A novel nonhistone protein (MENT) promotes nuclear collapse at the terminal stage of avian erythropoiesis.

The terminal stage of differentiation of nucleated chicken erythrocytes is associated with an overall gene repression and a condensation of the repressed chromatin portion. Two-dimensional DNP electrophoresis has been used to separate transcriptionally active and repressed chromatin of mature chicken erythrocytes. The repressed chromatin fraction is shown to be enriched with histone H5 as well as with a 42-kDa nonhistone chromosomal protein. The 42-kDa protein designated here as MENT (mature erythrocyte nuclear termination stage-specific protein) is hyperexpressed at the terminal stage of chicken erythropoiesis and is accumulated in adult chicken erythrocyte nuclei. This protein was purified by ion-exchange chromatography from 0.4 M NaCl extracts of the erythrocyte nuclei. It appeared to be a basic polypeptide (pI 9.2) which, however, precipitated at low pH. When reconstituted in vitro with immature erythrocyte nuclei, MENT promoted condensation of intact nuclear chromatin and enhanced the solubilization of nuclease-digested polynucleosomes, thus mimicking the processes occurring in vivo at the final stage of erythrocyte maturation. The extent of dissociation of specific gene sequences from the nuclear matrix in MENT-treated nuclei is in striking correlation with their transcriptional activity. No other basic proteins (H5, cytochrome c, RNase A) added to the nuclear preparation at the same level as MENT (protein/DNA = 0.005) caused any effect on nuclear organization. No alterations were observed when MENT was mixed with erythroblasts and nonerythroid nuclei having little or no histone H5. We propose that MENT cooperates with histone H5 to complete the nuclear collapse in mature nucleated erythrocytes.