EFFECTS OF MACROMOLECULAR CROWDING ON NUCLEAR SIZE

The concentration of macromolecules inside cells is high, and the resu ltant crowding of cytoplasm can be expected to affect many interaction s involving macromolecular assemblies, Here, we have examined the effe ct of solute size and concentration on nuclear volume in saponin-perme abilized macrophages. Nuclei swelled in the presence of small solutes and shrank reversibly in the presence of larger permeant solutes. Rema rkably, the smallest solutes capable of shrinking the nucleus were not excluded by the pores in the nuclear envelope, Indeed, nuclei shrank in the presence of such solutes even after the nuclear envelope had be en sheared mechanically or permeabilized with detergent. Nuclei extrac ted with 1% Triton X-100 shrank in the presence of very high concentra tions of small solute molecules (30% w/v) as well as in lower concentr ations of larger solutes. Consistent with a macromolecular crowding ef fect, changes in nuclear volume were dependent on solute size and not simply dependent on the colligative properties of solutes or the exclu sion of solutes by the nuclear envelope. Solute size-dependent changes in nuclear volume were independent of the chemical nature of the solu tes and of the activity of the ions in the buffer. Together, these obs ervations indicate that high concentrations of macromolecules such as those found inside cells can influence the size of the nucleus by dire ctly affecting nuclear structure. (C) 1995 Academic Press, Inc.