Chromatin structure in granulocytes. A link between tight compaction and accumulation of a heterochromatin-associated protein (MENT)

To study the mechanism of heterochromatin formation in vertebrate cells, we isolated nuclei from chicken polymorphonuclear granulocytes and examined the chromatin organization. We found granulocyte chromatin to remain insoluble after nuclease digestion and to be resistant to swelling in low salt/high pH media. Both insolubility and resistance to swelling were lost after washing with 0.3 M NaCl, a procedure that released two abundant tissue- specific proteins from granulocyte nuclei. One of them (42 kDa) is identified as MENT, a protein previously shown to be associated with repressed chromatin from mature chicken erythrocytes. We show that MENT is immunolocalized in granulocyte heterochromatin, where it is one of the most abundant chromatin proteins (~2 molecules/200 base pairs of DNA). MENT is the first nuclear protein structurally related to the serine protease inhibitor family. The other abundant protein is similar to or identical with mim-1, a myeloid- specific protein that is known to be stored in cell granules and to associate with isolated nuclei. MENT (but not mim-1) binds chromatin and free DNA, and, at its physiological protein/DNA ratio, enhances compaction and the reversible Mg²⁺-dependent self-association of nucleosome arrays. MENT appears to promote the formation of heterochromatin by acting as a 'glue' within and between chains of nucleosomes.