Effect of growth state and heat shock on nucleolar localization of the 110,000-Da heat shock protein in mouse embryo fibroblasts.

We have shown previously that the mammalian 110,000-Da heat shock protein (hsp110) associates with nucleoli in several cell types and that in 2-day postconfluent mouse 10T1/2 cells, a segregation of the antigen from the nucleolar phase-dense body is seen (J.R. Subjeck, T. Shyy, J. Shen, and R.J. Johnson. J. Cell Biol. 97: 1389-1395, 1983). Here we further characterize the nucleolar segmentation of hsp110 in mouse 10T1/2 and 3T3 cells with respect to the formation of this structure in dense cultures and investigate the behavior of this protein following conditions (serum deprivation, actinomycin D, and heat shock) known to affect the functional and morphological integrity of the nucleolus. It is shown that in addition to its nucleolar locale, an affinity of hsp110 for the nonnucleolar, nuclear compartment in actively proliferating cells is also observed. When proliferating cells are treated with actinomycin D (1 microgram/ml) for 8 h, hsp110 separates from the nucleolar phase-dense body to form a fluorescent nucleolar cap which resembles that seen in confluent cultures. This drug also results in a disappearance of hsp110 from the nucleoplasm. Incubation of cells for 24 h in media without serum also results in the nucleolar segmentation of hsp110 and a reduction in nucleoplasmic staining. A moderate nonlethal heat treatment does not lead to segmentation of hsp110 in proliferating cells but conversely results in a transient reversal of segmentation in confluent cultures. Examination of segmented nucleoli of postconfluent cells by immunoelectron microscopy reveals that hsp110 is associated with the fibrillar component of these nucleoli, the site of ribosomal DNA.