Year of Publication

2016

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Molecular and Cellular Biochemistry

First Advisor

Dr. Trevor P. Creamer

Abstract

Calcineurin is a Ser/Thr phosphatase whose function is implicated in critical physiological processes such as immune system activation, fetal heart development, and long-term depression in neurons. Calcineurin has been implicated in the progression of Alzheimer’s disease and cardiac hypertrophy. It is not well understood how calcineurin is activated on a molecular level by Ca2+ and its activating protein calmodulin. Previous data from our lab show that calmodulin interaction induces the folding of the intrinsically disordered regulatory domain of calcineurin in two discrete and distant regions into α-helical conformations and that this folding is critical for complete activation of calcineurin. It was also discovered that one of the helical elements which we call the “distal helix” was unstable at a human body temperature of 37°C in dilute buffer. This raises the question; how can a structure critical for the complete activation of calcineurin be unstable at average human body temperature? Proteins do not exist in solutions of the dilute buffer, but rather in a crowded cosmos that ranges between 200 and 400 g/L of macromolecules such as proteins, DNA, and other cellular components. We show here that phenomenon known as macromolecular crowding can stabilize the distal helix and that stabilization increases the activity of calcineurin at human body temperature.

Much about intrinsically disordered proteins (IDPs) remains a mystery, especially what influences the rate at which they interact with their target molecules. IDPs lack any sort of stable three-dimensional structure because of their lack of sufficient hydrophobic or aromatic amino acids while having a large proportion of polar and charged amino acids. Because of the high degree of charged amino acids, electrostatic forces play a significant role in their interaction other proteins. This is known to be the case for calmodulin which is net negatively charged protein that has over 300 binding targets of which are usually basic amphipathic alpha-helices. The calmodulin-binding site located in the intrinsically disordered regulatory domain of calcineurin is net positively charged, and, interestingly, is flanked by acidic patches on either side. These acidic patches might perturb attractive electrostatic forces between the calmodulin-binding site and calmodulin. Using fluorescence spectroscopy in conjunction with a stopped-flow apparatus to measure the kinetics between calmodulin and calcineurin we seek to characterize the influence of the steric and electrostatic forces between the two proteins.

Also, we present data on RCAN1-4 (Regulator of Calcineurin Isoform 1-4) which has been shown to be an inhibitor in some contexts and an activator of calcineurin in other. RCAN1-4 is expressed in the heart and its upregulation has been shown to prevent calcineurin-mediated pathological cardiac hypertrophy suggesting that it plays an inhibitory role in this context. The work shown demonstrates that RCAN1-4 is a competitive inhibitor of calcineurin and whose binding affinity is modulated by Ca2+/calmodulin. These data unveil a binding site utilized by RCAN1-4 which is commonly used among other calcineurin substrates.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2016.395

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