Understanding the mechanism of self-limited assembly of tubules using Monte Carlo simulations

Self-assembly of biomacromolecules is an important approach that nature uses to build biological structures, such as ribosomes, microtubules and viral capsids. Unlike the self-assembly of most synthetic materials, which grow in an unbounded manner, many biological structures undergo self-limited assembly: The assembled structures have at least one finite dimension. An essential question is how such assemblies terminate their growth to select a well-controlled size. To answer this question, we use Monte Carlo simulations to study the kinetic pathways of a class of self-limited structures: cylindrical tubules that are assembled from triangular monomers. In addition to assembly of the target geometry, we observe that the same monomers also assemble into tubules with different widths and chiralities, due to thermal fluctuations and assembly-pathway-dependent kinetic effects. We show that the width distribution is determined by a competition between the monomer insertion rate and the structure closure rate, as well as the underlying free energy landscape. These results elucidate design principles for assembling self-limited structures from synthetic components, such as artificial microtubules that have a desired width and chirality.