Wilhelm bernhard workshop on the cell nucleus

H1 SUBTYPE COMPOSITION AND AFFINITY FOR CHROMATIN IN SITU IN NUCLEATED MATURE ERYTHROCYTES

Koutzamani E.¹, Loborg H.¹, Sarg B.², Lindner H.H.², and Rundquist I¹.

¹Department of Biomedicine and Surgery, Division of Cell Biology, Faculty of Health Sciences, Linköpings Universitet, Sweden, and ²Department of Medical Chemistry and Biochemistry, University of Innsbruck, Austria
The linker histones, commonly referred to as H1, are involved in the formation and maintenance of the higher order structure of the chromatin fiber and most likely also in epigenetic modulation of gene expression. This family consists of seven subtypes of which the highly specialized isoforms H5 and H1° accumulate in some terminally differentiated cells. In avian and amphibian erythrocytes, expressing H5 and H1° respectively, these proteins have been linked to cessation of cell proliferation and condensation of chromatin. The aim of this study was to analyse the affinity of linker histones for chromatin in these cell types, having similar biological features, but containing different subsets of linker histones. Using 4´6-diamidino-2-phenylindole (DAPI) as an indirect cyto-chemical probe for linker histone affinity in situ, we detected a substantial difference in the association of linker histones for chromatin between these two cell types. Linker histones in chicken erythrocytes were more strongly bound to chromatin than H1 in frog erythrocytes. Linker histone dissociation from chromatin was also analysed using electrophoretic and chromatrographic methods. The results showed that H5 dissociated at higher ionic strength than H1°, indicating a stronger affinity for chromatin. We also found that the amount of H5 in chicken erythrocyte nuclei was about 60% of the total amount of linker histones. In frog erythrocytes, H1° represented about 15% of the total amount of linker histones. Taken together, our results show that linker histones have a substantially higher affinity for chromatin in chicken erythrocytes than in frog erythrocytes. This difference can be explained by the high content of arginine-rich H5 in chicken erythrocytes. Our results also show a lack of correlation between linker histone affinity and chromatin condensation.