This entry represents the GAF domain and domains sharing protein structural similarity, such as the C-terminal domain of the transcription regulator IclR, the C-terminal domain of the heat-inducible transcription repressor HrcA, and the phytochrome chromophore attachment domain.

The GAF domain is named after some of the proteins it is found in, including cGMP-specific phosphodiesterases, adenylyl cyclases and FhlA. It is also found in guanylyl cyclases and phytochromes [1, 2]. The structure of a GAF domain shows that the domain shares a similar fold with the PAS domain [3]. Adenylyl and guanylyl cyclases catalyse ATP and GTP to the second messengers cAMP and cGMP respectively; these products up-regulate catalytic activity by binding to the regulatory GAF domain(s). The opposite hydrolysis reaction is catalysed by phosphodiesterase. cGMP-dependent 3',5'-cyclic phosphodiesterase catalyses the conversion of guanosine 3',5'-cyclic phosphate to guanosine 5'-phosphate. Here too, cGMP regulates catalytic activity by GAF-domain binding.

Phytochromes are regulatory photoreceptors in plants and bacteria which exist in two thermally stable states that are reversibly inter-convertible by light, the Pr state absorbs maximally in the red region of the spectrum, while the Pfr state absorbs maximally in the far-red region [4].

1. The GAF domain: an evolutionary link between diverse phototransducing proteins. Trends Biochem. Sci. 22, 458-9
2. Cyclic nucleotide binding GAF domains from phosphodiesterases: structural and mechanistic insights. Structure 17, 1551-7
3. Structure of the GAF domain, a ubiquitous signaling motif and a new class of cyclic GMP receptor. EMBO J. 19, 5288-99
4. Phytochrome three-dimensional structures and functions. Biochem. Soc. Trans. 38, 710-6

This entry represents the GAF domain and domains sharing protein structural similarity, such as
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