Murine erythroleukemia (MEL) cells, in addition to an mRNA coding for a 30 kDa high mobility group (HMG)-1 protein, contain an mRNA coding for a 6 kDa HMG1 protein having the following structural properties: (1) its primary structure has 90% homology with the N-terminal sequence of the 30 kDa HMG1 protein; (2) it contains a consensus region of the HMG1 protein family; (3) it is deprived of the cluster of acidic amino acids that characterizes the C-terminal region of the 30 kDa HMG1 protein. This novel small M r HMG1 protein has been expressed in prokaryotic cells and tested to establish similarities and differences in activity compared to the homologous higher M r HMG1 protein. It has been found that the low M r HMG1 form is not released from MEL cells following induction to erythroid differentiation, but is still effective, although with much less efficiency, when added to the external medium, in promoting acceleration in the rate of MEL cell differentiation as well as in activation of α-protein kinase C. Altogether these results provide evidence for the presence in MEL cells of a multigene family that encodes at least two different HMG1-type sequences most presumably involved, at distinct cellular sites, in different functions although commonly related to the promotion of cell differentiation. Additional information can be considered concerning the relationship between the characteristic N-terminal sequence of HMG1 protein and the extracellular activity on MEL cell differentiation.
