New work designates the protein RCN1 as a harness responsible for restraining the protein calcineurin, whose activity is essential for proper regulation of immune, muscle and brain cells. However, as evidenced by links to osteoporosis and Down’s syndrome, unleashed or excessive calcineurin function can result in devastating biological consequences. Thus, calcineurin acts as a delicate pivot point on which physiological homeostasis rests.
Prior work aligned calcium ‘upstream’ of calcineurin, and RCN proteins ‘downstream’ of calcineurin in intracellular signaling pathways. However, confusing data indicating that RCN proteins can both inhibit and enhance calcineurin activity remained unresolved.
A group led by Tsutomu Kishi, a scientist at the RIKEN Frontier Research System in Wako, set out to understand the molecular process through which RCN proteins influence calcineurin activity. Their findings were published in a recent issue of the Proceedings of the National Academy of Sciences of the USA1.
The researchers hypothesized that RCN proteins might be controlled by SCFCdc4, a protein complex that binds to and routes substrate proteins for destruction. Conducted in yeast cells, an unbiased screen for proteins interacting with SCFCdc4 confirmed their suspicions. Experiments using mutant yeast cells firmly designated SCFCdc4 as essential for destabilization and degradation of RCN1, and showed that RCN1 destruction is required for calcineurin activation.
As RCN1 is degraded in a dynamic rather than constitutive manner, the team sought to identify the mechanism responsible for ‘tagging’ RCN1 for destruction. Their focus on Mck1, a protein already established as capable of phosphorylating RCN1, proved fruitful. Mutant RCN1 proteins lacking the serine residues phosphorylated by Mck1 were resistant to SCFCdc4-mediated degradation.
Adding to the complexity of calcineurin control mechanisms, the researchers noted that calcineurin, a phosphatase capable of dephosphorylating proteins, effectively counteracted Mck1-mediated RCN1 phosphorylation.
Thus it appears that a ‘feedback loop’ regulates calcineurin activity (Fig. 1). Calcium flux stimulates Mck1-mediated phosphorylation of RCN1. SCFCdc4 targets phosphorylated RCN1 for destruction and releases calcineurin function. By synthesizing, dephosphorylating and stabilizing RCN1, activated calcineurin then suppresses its own activation.
Additional work is needed to understand the factors capable of influencing the direction in which this regulatory cycle spins. “We believe that cellular regulation by this feedback loop and the selective degradation of feedback inhibitors might be a fundamental strategy to control cellular signals,” says Kishi.
1. Kishi, T., Ikeda, A., Nagao, R. & Koyama, N. The SCFCdc4 ubiquitin ligase regulates calcineurin signaling through degradation of phosphorylated Rcn1, an inhibitor of calcineurin. Proceedings of the National Academy of Sciences USA 104, 17418–17423 (2007).
