FUT4 - fucosyltransferase 4 |Elisa - Clia - Antibody - Protein

Background

FUT4 (fucosyltransferase 4) is a crucial enzyme in the glycosylation process, specifically responsible for catalyzing the transfer of fucose, a hexose sugar, to target glycoproteins and glycolipids. This process is essential in forming fucosylated glycans, such as the Lewis antigens. FUT4 belongs to the broader family of fucosyltransferases, enzymes that facilitate the addition of fucose residues to various substrates. These glycosylation modifications are important for many cellular functions, including cell adhesion, signal transduction, immune response, and development.

One of the primary roles of FUT4 is in the synthesis of Lewis X antigen (CD15), a fucosylated oligosaccharide that plays a critical role in mediating cell-cell interactions. FUT4 is encoded by the FUT4 gene, which is located on chromosome 11q21-22 in humans. It is expressed in a variety of tissues but is especially prominent in the bone marrow, placenta, and immune cells. The enzyme's activity has been linked to processes such as leukocyte trafficking, inflammation, and tumor metastasis, due to its ability to regulate the presentation of fucosylated antigens on the cell surface.

Protein Structure

FUT4 is a type II transmembrane protein, a classification that reflects its orientation in the membrane and structural characteristics. The enzyme consists of approximately 576 amino acids and can be divided into several distinct domains that are critical for its enzymatic activity and function.

N-terminal Transmembrane Domain:

• The N-terminal region of FUT4 contains a single-pass transmembrane helix, which anchors the protein in the Golgi membrane, where fucosylation occurs. This orientation allows the enzyme's active site to face the lumen of the Golgi apparatus, where it catalyzes the transfer of fucose to glycoconjugates.

Catalytic Domain:

• The main body of the FUT4 protein is located in the luminal portion of the Golgi. This region contains the catalytic domain, responsible for recognizing the substrate and transferring fucose from GDP-fucose (guanosine diphosphate-fucose) to acceptor molecules, typically glycoproteins or glycolipids. This transfer typically occurs at the N-acetylglucosamine (GlcNAc) residue of N-glycans or O-glycans.
• The active site of FUT4 contains a DxD motif (Asp-X-Asp), which is common in glycosyltransferases and is involved in coordinating metal ions like Mn²⁺ or Mg²⁺. These ions are essential for stabilizing the enzyme-substrate complex and facilitating the nucleophilic attack on GDP-fucose, leading to the formation of a fucosylated bond.

Substrate Recognition and Specificity:

• FUT4 exhibits substrate specificity for GlcNAc residues on glycoproteins and glycolipids, with the α-1,3 linkage being the predominant bond formed. This specificity is governed by residues surrounding the catalytic site that interact with both the donor substrate (GDP-fucose) and the acceptor substrate (the GlcNAc residue). These interactions are crucial for the proper positioning of the substrates and the efficient catalysis of the fucosylation reaction.

Glycosylation Sites:

• FUT4, like many other enzymes involved in glycosylation, is itself glycosylated. Glycosylation of the enzyme ensures proper folding, stability, and trafficking to the Golgi apparatus, where its activity takes place. These glycosylation sites help stabilize the protein in the membrane and may influence its interaction with substrates or other enzymes within the glycosylation machinery.

Classification and Subtypes

Fucosyltransferases are a large family of enzymes, divided based on the linkage specificity of the fucosylation reaction. FUT4 is a member of the α-1,3-fucosyltransferase family. Other fucosyltransferases include:

FUT1 and FUT2:

• These are responsible for adding fucose in an α-1,2 linkage to substrates, particularly in the synthesis of ABO blood group antigens.

FUT3:

• Also known as Lewis fucosyltransferase, it adds fucose in both α-1,3 and α-1,4 linkages and is involved in the synthesis of the Lewis antigens.

FUT5, FUT6, FUT7, FUT9:

• These enzymes share structural similarities with FUT4 and catalyze the addition of fucose in α-1,3 linkages to different glycan substrates, with slight variations in tissue expression and functional roles.

Among these, FUT4 is particularly important in generating Lewis X antigen (CD15), a carbohydrate antigen involved in immune cell migration, inflammation, and tumor progression.

Function and Biological Significance

The primary function of FUT4 is the transfer of fucose to glycoproteins and glycolipids, contributing to the formation of the Lewis X antigen (CD15). This antigen plays a critical role in several biological processes:

Leukocyte Adhesion and Migration:

• The Lewis X antigen, produced by FUT4 activity, is a key component of selectin ligands on the surface of leukocytes. These ligands interact with selectins expressed on endothelial cells during the process of leukocyte extravasation, enabling immune cells to migrate from the bloodstream to sites of inflammation or infection. This process is crucial for the proper functioning of the immune system and for maintaining tissue homeostasis during inflammatory responses.

Immune Response Modulation:

• FUT4 is expressed in myeloid cells, where it modulates immune responses. The presence of Lewis X antigens on immune cells like neutrophils, monocytes, and macrophages enhances their ability to adhere to endothelial cells, facilitating their recruitment to inflamed tissues. This glycosylation also impacts cell signaling pathways that control immune cell activation, cytokine secretion, and phagocytosis.

Tumor Progression and Metastasis:

• The expression of FUT4 and the Lewis X antigen has been correlated with increased tumor invasiveness and metastasis. In various cancers, including colon, breast, and gastric cancer, elevated FUT4 expression promotes tumor cell adhesion to endothelial cells and enhances their ability to migrate, invade tissues, and form metastases. This makes FUT4 a potential marker for tumor aggressiveness and a therapeutic target in cancer treatment.

Embryonic Development:

• During embryogenesis, FUT4 is involved in cell differentiation and development. The Lewis X antigen has been shown to play roles in cell recognition, adhesion, and migration during the early stages of embryonic development, suggesting a role for FUT4 in tissue patterning and organogenesis.

Clinical Issues

FUT4 has been implicated in several diseases, particularly in the context of inflammation, cancer, and immune dysfunction:

Cancer:

• Overexpression of FUT4 is associated with poor prognosis in multiple cancers, including colorectal cancer, gastric cancer, and non-small cell lung cancer. FUT4-mediated fucosylation facilitates tumor cell adhesion to the vascular endothelium, promoting metastasis. Its role in enhancing selectin-mediated interactions makes it a target for therapeutic interventions aimed at blocking tumor dissemination.

Inflammatory Diseases:

• Since FUT4 plays a role in immune cell trafficking, dysregulation of FUT4 expression or activity can lead to inappropriate immune responses. For example, in autoimmune diseases like rheumatoid arthritis, overactive immune cells expressing high levels of FUT4 can exacerbate inflammation by infiltrating tissues excessively.

Immunodeficiency:

• Defective fucosylation due to mutations in FUT4 or related enzymes can lead to immune deficiencies, where leukocyte migration is impaired. This can result in an inability to mount effective immune responses to infections.

Congenital Disorders of Glycosylation (CDG):

• Though less commonly reported with FUT4 mutations, defects in glycosylation pathways, including fucosylation, are linked to a group of metabolic disorders known as CDGs. These disorders can result in developmental delays, immune dysfunction, and other systemic effects due to improper glycan processing.

Summary

FUT4 (fucosyltransferase 4) is an essential enzyme that catalyzes the addition of fucose residues to glycoproteins and glycolipids, producing the Lewis X antigen (CD15). This antigen plays a critical role in cell-cell interactions, particularly in the immune system, where it mediates leukocyte adhesion and migration during inflammatory responses. The enzyme's activity also contributes to tumor progression by enhancing cancer cell adhesion and metastasis. Structurally, FUT4 is a type II transmembrane protein with a zinc-dependent catalytic domain responsible for fucose transfer. The enzyme's involvement in inflammation, immune modulation, and cancer makes it a key target for therapeutic interventions. Dysregulation of FUT4 activity is associated with various clinical conditions, including cancer, autoimmune diseases, and congenital glycosylation disorders, highlighting its broad biological significance and clinical relevance.