Citron/A0276

Rho-interacting serine/threonine kinase 21, also known as Citron (CIT or A0276), has kinase activity at tyrosine amino acids.

Rho-interacting serine/threonine kinase 21, also known as Citron (CIT or A0276), has kinase activity at tyrosine amino acids. It is highly expressed in the mouse embryonic brain, and after birth it is restricted to neuronal precursors. It is found in the cell cytosol. Citron binds the guanosine triphosphate (GTP)-bound form of the small monomeric G proteins RHO and RAC and is involved in cytokinesis (cell division) and apoptosis. Citron contains 2055 amino acids with a molecular weight of 235 kDa. The structure includes kinase, plekstrin homology, RHO/RAC-interacting and SH3-binding domains. A short isoform of 54 kDa is also expressed, comprized mainly of the kinase domain. The gene is found on chromosome 5 in the mouse. As expected from its expression and function, Citron plays a major role in neurogenesis and brain development. Citron knock-out mice die young, exhibit tremors and seizures and show severe defects in many brain structures, including the hippocampus and cerebellum and an overall reduction in brain size. Owing to similarity with human autosomic recessive lissencephalies (meaning ‘smooth brains’), Citron has been proposed as possible molecular basis for these syndromes.