Date of Award


Publication Type

Doctoral Thesis

Degree Name



Biological Sciences


Biology, Cell.


Dr. Lisa Porter (Department of Biological Sciences)




Cell growth and proliferation are tightly controlled by the cyclic regulation of the cyclin dependent kinases (Cdks). Cdks are positively regulated through interactions with regulatory Cyclin partners as well as being negatively regulated through interactions with families of Cdk inhibitors (CKIs). The Spy1/RINGO family of proteins have emerged as a unique class of Cyclin-like proteins capable of directly binding both to the Cdks, as a positive regulatory partner, as well as to at least one member of the CKI's, p27Kip1, as a negative regulator. Abnormally elevated levels of Spy1 promote cell proliferation, nhibit apoptosis and are implicated in aggressive tumorgenesis in all cell/tissue types studied to date. Understanding how Spy1 protein is regulated is essential in resolving how it contributes to normal and abnormal growth processes. Herein, we demonstrate that Spy1 is degraded in a cell-cycle-dependent manner via the ubiquitin-proteasome system. We have resolved the E3 ligase and essential phosphorylation sites mediating Spy1A degradation. Furthermore, we show that Spy1 protein is stabilized in subsets of human breast cancer samples. Using a stable mutant of Spy1 we demonstrate that this represents an oncogenic modification in vitro and accelerates tumor formation in vivo. We further show that these oncogenic properties are largely dependent upon the unique activation of Cdk1 and the subsequent inhibition of the anti-apoptotic regulator FOXO1. Utilizing Spy1 mutants unable to bind to the primary effectors p27 and Cdk2, we have found that Spy1-mediated effects in the breast rely on direct interactions with each of these effectors via separable functional mechanisms. This work reveals novel mechanisms regulating the progression, and potentially the etiology, of human breast cancers and may be of considerable therapeutic relevance.