FIGURE 1.

USF is subject to proteolytic cleavage by the calcium-sensitive protease m-calpain during erythroid differentiation. A, Western blot analysis of USF2 in uninduced MEL and DMSO (2%)-induced MEL cells. Whole cell protein extracts were isolated from the cells and subjected to Western blot analysis using antibodies specific for USF2. B, Western blot analysis of protein extracts from DMSO (2%)-induced MEL cells treated with recombinant m-calpain in the presence or absence of CaCl₂ and MgCl₂. MEL cells were incubated for 2 days in 2% DMSO to induce differentiation. Whole cell protein extracts from differentiated MEL cells were incubated in the absence or presence of different amounts of m-calpain as indicated as well as in the absence and presence of 5 mm CaCl₂ and MgCl₂. The protein extracts were subjected to Western blot analysis using a USF2-specific antibody.