Structural properties of Sp1-like/KLF proteins. Sp1-like/KLF proteins have highly homologous carboxy-terminal DNA-binding domains characterized by three Cys2His2 zinc-finger motifs and recognizing GC-rich DNA elements, and variant amino termini. The members of the family can be classified into subgroups on the basis of common structural and functional features of the amino termini; these correlate well with the subgroups predicted by sequence similarities in Figure 1a. Some members of subgroup I (Sp1, Sp2, Sp3, and Sp4) contain glutamine-rich (Q) and serine/threonine-rich (S/T) amino-terminal transcription activation domains. Two members of subgroup III, KLF10 and KLF11, are TGFβ-inducible repressors and have three conserved amnio-terminal repression domains, including the Sin3 interaction domain (SID), which mediates interaction with the corepressor mSin3A. Three other members of subgroup III, KLF9, KLF13 and KLF16, are also characterized by a functional SID domain. KLF1, KLF2 and KLF4, which belong to subgroup II, are characterized by amino-terminal acidic activation domains, inhibitory regions adjacent to the zinc fingers and a conserved nuclear localization signal (NLS) sequence. In addition, KLF13 contains a similar nuclear localization sequence. Other members of subgroup II, KLF3, KLF8 and KLF12, have a conserved repression motif (PVALS/T) that interacts with the corepressor CtBP2.