation broadly affects protein functions regardless of protein domain or family, we have sought to identify other types of C-mannosylated protein and analyse their functions. In this study, we focused on receptor activity modifying protein 1, which neither contains thrombospondin type 1 repeat domain nor belongs to the type I cytokine receptors. Our mass spectrometry analysis demonstrated that RAMP1 is C-mannosylated at Trp⁵⁶ . It has been shown that RAMP1 transports to the plasma membrane after dimerization with calcitonin receptor-like receptor and is important for ligand-dependent downstream signalling activation. Our results showed that C-mannosylation has no effect on this transport activity. On the other hand, C-mannosylation did enhance protein stability and cell migration activity. Our data may provide new insight into both C-mannosylation research and novel RAMP1 analysis.

Biological evaluation for anti-inflammatory effect of africane-type sesquiterpenoids

Africane-type sesquiterpenoids are a unique tricyclic carbon architecture sesquiterpenoid isolated as natural products. Δ⁹⁽¹⁵⁾ -africanene has been reported to exhibit anti-inflammatory activity for carrageenan-induced rat foot edema. In this study, we reported structure-activity relationship study of africane-type sesquiterpenoids and found that some africane-type sesquiterpenoid analogs and their synthetic intermediate showed potent anti-inflammatory activity. To identify the mode of action of africane-type sesquiterpenoids and their synthetic intermediate, we evaluated the anti-inflammatory activity using lipopolysaccharide (LPS)-stimulated mouse macrophage RAW264.7 cells. Treatment with the africane-type compounds and their synthetic intermediate suppressed LPS-induced expressions of Cox-2 protein and mRNAs of the inflammatory cytokines IL-1β and IL-6 at the concentrations that did not affect cell viability. Interestingly, although these africane-type compounds and their synthetic intermediate suppressed the pro-inflammatory cytokines' expressions, the compounds did not modulate NF-κB activation. These results suggest that the africane-type compounds and their synthetic intermediate are anti-inflammatory compounds that suppress the expression of LPS-induced inflammatory mediators independently of NF-κB activation.

Involvement of DPY19L3 in Myogenic Differentiation of C2C12 Myoblasts

DPY19L3 has been identified as a C-mannosyltransferase for thrombospondin type-1 repeat domain-containing proteins. In this study, we focused on the role of DPY19L3 in the myogenic differentiation of C2C12 mouse myoblast cells. We carried out DPY19L3 gene depletion using the CRISPR/Cas9 system. The result showed that these DPY19L3-knockout cells could not be induced for differentiation. Moreover, the phosphorylation levels of MEK/ERK and p70S6K were suppressed in the DPY19L3-knockout cells compared with that of parent cells, suggesting that the protein(s) that is(are) DPY19L3-mediated C-mannosylated and regulate(s) MEK/ERK or p70S6K signaling is(are) required for the differentiation.

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

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.

Involvement of LH3 and GLT25D1 for glucosyl-galactosyl-hydroxylation on non-collagen-like domain of FGL1

Glucosyl-galactosyl-hydroxylation (GGH) is one type of post-translational modification, which is mainly observed in collagen-like domain-containing proteins. Using LC-MS/MS analysis, we found a GGH-like modification at Lys65 of fibrinogen-like protein 1 (FGL1), although it does not contain a collagen-like domain. To identify the glycosyltransferases responsible for this modification, we established LH3/GLT25D1-knockout FGL1-overexpressing HT1080 cell lines. The result showed that knockout of LH3 or GLT25D1 significantly inhibited the glycosylation. Furthermore, deficiency of GGH by point mutation of the FGL1 protein or knockout of the GGH-related glycosyltransferase reduced FGL1 protein levels. Taken together, these data indicate that Lys65 of FGL1 is glucosyl-galactosyl-hydroxylated by LH3 and GLT25D1. Our results provide novel insights to regulate various FGL1 functions.

Identification of madangamine A as a novel lysosomotropic agent to inhibit autophagy

Madangamines are marine natural products isolated from Xestospongia ingens, and madangamine A-E with a different D-ring structure have been reported. We have reported that madangamine A has strong anti-proliferative activity against various human cancer cell lines. In this study, to clarify the anti-proliferative activity of madangamine A, we searched for molecular target of the madangamine A in human cells. Treatment with madangamine A increased the levels of LC3-II and p62, autophagy-related proteins, concomitant with growth inhibition. Moreover, madangamine A resulted in lysosome enlargement and increase in lysosomal pH, which are same phenomena observed in chloroquine-treated cells. These results suggest that madangamine A is a novel lysosome inhibitor, and the anti-proliferative activity of madangamine A is due to the inhibition of lysosome function.

Regulation of N-glycosylation and secretion of Isthmin-1 by its C-mannosylation

BACKGROUND

C-mannosylation is a type of protein glycosylation. Human Isthmin-1 (ISM1) is a 52-kDa secreted protein with a thrombospondin type 1 repeat (TSR) domain, containing two consensus C-mannosylation sequences at Trp²²³ and Trp²²⁶. In this study, we sought to examine the role of C-mannosylation in the secretion of ISM1.

METHODS

We established and cultured an ISM1-overexpressing HT1080 cell line and purified recombinant ISM1 for analysis from the conditioned medium by LC-MS/MS. Subcellular localization of ISM1 was observed by confocal fluorescence microscopy.

RESULTS

We found that ISM1 is C-mannosylated at Trp²²³ and Trp²²⁶ in the TSR domain. To determine the functions of the C-mannosylation of ISM1, we established a C-mannosylation-defective mutant ISM1-overexpressing HT1080 cell line and measured its secretion of ISM1. The secretion of ISM1 decreased significantly in this mutant ISM1-overexpressing line compared with wild-type cells. Furthermore, ISM1 was N-glycosylated only in these C-mannosylation-defective cells.

CONCLUSIONS

ISM1 is C-mannosylated in its TSR domain, and the status of the C-mannosylation of ISM1 affects its N-glycosylation.

GENERAL SIGNIFICANCE

The C-mannosylation of ISM1 regulates its N-glycosylation status.

Requirement for C-mannosylation to be secreted and activated a disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS4)

BACKGROUND

C-mannosylation is a unique type of glycosylation. A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS4) is a multidomain extracellular metalloproteinase that contains several potential C-mannosylation sites. Although some ADAMTS family proteins have been reported to be C-mannosylated proteins, whether C-mannosylation affects the activation and protease activity of these proteins is unclear.

METHODS

We established wild-type and mutant ADAMTS4-overexpressing HT1080 cell lines. Recombinant ADAMTS4 was purified from the conditioned medium of the wild-type ADAMTS4-overexpressing cells, and the C-mannosylation sites of ADAMTS4 were identified by LC-MS/MS. The processing, secretion, and intracellular localization of ADAMTS4 were examined by immunoblot and immunofluorescence analyses. ADAMTS4 enzymatic activity was evaluated by assessing the cleavage of recombinant aggrecan.

RESULTS

We identified that ADAMTS4 is C-mannosylated at Trp⁴⁰⁴ in the metalloprotease domain and at Trp⁵²³, Trp⁵²⁶, and Trp⁵²⁹ in the thrombospondin type 1 repeat (TSR). The replacement of Trp⁴⁰⁴ with Phe affected ADAMTS4 processing, without affecting secretion and intracellular localization. In contrast, the substitution of Trp⁵²³, Trp⁵²⁶, and Trp⁵²⁹ with Phe residues suppressed ADAMTS4 secretion, processing, intracellular trafficking, and enzymatic activity.

CONCLUSIONS

Our results demonstrated that the C-mannosylation of ADAMTS4 plays important roles in protein processing, intracellular trafficking, secretion, and enzymatic activity.

GENERAL SIGNIFICANCE

Because C-mannosylation appears to regulate many ADAMTS4 functions, C-mannosylation may also affect other members of the ADAMTS superfamily.