Clusters of nucleosomes containing chromosomal protein HMG-17 in chromatin.

Chromosomal proteins HMG-14 and HMG-17 are nucleosome binding proteins which can function as architectural elements to alter the structure of the chromatin fiber and enhance transcription from chromatin templates. Here we study the spatial organization of these HMG proteins in the nucleus and the distribution of nucleosomes containing HMG-17 in the chromatin fiber. By confocal immunofluorescence microscopy we find that HMG-14/17 proteins are clustered into foci containing either HMG-14 or HMG-17. These results suggest that HMG-14/17 proteins segregate into distinct nuclear domains. Indeed, immunofractionation of defined length oligonucleosomes, with affinity pure antibodies to HMG-17, indicates that oligonucleosomes containing HMG-17 are devoid of HMG-14. Quantitative analysis indicates that in cellular chromatin nucleosomes containing HMG-17 are clustered. The average size of the cluster is six contiguous HMG-17-containing nucleosomes. The nucleosomes in this cluster contain either two or zero molecules of HMG-17 and a complete set of four core histones. We suggest that HMG-14/17 proteins modify the nucleosomal organization of the 30 nm chromatin fiber, to unfold the higher order chromatin structure and facilitate access to the underlying DNA sequence. Clustering of architectural elements, such as HMG proteins and linker histone subtypes into distinct domains, may lead to structural and functional heterogeneity along the chromatin fiber.