Nonspecific colloidal forces in biomacromolecular systems

Phosphorylation-dependent conformational changes in unstructured proteins. Increased phosphorylation produces increased intramolecular charge-charge repulsion, increasing the effective radius of the polypeptide (from Kumar and Hoh, 2004).

In a biological context, when two macromolecules are said to "interact," the usual implication is that they bind to one another in a highly specific manner facilitated by tight shape complementarity. In the tradition of polymer physics and complex fluids, however, "interactions" are frequently nonspecific and even repulsive (mediated, for example, by electrostatic, steric, and hydrophobic forces). While these forces nominally act on the nanoscale, in a concentrated environment they can drive self-assembly that produces structure on the microscale. These nonspecific colloidal forces are gaining increasing recognition as important players in biological systems.

For example, several structural proteins derive function from the fact that they are charged polymers, including neurofilaments, microtubule-associated proteins, milk caseins, and nucleporins. The importance of these nonspecific forces is implicit in the fact that the cytoplasm is literally stuffed with proteins and nucleic acids at concentrations approaching hundreds of grams per liter. Many disease states are characterized by the pathological aggregation of intracellular macromolecules. Understanding the nature of the forces that prevent aggregation and allow biochemistry to occur in highly congested environments bears fundamental implications for cell biology.

Atomic force microscopy imaging of neurofilaments (NFs). The dark regions around the NFs are zones from which contaminants in the preparation are excluded due to the brushlike motion of the unstructured NF sidearm domains. When the sidearms are dephosphorylated, the sidearms condense and the excluded volume falls dramatically (from Kumar and Hoh, 2004).

Nonspecific colloidal forces are tremendously important to biotechnological design, as well. Any technology intended to interface with the cell by mimicking the cell's environment must confront and integrate the electrostatic and steric properties of the cell surface. Moreover, protein-based biomaterials have been widely promoted as biocompatible alternatives to synthetic polymers. The effective design of these biomaterials demands an understanding of how chemical properties (i.e., primary sequence) gives rise to colloidal properties.

Thus, we would like to understand how the primary sequence and posttranslational modification of "colloidally-active" proteins gives rise to their physical properties. We are especially interested in whether multiphosphorylation domains may serve as a useful modules for graded control of conformational properties. The long-term goal is to develop a set of chemical rules that facilitates the de novo design of these proteins for specific technological applications. Our key tools here include atomic force microscopy imaging and force spectroscopy, scattering, and computational methods. Because nonspecific interactions between cytoskeletal polymers figure critically into their ability to organize into a three-dimensional network, we expect that an improved understanding of how polypeptide biochemistry translates into colloidal biophysics will feed back to our understanding of cell shape and mechanics.

Measures of organization in distributions of biomacromolecules. Structure in biomacromolecular distributions may be quantified through two useful metrics: the radial distribution function [g(r), top)], and the occupancy probability distribution [pn, bottom]. Together with computer simulations, these metrics enable us to deduce information about intermolecular forces.