Attempts to elucidate the mechanisms of intracellular signal transduction at the molecular level

-Calcineurin and Ca2+ mediated intracellular signal transduction

Calcineurin is a serine/threonine phosphatase whose enzymatic activation is directly regulated by Ca2+/calmodulin. This enzyme is responsible for a variety of important roles in Ca2+-mediated intracellular signaling pathways, from control of gene expressions to regulation of protein functions. It also contributes to the regulation of immune response, neurotransmitter release, stress response, muscle remodeling, and other various biological functions. Calcineurin can be found ubiquitously in eukaryotic organisms from yeast to humans, and it is known that the intracellular signal transduction pathways involving calcineurin utilize a universal mechanism.

-Calcineurin and calcineurin-regulating proteins

Compared to over 400 types of serine/threonine kinases, only 25 types of serine/threonine phosphatases exist. However, recent studies suggest that serine/threonine phosphatases are capable of forming complexes with regulatory proteins to generate functional diversity. One enzyme can thus bind to different regulatory proteins to control its activity, subcellular localization, and tissue specificity. In the laboratory of bioorganic chemistry, we focus on the regulatory proteins of calcineurin to determine the mechanism by which these proteins influence the function of calcineurin at only the specified time and specified place.

(1) Regulatory proteins that link calcineurin and small G-protein

There is a testis-specific isoform of calcineurin that is assumed to have an important role in the testis; however, the details of its function are unknown. We have identified the calcineurin-binding protein (CaNBP: CaNBP75 or RanBP3) in the testes. This protein can also bind to the small G protein, Ran, which is known as a regulator of nucleocytoplasmic transport. Recently, Ran has been shown to act as a molecular switch to a variety of processes, including spindle formation and reconstruction of the nuclear membrane after cell division. We believe that CaNBP serves as a linker between Ca2+/calmodulin signal transduction pathways through CaN and GTP/GDP-Ran signaling pathways.

(2) Regulatory proteins that act as “activator” and “inhibitor” of calcineurin

RCAN1, a regulatory protein of calcineurin, is a eukaryotic protein that is involved in calcineurin pathways. RCAN1 has been shown to activate and inhibit its target, calcineurin; however, the regulatory mechanisms for these opposing functions remain elusive. It has been suggested that one of the causes of Down’s syndrome in humans is the inhibition of calcineurin by RCAN1. Our bioorganic chemistry laboratory is attempting to elucidate the mechanisms of positive/negative calcineurin regulation by RCAN1 using both functional and structural analysis.