InterPro domain: IPR001180

Based on sequence similarities a domain of homology has been identified in the following proteins [ 1 ]:

• Citron and Citron kinase. These two proteins interact with the GTP-bound forms of the small GTPases Rho and Rac but not with Cdc42.
• Myotonic dystrophy kinase-related Cdc42-binding kinase (MRCKalpha). This serine/threonine kinase interacts with the GTP-bound form of the small GTPase Cdc42 and to a lesser extent with that of Rac.
• NCK Interacting Kinase (NIK), a serine/threonine protein kinase.
• ROM-1 and ROM-2, from yeast. These proteins are GDP/GTP exchange proteins (GEPs) for the small GTP binding protein Rho1.

This domain, called the citron homology domain, is often found after cysteine rich and pleckstrin homology (PH) domains at the C-terminal end of the proteins [ 2 ]. It acts as a regulatory domain and could be involved in macromolecular interactions [ 2 , 2 ].

1. The myotonic dystrophy kinase-related Cdc42-binding kinase is involved in the regulation of neurite outgrowth in PC12 cells. J. Biol. Chem. 274, 19901-5
2. NIK is a new Ste20-related kinase that binds NCK and MEKK1 and activates the SAPK/JNK cascade via a conserved regulatory domain. EMBO J. 16, 1279-90