Both civil engineering and bioengineering approaches are being used by investigators at MIT and Carnegie Mellon University to examine the behavior of a protein connected with progeria, a rare disorder in children that causes them to age extremely rapidly and generally results in death from cardiovascular disease before the age of 16. Progeria is marked by the loss of 50 amino acids near the end of the lamin A protein, which helps support a cell’s nuclear membrane. The findings are published in the September issue of the Journal of Structural Biology.

Using molecular modeling – which obeys the laws of physics at the molecular scale – the investigators created exact replicas of healthy and mutated lamin A protein tails and pulled on them to simulate the behavior of them under stress, in much the same way a traditional civil engineer might apply pressure to test the strength of a beam.

Markus Buehler, a professor in MIT’s Department of Civil and Environmental Engineering who also studies structural proteins found in bone and collagen, explained: “The application of engineering mechanics to understand the process of rapid aging disease may seem odd, but it actually makes a lot of sense.” In this new investigation, Buehler worked together with Kris Dahl, professor of biomedical engineering and chemical engineering at Carnegie Mellon, and graduate students Zhao Qin of MIT and Agnieszka Kalinowski of Carnegie Mellon.

In its natural state, a protein (and its tail) exist in complex folded configurations that alter for each protein type. Several misfolded proteins are linked with diseases. Qin and Buehler discovered in molecular simulations, that the healthy lamin A protein tail unravels sequentially along its backbone strand, one amino acid at a time.

Qin said:

“It behaved much as if I pulled on a loose thread on my shirt cuff and watched it pull out stitch by stitch.”

When pulled, the mutant protein first breaks almost in half, marking a big gap near the middle of its folder structure, and then starts unfolding in sequence. The MIT investigators concluded that an extra 70 kilocalories per mole (a unit of energy) is needed in order to straighten the mutant tails, which means that the mutant protein is actually more stable than healthy proteins.

At Carnegie Mellon, Dahl and Kalinowski also investigated this topic by subjecting lamin A protein tails to heat, which causes proteins to denature or unfold. And like the MIT engineers they saw the same pattern of unraveling in healthy and mutated proteins.

After, Qin wrote a mathematical equation to convert the temperature differential observed in denaturing the mutant and healthy proteins (4.7 degrees Fahrenheit) to the unit of energy discovered in the atomistic simulations, he found that the rise in temperature almost matched the increase in energy. The investigators say: “This agreement validates the application of the civil engineering methodology to the study of the mutated protein in diseased cells.”

However, to the civil engineers who are used to flawed materials being weaker – not stronger – than their unimpaired counterparts, the results were counterintuitive.

As a component of the cell’s nucleoskeleton, lamin A plays an crucial role in defining the mechanical properties of a cell’s nuclear membrane, which must stay flexible enough to easily withstand deformation. In prior studies, Dahl had seen that nuclear membranes built from the mutated proteins become very stiff and brittle, which may explain the altered protein-DNA and protein-protein interactions seen in diseased cells.

Buehler explained:

“Our surprising finding is that the defective mutant structure is actually more stable and more densely packed than the healthy protein. This is contrary to our intuition that a ‘defective’ structure is less stable and breaks more easily, which is what engineers would expect in building materials. However, the mechanics of proteins is governed by the principles of nanomechanics, which can be distinct from our conventional understanding of materials at the macro scale.”

Written by Grace Rattue