Extracellular matrix: from atomic resolution to ultrastructure
Introduction
The extracellular matrix (ECM) is a complex network of glycoproteins and proteoglycans that originated with the advent of multicellular organisms [1]. Cells generate well-ordered ECM-complexes to surround and support themselves. The ECM then plays an essential role in the survival, migration and proliferation of these adjoining cells. Continuous ECM remodeling, catalyzed by various degradation enzymes, is common, and the arrangement and concentration of different macromolecules gives rise to a wide diversity of ECM forms in the various tissue types (skin, bone, cornea of the eye, etc.) [2]. The structure of the constituent polymers is rather well known at the domain or fragment level but is less well known at the levels of intact molecule or higher. The monomeric subunits are large, multi-domain and often inherently flexible, thus presenting problems to atomic resolution techniques such as single crystal diffraction or NMR. The polymers formed retain substantial heterogeneity and are cross-linked and difficult to extract from the matrix in undamaged form. Although progress is relatively slow, new approaches and tools are beginning to have an impact.
In this brief review, we have chosen to illustrate this field by discussing recent structural studies and current understanding of three archetypal ECM proteins: collagen (the most abundant protein in mammals), fibrillin and fibronectin. The first is a biopolymer of characteristic amino acid sequence, while the last two are modular proteins, constructed from repeating, autonomously folding domains with a high degree of structural similarity [3]. A major remaining structural problem is to define the various inter and intramolecular interactions made by molecules, especially in the context of structure at the μm level. We appreciate that this selection, with only three proteins, neglects many other important ECM molecules and provides only a part of the picture, however, space is limited. A particular area of neglect is polysaccharides, such as hyaluronan [4] which, with its receptors [5], plays a pivotal role in ECM hydration and elasticity.
Section snippets
Collagen
Collagen has a characteristic three residue repeat, Gly-Xaa-Yaa, in its primary structure, which results in a stable triple-helical conformation with the glycine residues at the core of the helix [6, 7, 8]. Proline and 4-hydroxyproline residues, usually found in positions Xaa and Yaa, function to stabilize the three individual polyproline II-like helices. After post-translational modification, secreted collagen helices self-assemble to cross-linked microfibrils and eventually μm-long fibrils.
Fibrillin
Fibrillin-1 is a large (350 kDa) multidomain glycoprotein that forms the major structural component of 10–12 nm elastic microfibrils in the ECM [22, 23]. With elastin, it provides the necessary elasticity and resilience of a variety of tissues. A large number of matrix components that interact with fibrillin-1 have been identified, including integrins [24•], heparin [25•], latent transforming growth factor β-binding proteins and fibulin-2 [26]. These, together with homotypic fibrillin
Fibronectin
Fibronectin (FN) is a large dimeric plasma glycoprotein found only in vertebrates. FN is composed of three different domain types, FNI, FNII and FNIII, high-resolution structures of which have been available for some time [35]. A tightly controlled process transforms plasma FN to a fibrillar form within the ECM. Little is known about the fibrillogenesis process, or the structure of the fibrillar matrix formed. It is, however, believed that FN interdomain interactions [36, 37, 38, 39, 40, 41, 42•
Conclusion
In all three ECM molecules visited, association interactions are the key to understand higher order structures. Intermolecular interactions define the packing order of collagen [18••], inter-domain interactions have a major role in models of fibrillin [27•] (whether ‘pleated’ or staggered) and interdomain association sites lead directly to FN fibrillogenesis models [50••]. Detailed structural descriptions of module interactions are, comparatively, fewer than structures of individual modules [3
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgement
Financial support was provided by the Wellcome Trust.
References (51)
- et al.
Domain structure and organisation in extracellular matrix proteins
Matrix Biol
(2002) - et al.
Molecular structure of the collagen triple helix
Adv Protein Chem
(2005) - et al.
Position of single amino acid substitutions in the collagen triple helix determines their effect on structure of collagen fibrils
J Struct Biol
(2004) - et al.
Thermostability gradient in the collagen triple helix reveals its multi-domain structure
J Mol Biol
(2004) - et al.
Collagen fibrils: nanoscale ropes
Biophys J
(2007) Building collagen molecules, fibrils, and suprafibrillar structures
J Struct Biol
(2002)- et al.
Type I collagen N-telopeptides adopt an ordered structure when docked to their helix receptor during fibrillogenesis
Proteins
(2004) - et al.
Fibrillin-1 interactions with heparin. Implications for microfibril and elastic fiber assembly
J Biol Chem
(2005) - et al.
Fibrillin: from domain structure to supramolecular assembly
Matrix Biol
(2000) - et al.
Evidence for the intramolecular pleating model of fibrillin microfibril organisation from single particle image analysis
J Mol Biol
(2005)