zinc finger (thing)

The zinc finger is a sequence motif involved in binding of DNA:

The C₂H₂ class

 	Cys-(Xaa)2-Cys-(Xaa)12-His-(Xaa)3-His.

Xaa - nonspecific amino acid.

This motif was first discoved in TFIIIA ( an RNA polymerase III associated transcription factor) isolated from Xenopus laevis (African clawed toad). In TFIIIA, this sequence is repeated nine times in the protein. Each repeat can coordinate a zinc ion with the two cysteines and two histidines:

        ======
       =      =
      =        =
       =      =
      CYS    HIS
       = \  / =
       =  Zn   =
      CYS/  \ =
       =     HIS
       =      =
      =        =
 =====          =====>

The twelve residues between the cysteine and histidine loop out to form a DNA binding interface. When TFIIIA associates with DNA, it binds along the major groove with each finger binding about five nucleotides of DNA. In all cases known, the finger is repeated at least two times. Successive fingers are linked by seven or eight amino acids which form a helical spacer. These modular units may have evolved to make DNA binding surfaces more adaptable to changes in DNA sequence.

The C_x class

The C_x class of zinc fingers have a variable number of cysteines that can chelate a Zn ion. These are also involved in DNA binding such as the GAL4 protein (yeast transcripiton factor involved in galactose metabolism.) They're also found in a number of steriod binding proteins which act as transcription factors. The cysteines are closely spaced and can vary from 4 to 6 in number.

        ======
       =      =
      =        =
       =      =
      CYS    CYS
       = \  / =
       =  Zn   =
      CYS/  \ =
       =     CYS
       =      =
      =        =
 =====          =====>