A high-mobility group 1 (HMG1) protein type isolated from murine erythroleukaemia (MEL) cells promotes acceleration of the differentiation process when added to a MEL cell culture together with the inducer hexamethylene bisacetamide. We now provide direct evidence that the presence of HMG1 protein in the extracellular medium is essential for terminal erythroid differentiation. An extracellular function for HMG1 protein in MEL cell is further supported by a demonstration that this protein is released from MEL cells exposed to the chemical inducer and that the addition of an anti-(HMG1 protein) monoclonal antibody to the cell culture inhibits the differentiation process almost completely. The release of HMG1 protein from MEL cells is modulated by compounds affecting cell calcium homoeostasis, such as a calcium ionophore or verapamil. In fact, in the presence of the ionophore an increased rate of differentiation is accompanied by an enhanced extracellular release of HMG1 protein, whereas in the presence of verapamil both phenomena are significantly decreased.

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