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:
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= =
= =
= =
CYS HIS
= \ / =
= Zn =
CYS/ \ =
= HIS
= =
= =
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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 Cx 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.
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= =
= =
= =
CYS CYS
= \ / =
= Zn =
CYS/ \ =
= CYS
= =
= =
===== =====>