Shape of tropoelastin, the highly extensible protein that controls human tissue elasticity

Abstract

Elastin enables the reversible deformation of elastic tissues and can withstand decades of repetitive forces. Tropoelastin is the soluble precursor to elastin, the main elastic protein found in mammals. Little is known of the shape and mechanism of assembly of tropoelastin as its unique composition and propensity to self-associate has hampered structural studies. In this study, we solve the nanostructure of full-length and corresponding overlapping fragments of tropoelastin using small angle X-ray and neutron scattering, allowing us to identify discrete regions of the molecule. Tropoelastin is an asymmetric coil, with a protruding foot that encompasses the C-terminal cell interaction motif. We show that individual tropoelastin molecules are highly extensible yet elastic without hysteresis to perform as highly efficient molecular nanosprings. Our findings shed light on how biology uses this single protein to build durable elastic structures that allow for cell attachment to an appended foot. We present a unique model for head-to-tail assembly which allows for the propagation of the molecule's asymmetric coil through a stacked spring design.AFM | SAXS | atomic force microscopy A ll mammals rely on elastin to convey extensional elasticity to their tissues. Elastin dominates the mass of the aorta where it encounters the peaks and troughs of systole and diastole over the course of two billion heartbeats in a lifetime (1). The lung expands with each intake of breath and elastically contracts on exhalation. The function of these tissues benefits from minimal energy loss during elastic return in each cycle of expansion and contraction. Additionally, elastin is required to function in an environment that relies on cellular contact (2-4) without compromising persistent elasticity. This high level of physical performance demanded of elastin vastly exceeds and indeed outlasts all human-made elastomers (5).Elastin is constructed by the hierarchical assembly and crosslinking of many tropoelastin monomers that accumulate on a microfibrillar skeleton. Tropoelastin is encoded by a single gene in humans and predominantly laid down in utero and early childhood, providing a durable resource that is intended to elastically serve until old age. This exquisite assembly helps to generate elastic tissues as diverse as artery, lung, and skin (4). Consequences of elastolytic damage in aortic aneurysms, emphysema, and solar elastosis confirm the key roles of elastin in structure and cellular interactions (6-8). These tissues rely on this paradoxical combination of organized tissue structures built from an intrinsically unstructured protein. Tropoelastin serves as a component of rigidly organized assemblies, yet enables the formation of dynamically distensible, elastic tissues.Tropoelastin is frequently described in the literature as an unstructured protein, mainly because models of elasticity invoke an element of disorder within the structure (4, 9, 10). While this concept appears to be the case at the fine, more subtle intramolecular leve...

Elastin Based Constructs

“…Although the mechanism for elasticity has not been fully elucidated, elastic recoil likely to be entropically driven whereby extension of the protein results in a more ordered structure and thus recoil occurs so the protein can return to a disorder state (reviewed by (Rosenbloom, Abrams et al 1993;Vrhovski and Weiss 1998). This elasticity is due to the inherent elastic properties of the monomer (Holst, Watson et al 2010;Baldock, Oberhauser et al 2011). …”

Perspectives on the Molecular and Biological Implications of Tropoelastin in Human Tissue Elasticity

“…We discovered that tropoelastin has a defined monomer shape in solution that is needed for assembly, [9,10,13] but also displays a large percentage of flexible, disordered regions needed for molecular elasticity. [14,15] The tertiary structure of human tropoelastin represents an ensemble of elastic conformers, [10,16] yet occasional conserved sequence elements hint at requirements for functional demands in one or more key parts of this molecule. [17][18][19] Tropoelastin is a 20-nm-long asymmetric protein monomer with a slightly curved nanospring shaft that extends from its N-terminus to a bifurcating hinged foot region towards the C-terminus.…”

“…This model of tropoelastin dynamics describes a characteristic scissors-like motion between the hinge and foot regions, as well as a twisting motion in the N-terminal coil region, in which the N-terminus experiences highest displacement. [9,10,13,20] The geometric distribution of the tropoelastin molecular volume intrinsically prompts a cohesive motion pattern between the upper and lower regions of the molecule, giving rise to local regions of conformational flexibility thought to be responsible for elastic deformation, while maintaining a defined tertiary shape with distinct functional segments for protein and cellular interactions. Despite the intrinsic flexibility of tropoelastin as a supposedly disordered elastomeric protein, [21] local changes to its native shape, manifested by a perturbed hinge region in human tropoelastin, can have a substantial impact on function, including its assembly into larger-scale structures.…”

“…[17][18][19] Tropoelastin is a 20-nm-long asymmetric protein monomer with a slightly curved nanospring shaft that extends from its N-terminus to a bifurcating hinged foot region towards the C-terminus. [10] Small-angle X-ray (SAXS) and neutron scattering (SANS), combined with antibody access, assembly, and cell interaction data reveal that tropoelastin has defined shape requirements, yet operates as a kinetically dynamic molecule. The protein's dynamics can be described as an ensemble of conformational microstates that are intrinsically accessible to the molecule under physiological conditions.…”

“…[7] Although other proteins are present, elastic tissue is predominantly made of one protein, tropoelastin, [8] which dominates molecular contributions to the architecture and function of elastin. [9,10] For this reason, the present review considers the perspective of tropoelastin.…”