In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Biology

In the

School of Biological Sciences

Zahra Nassiri Toosi

Will defend her dissertation

Functional roles and underlying mechanism of site-specific N-terminal phosphorylation in a heterotrimeric G protein gamma subunit

Tuesday, April 13th, 2021

1:00 PM

https://bluejeans.com/4537128340

  Thesis Advisor:

Dr. Matthew Torres, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Committee Members:

Dr. Liang Han, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Dr. Brian Hammer, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Dr. Amit Reddi, Ph.D.

School of Chemistry and Biochemistry

Georgia Institute of Technology

Dr. Randy Hall, Ph.D.

Department of Pharmacology and Chemical Biology

Emory University School of Medicine

ABSTRACT: Heterotrimeric G-proteins (consisting of Ga, Gb, and Gg subunits) transduce extracellular signals such as hormones and neurotransmitters into intracellular responses that enable cells to communicate with their environment. Dysregulation of G-protein signaling pathways have significant implications in cardiovascular disease, diabetes, and cancer. Therefore, understanding the underlying signaling and regulatory mechanisms of the G-protein signaling systems is of prime importance. While several regulatory roles have been associated with Ga and Gb subunits, Gg subunits, the smallest components of the heterotrimeric G-protein complex, have been long recognized solely as membrane anchors for Gb subunits. Here, I will demonstrate a novel regulatory role for Gg subunits that is mediated through combinatorial phosphorylation of their intrinsically disordered N-terminal (Nt) tails. Using the yeast model system, I show that the Gg subunit (Ste18) undergoes multi-site phosphorylation within its disordered Nt tail in response to a range of different stimuli, such as G-protein activation and cellular stress. Phosphorylation exhibits inter-site interactivity that is controlled by multiple kinases that produce differential effects on the structure and function of Ste18-Nt subunits and output from the G protein signaling pathway. Taken together, my work sheds light on a new multi-site phosphorylation code in Gg subunits that facilitates their function as governors of G-protein signaling. Similar such codes have been described for other important proteins including histones, RNA polymerases, and G protein coupled receptors, thus placing Gg subunits among the list of intrinsically disordered proteins that exploit combinatorial post-translational modification to govern signaling pathway outputs.