Sp1 binding to the α2 integrin promoter requires phosphorylation. (A) Immunoblot analysis was performed by SDS-PAGE on 100 μg of total protein from cell lysates of uninduced K562 cells and K562 cells induced for 4 days with phorbol dibutyrate using a polyclonal antiserum specific for the Sp1 protein. A triplet of Sp1 bands was shown. (B) Gel mobility shift analysis was performed using 32P-end–labeled α2 integrin core promoter (−32 to −92 bp) and 2 μg of nuclear extracts from uninduced or phorbol ester-induced K562 cells (untreated), nuclear extracts dephosphorylated by calf intestinal phosphatase (dephosph), or nuclear extracts treated with 2 μg of calf intestinal alkaline phosphatase in the presence of phosphatase inhibitors (control). The two specific DNA-protein complexes are indicated with arrows.
