BPS2026 – The function and conformation of protein disordered regions is encoded in their modular architecture

The structure-function paradigm affords interpretability to protein folded regions. However, greater than 30% of the human proteome consists of intrinsically disordered regions (IDRs) which lack stable secondary structure and yet carry out essential biological functions. It is currently unknown whether IDRs are subject to generalizable organizational principles that could provide a priori mechanistic or functional information. Inspired by the conspicuous sequence organization of low-complexity IDRs, we recently developed a statistical approach to quantify linear variance in amino acid composition. Applying this algorithm on a proteome-wide level reveals that IDRs are built from juxtaposed “modules” of distinct compositional bias that provide a sequence-spanning level of organization. We hypothesized that IDR functionality and conformation derive from their modular architecture. To test this, we have determined how modularity impacts the function of LCE3A, an anti-microbial IDR produced in the epidermal layer of human skin. Using functional assays, we find that LCE3A’s anti-microbial activity is conferred by a single internal module, the so-called “basic” module. To test if composition—the criterion upon which modules are identified—is alone sufficient to define function, we assayed the anti-microbial activity of randomly scrambled variants of the basic module and of compositionally similar sequences found in the human proteome. Each of these variants exhibited similar anti-microbial activity as the basic module. Finally, we took advantage of the high-cysteine content of LCE3A (∼20%) to map disulfides and determine how modularity impacts IDR conformation. Akin to the domain architecture of globular proteins, these studies demonstrate that modules represent functionally separable units of an IDR and that module function is encoded simply by sequence composition. Altogether, these studies provide a new paradigm for dissecting and predicting the functional elements of protein intrinsically disordered regions.