Life in a crowded world.

Introduction Macromolecules are present as soluble species and/or structural arrays at total concentrations of up to several hundred grams per litre in essentially all physiological compartments. Although local composition varies widely between different systems, it is evident that most macromolecular reactions and processes in vivo—as opposed to typical experiments in vitro in which the total concentration of macromolecules rarely surpasses 1gl−1 (Ralston, 1990; Ellis, 2001)— take place in environments in which macromolecules occupy a considerable fraction (between 10% and 40%) of the total volume (Fulton, 1982; Record et al, 1998). These media are termed ‘crowded’ or ‘volume-occupied’ rather than ‘concentrated’, because no single species of macromolecule is necessarily present at a high concentration. The term ‘macromolecular crowding’ connotes the non-specific influence of steric repulsions on specific reactions and processes that occur in highly volume-occupied media. During recent decades it has gradually become recognized that crowding can considerably alter the reactivity of individual macromolecules, both qualitatively and quantitatively. Crowding can be mimicked experimentally by adding high concentrations of inert synthetic or natural macromolecules, termed crowding agents or crowders, to the system in vitro (Ellis, 2001). Experimental and theoretical work has demonstrated substantial (order-of-magnitude) effects of crowding on a broad range of biochemical, biophysical and physiological processes, including—but not limited to—nucleic acid and protein conformation and stability, protein–protein and protein–DNA association equilibria and kinetics (including protein crystallization, protein fibre formation and bundling), catalytic activity of enzymes and cell volume regulation (Zimmerman & Minton, 1993; Minton 1997, 2001; Ellis, 2001). The fact that biological macromolecules have evolved to function in such crowded environments raises biologically important questions. Why did cells evolve such a highly packed interior? Are there any advantages in being crowded? How do macromolecules fold, associate and travel through the crowded intracellular medium? (Fulton, 1982; Zimmerman & Minton, 1993; Bray, 1998). The first international workshop on the Biological Implications of Macromolecular Crowding brought together a group of 60 theoreticians and experimentalists to exchange ideas on topics that fell into three categories: first, theoretical predictions of the effects of excluded volume on the rates and/or equilibria of macromolecular reactions; second, experimental and simulation approaches that can be applied to the quantitative characterization of biochemical and biophysical processes in crowded media, both in vitro and in vivo; and last, experimental findings that indicate that a broad variety of biological phenomena and systems are likely to be substantially influenced by crowding. As many aspects of the theory of crowding are already well developed, this opportunity for interdisciplinary exchange came at a particularly appropriate time. Some conferences resemble medieval jousting matches during which opponents try to unseat their counterparts in well-known controversies; others are like marketplaces where proponents of new ideas, new theories and new experimental techniques present their ‘wares’ for acquisition by others who may find that they fill a particular need. Despite the dramatic venue of the conference (namely, a castle in Avila), this meeting had more in common with the market than the tournament. A community of interest that really did not exist before the workshop was assembled, and the