Facilitated nuclear transport of histone H1 and other small nucleophilic proteins.

Upon microinjection into the cytoplasm, three small nonnuclear (extracellular or mitochondrial) proteins diffused into nuclei of chilled or energy-depleted cells. In contrast, the facilitated transport of two large nuclear localization signal (NLS)-containing proteins was reversibly arrested by chilling or energy depletion. Surprisingly, the transport of two small nucleophilic proteins, histone H1 and P(Lys)-cytochrome c (cytochrome c cross-linked with synthetic peptide NLSs), was also arrested by either chilling or energy depletion. In situ titration studies indicate that the transport arrest of H1 in chilled cells is mediated by a cytoplasmic receptor. Therefore, even though they are potentially able to diffuse into nuclei, histones and other small NLS-containing proteins are localized by a receptor-mediated process that precludes their diffusion through the nuclear pores.