N-terminal domain of transcription factor Specificity Protein (SP) 3Specificity Proteins (SPs) are transcription factors that are involved in many cellular processes, including cell differentiation, cell growth, apoptosis, immune responses, response to DNA damage, and chromatin remodeling. SP1 and SP3 can interact with and recruit a large number of proteins including the transcription initiation complex, histone modifying enzymes, and chromatin remodeling complexes, which strongly suggest that SP1 and SP3 are important transcription factors in remodeling chromatin and the regulation of gene expression. SP3 belongs to a family of proteins, called the SP/Kruppel or Krueppel-like Factor (KLF) family, characterized by a C-terminal DNA-binding domain of 81 amino acids consisting of three Kruppel-like C2H2 zinc fingers. These factors bind to a loose consensus motif, namely NNRCRCCYY (where N is any nucleotide; R is A/G, and Y is C/T), such as the recurring motifs in GC and GT boxes (5'-GGGGCGGGG-3' and 5-GGTGTGGGG-3') that are present in promoters and more distal regulatory elements of mammalian genes. SP factors preferentially bind GC boxes, while KLFs bind CACCC boxes. Another characteristic hallmark of SP factors is the presence of the Buttonhead (BTD) box CXCPXC, just N-terminal to the zinc fingers. The function of the BTD box is unknown, but it is thought to play an important physiological role. Another feature of most SP factors is the presence of a conserved amino acid stretch, the so-called SP box, located close to the N-terminus. SP factors may be separated into three groups based on their domain architecture and the similarity of their N-terminal transactivation domains: SP1-4, SP5, and SP6-9. The transactivation domains between the three groups are not homologous to one another. SP1-4 have similar N-terminal transactivation domains characterized by glutamine-rich regions, which, in most cases, have adjacent serine/threonine-rich regions. This model represents the N-terminal domain of SP3.