Progress in research on bacterial HD-GYP domain proteins
Bis-(3′, 5′) cyclic di-guanylate (c-di-GMP) is an almost ubiquitous intracellular second messenger in bacteria. Now it is known to regulate complex physiological processes, including mobility, adhesion, virulence and biofilm formation. The level of c-di-GMP is regulated by diguanylate cyclases (DGCs) containing GGDEF domains and phosphodiesterases (PDEs) containing EAL or HD-GYP domains. Recent studies have demonstrated that HD-GYP domain protein is a novel phosphodiesterase, which is also involved in the regulation of c-di-GMP degradation. This review highlights recent advances in the structure and biochemical functions of HD-GYP domain proteins, which might help to further clarify the mechanism of c-di-GMP signal system. 
 
 
Key words: 
Cyclic diguanylate; Phosphodiesterase; HD-GYP domain