Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular Biophysics and Biochemistry

First Advisor

Xiong, Yong

Abstract

Sterile α-motif and HD domain-containing protein 1 (SAMHD1) is a pivotal enzyme that maintains cellular nucleotide homeostasis. SAMHD1 forms a tetramer upon binding GTP and a dNTP at allosteric sites, activating its triphosphohydrolase activity against substrate deoxynucleoside triphosphates (dNTPs). This dNTPase activity, in combination with other functions in DNA repair, cell cycle regulation, and nucleic acid binding, is important to other facets of SAMHD1 biology as a critical mediator of antiviral defense, cancer suppression, and autoimmune regulation. An array of post-translational modifications on SAMHD1 present an additional layer of functional regulation – most notably phosphorylation at residue T592, a master switch that generally attenuates dNTPase activity and modulates SAMHD1’s other functions. Yet, precise mechanisms for SAMHD1 cellular functions remain unclear and previous efforts examining structural consequences of T592 phosphorylation are incomplete. Some clinically-relevant mutations decouple SAMHD1’s intertwined activities and allow us to parse the molecular determinants that contribute to disease. Here I present a structural analysis of cancer-associated SAMHD1 mutants that, in combination with functional data, show the importance of SAMHD1 dNTPase activity to avoid cancer proliferation. Furthermore, I present the first high resolution cryo-electron microscopy (cryo-EM) structures of full-length human SAMHD1 and a T592D phosphomimetic which reveal little difference and show extreme SAM domain flexibility. Collectively, these studies provide deeper insight into determinants of SAMHD1 function.

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