The ubiquitin system plays a pivotal role in regulating protein degradation, endocytosis and numerous other cellular functions. E3 ubiquitin ligases, which mediate the final step in the ubiquitylation reaction cascade, are responsible for substrate recognition and ubiquitin attachment to them, underscoring the importance of identifying their substrates. Nedd4 family members are E3 ubiquitin ligases comprised of a C2-WW-HECT domain architecture. This thesis was aimed at first globally delineating the substrate specificity of the closely related Nedd4 family members in humans, hNedd4-1 (Nedd4) and hNedd4-2 (Nedd4L), and second, to follow up on one of the novel hits identified, the FGFR1, and study in detail how it is regulated by its E3 ligase hNedd4-1. To globally identify substrates for Nedd4 proteins, a high throughput proteomic screening technology using protein microarrays was employed. Despite the obvious homology in their domain architecture, the results presented here suggest that these Nedd4 family members may function non-redundantly, since they demonstrate a selective preference towards substrate ubiquitylation. Our focus on FGFR1, a substrate identified for hNedd4-1, has revealed an important functional role for this ubiquitin ligase in promoting FGFR1 endocytosis and downregulation of its signaling activity. The evidence presented indicates that this interaction has important consequences for developmental processes that are dependent on FGF signaling: human neural stem cell differentiation and zebrafish embryonic patterning and brain development. We demonstrate that the WW3 domain of Nedd4-1 recognizes a novel, non-canonical binding surface (peptide2) within the juxtamembrane region of FGFR1, and we are currently in the process of solving the 3 dimensional structure of the hNedd4-1 WW3: FGFR1 peptide2 complex using X-ray crystallography. Furthermore, in characterizing the interaction between hNedd4-1 and FGFR1, we have provided evidence for a novel mechanism for regulating the catalytic activity of hNedd4-1 by FGFR1 activation. This involves the formation of hNedd4-1 dimers upon removal of the autoinhibitory C2 domain from the HECT domain. Dimerized hNedd4-1, in turn, exhibits enhanced interactions with FGFR1 and enhanced receptor ubiquitylation. From these data, we proposed a negative feedback inhibitory model for FGFR1 downregulation, whereby activated receptor enhances the activation of its suppressor, hNedd4-1, to ensure timely termination of receptor signaling.

The ubiquitin system plays a pivotal role in regulating protein degradation, endocytosis and numerous other cellular functions. E3 ubiquitin ligases, which mediate the final step in the ubiquitylation reaction cascade, are responsible for substrate recognition and ubiquitin attachment to them, underscoring the importance of identifying their substrates. Nedd4 family members are E3 ubiquitin ligases comprised of a C2-WW-HECT domain architecture. This thesis was aimed at first globally delineating the substrate specificity of the closely related Nedd4 family members in humans, hNedd4-1 (Nedd4) and hNedd4-2 (Nedd4L), and second, to follow up on one of the novel hits identified, the FGFR1, and study in detail how it is regulated by its E3 ligase hNedd4-1. To globally identify substrates for Nedd4 proteins, a high throughput proteomic screening technology using protein microarrays was employed. Despite the obvious homology in their domain architecture, the results presented here suggest that these Nedd4 family members may function non-redundantly, since they demonstrate a selective preference towards substrate ubiquitylation. Our focus on FGFR1, a substrate identified for hNedd4-1, has revealed an important functional role for this ubiquitin ligase in promoting FGFR1 endocytosis and downregulation of its signaling activity. The evidence presented indicates that this interaction has important consequences for developmental processes that are dependent on FGF signaling: human neural stem cell differentiation and zebrafish embryonic patterning and brain development. We demonstrate that the WW3 domain of Nedd4-1 recognizes a novel, non-canonical binding surface (peptide2) within the juxtamembrane region of FGFR1, and we are currently in the process of solving the 3 dimensional structure of the hNedd4-1 WW3: FGFR1 peptide2 complex using X-ray crystallography. Furthermore, in characterizing the interaction between hNedd4-1 and FGFR1, we have provided evidence for a novel mechanism for regulating the catalytic activity of hNedd4-1 by FGFR1 activation. This involves the formation of hNedd4-1 dimers upon removal of the autoinhibitory C2 domain from the HECT domain. Dimerized hNedd4-1, in turn, exhibits enhanced interactions with FGFR1 and enhanced receptor ubiquitylation. From these data, we proposed a negative feedback inhibitory model for FGFR1 downregulation, whereby activated receptor enhances the activation of its suppressor, hNedd4-1, to ensure timely termination of receptor signaling.

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