Fibronectin Structure: Its Dimers, Repeats, and Shape

Fibronectin is a large glycoprotein and a principal component of the extracellular matrix, the network of molecules surrounding cells. It is also found in a soluble form in blood and other body fluids. This protein plays a part in numerous biological processes, including cell adhesion, cell migration during embryonic development, tissue repair, and blood clotting.

The structure of fibronectin enables its function, providing a scaffold for tissues and a communication link between a cell’s interior and its external environment. Understanding this structure reveals how one molecule can participate in such a wide array of biological events.

The Fibronectin Polypeptide and Dimer Formation

Functional fibronectin begins with the synthesis of individual polypeptide chains, which are long, linear sequences of amino acids. Each chain, with a molecular weight of approximately 230 to 275 kilodaltons (kDa), serves as a fibronectin subunit. Following synthesis, these polypeptides undergo modifications, such as glycosylation, where sugar molecules are attached at various points, contributing to the protein’s final architecture and stability.

The defining feature of fibronectin is its existence as a dimer, composed of two large polypeptide subunits. The two subunits are covalently linked near their C-termini by a pair of disulfide bonds. This dimerization creates the functional fibronectin molecule, which has a high molecular weight of around 500 to 600 kDa. The two polypeptide “arms” of the dimer are nearly identical, giving the molecule a symmetrical structure.

Modular Architecture: Type I, II, and III Repeats

Each arm of the fibronectin dimer is an assembly of repeating structural units, or modules, connected by flexible linker regions. These modules are often described as “beads on a string” that form the long polypeptide chain. There are three distinct types of these modules: Type I, Type II, and Type III fibronectin repeats, each with a characteristic structure and size.

Type I repeats are the smallest modules, each composed of about 40-45 amino acids. A feature of Type I modules is the presence of internal disulfide bonds, which create a stable, folded structure. There are twelve Type I repeats in each fibronectin subunit, concentrated at the N-terminal and C-terminal ends of the polypeptide arms. They are involved in binding to other proteins like fibrin and collagen.

Type II repeats are comprised of approximately 60 amino acids and, like Type I repeats, are stabilized by internal disulfide bonds that help fibronectin bind to collagen. In contrast, Type III repeats are the largest and most common, with each containing about 90 amino acids. Type III modules form a beta-sandwich fold but lack the internal disulfide bonds, which gives them structural flexibility to partially unfold under mechanical force. Each fibronectin arm has between 15 and 17 Type III repeats arranged along its central part.

Overall Three-Dimensional Shape and Dynamics

The long, modular arms of the fibronectin dimer fold to create a molecule with a “V” or “Y” shape. The two arms are joined at their C-termini, forming the base of the V-shape. This conformation is not rigid; fibronectin is a highly dynamic and flexible molecule whose structure can change depending on its environment and the forces acting upon it.

Fibronectin exists in two primary conformational states: a compact, soluble form and an extended, fibrillar form. In body fluids like blood plasma, fibronectin circulates in a compact state where the arms are folded back on themselves. This folded conformation is maintained by electrostatic interactions between modules and keeps certain binding sites hidden, preventing fibronectin from forming aggregates in the blood.

When fibronectin binds to cell surfaces and is assembled into the extracellular matrix, it undergoes a transformation. Cell-generated mechanical forces pull on the molecule, causing it to stretch and unfold into an extended, linear conformation. This stretching exposes the hidden binding sites, allowing fibronectin molecules to interact with one another and assemble into insoluble fibrils that form the backbone of the matrix.

Diversity in Fibronectin: Isoforms and Their Structural Distinctions

A single human gene, FN1, can produce up to 20 different versions of fibronectin, known as isoforms. This diversity arises from alternative splicing, a process where specific segments of precursor messenger RNA, called exons, are included or excluded from the final mRNA. This mechanism allows cells to produce structurally and functionally distinct fibronectin molecules tailored to specific tissues or situations.

The primary structural differences between isoforms occur in three regions: Extra Domain A (EDA), Extra Domain B (EDB), and the Variable (V) region. EDA and EDB are single Type III repeats that can be entirely included or skipped. For instance, plasma fibronectin, which circulates in the blood, is a soluble form that lacks both EDA and EDB domains. Cellular fibronectin, produced by cells like fibroblasts, often includes these domains and is assembled into insoluble matrix fibrils.

The V region exhibits more complex splicing patterns, with different portions of the segment being included or excluded, leading to several variants. These structural variations directly influence the isoform’s location and role. The inclusion of the EDA and EDB domains in cellular fibronectin is associated with tissue repair, embryonic development, and pathological conditions. These structurally distinct isoforms can interact with different cellular receptors and matrix components, enabling fibronectin to perform a wide range of specialized tasks.

How Structure Enables Fibronectin’s Interactions

Specific binding sites are located within particular modules, and their accessibility is regulated by the protein’s conformation. The arrangement of these sites along the flexible arms allows fibronectin to act as a multipurpose connector in the extracellular matrix.

An example is the Arg-Gly-Asp (RGD) sequence, a three-amino-acid motif located on a flexible loop within the tenth Type III repeat (III-10). The shape and exposure of this RGD loop allow it to be recognized and bound by integrins, receptor proteins on the surface of cells. This interaction is not solely dependent on the RGD sequence; a “synergy site” in the adjacent ninth Type III repeat (III-9) also participates.

Fibronectin possesses other structurally defined binding domains. For instance, Type I modules near the N-terminus are structured to bind to fibrin and collagen. Another interaction is with heparin, a type of glycosaminoglycan, at a site in the thirteenth Type III repeat (III-13). This site consists of a cluster of positively charged amino acids that form a “cationic cradle” to accommodate the negatively charged heparin molecule.