PROTEIN HETEROGENEITY IN THE COILED BODY COMPARTMENT

Coiled bodies are ubiquitous nuclear inclusions of unknown function. A lthough a considerable list of coiled body components has been assembl ed in recent years leading to several functional hypotheses, none have yet been borne out by experimentation. Pinpointing coiled body functi on is difficult in part because each known component molecule has been shown to be present at other sites in the nucleus. Using probes to in dividual coiled body molecules is therefore likely to yield ambiguous results. From direct observation of coiled body behavior we know that they are dynamic structures, changing in content, size, and number und er different physiological conditions. In our studies, we have found t hat the number of coiled bodies in mammalian endothelial cells is rela tively high. Depending on phenotype, quiescent or angiogenic, endothel ial cells can average as few as 4 or as many as 15 coiled bodies per n ucleus (as opposed to 2 or 3 for most cell types). This can provide ce rtain advantages in the analysis of their dynamics and composition. Mo reover, expression of the coiled body protein, pigpen, is sharply regu lated as endothelial cells toggle back and forth between the two pheno types. Using the endothelial cell system, we present several new obser vations in this report on the dynamics of coiled bodies and their cons tituent proteins and reinforce prior observations that we consider imp ortant but understated in the literature. With antibodies to p80-coili n, pigpen, and fibrillarin, we show that there may be heterogeneity in the coiled body population of individual cells. We demonstrate that t he coiled body marker protein p80-coilin can also be found distributed in the nucleoplasm and in apparent association with the nuclear envel ope. This suggests that coilin could play a role in some aspect of nuc leocytoplasmic exchange. Finally, we correlate the presence of pigpen in a diffuse nucleoplasmic pool with the expression of a phosphatase-s ensitive epitope, indicating that subnuclear localization may depend u pon the phosphorylation state of the protein. Our results suggest to u s that a viewpoint of coiled bodies as part of a fluid trafficking net work may be helpful in discerning their cellular functions. (C) 1998 A cademic Press.