A Negatively-Charged Residue Stabilizes the Tropoelastin N-terminal Region for Elastic Fiber Assembly.

Tropoelastin is an extracellular matrix protein that assembles into elastic fibers which provide elasticity and strength to vertebrate tissues. While the contributions of specific tropoelastin regions during each stage of elastogenesis are still not fully understood, studies predominantly recognize the central hinge/bridge and C-terminal foot as the major participants in tropoelastin assembly, with a number of interactions mediated by the abundant positively-charged residues within these regions. On the other hand, much less is known about the importance of the rarely-occurring negatively-charged residues and the N-terminal coil region in tropoelastin assembly. The sole negatively-charged residue in the first half of human tropoelastin is aspartate 72. In contrast, the same region comprises 17 positively-charged residues. We mutated this aspartate residue to alanine and assessed the elastogenic capacity of this novel construct. We found that D72A tropoelastin has a decreased propensity for initial self-association, and cross-links aberrantly into denser, less porous hydrogels with reduced swelling properties. While the mutant can bind cells normally, it does not form elastic fibers with human dermal fibroblasts, and forms fewer, atypical fibers with human retinal pigmented epithelial cells. This impaired functionality is associated with conformational changes in the N-terminal region. Our results strongly point to the role of the D72 site in stabilizing the N-terminal segment of human tropoelastin, and the importance of this region in facilitating elastic fiber assembly.