Structural Functional Investigation of HAP40

<p>STRUCTURAL FUNCTIONAL INVESTIGATION OF HAP40</p> <p>Amanda Marie Solbach</p> <p>Dissertation Advisor: Sheng Zhang, Ph.D.</p> <p>Huntington’s disease (HD) is a devastating neurodegenerative disorder caused by an expanded CAG trinucleotide repeat in the Huntingtin (HTT) gene, resulting in a mutant huntingtin (mHTT) protein with an elongated polyglutamine tract. While the genetic basis of HD is well established, the mechanisms underlying mHTT toxicity and its impact on cellular function remain incompletely understood. Recent discoveries implicate Huntingtin-associated protein 40 (HAP40) as a critical regulator of HTT stability and intracellular function. However, the physiological and pathological significance of this interaction has not been fully explored.</p> <p>This dissertation investigates the structure-function relationship of HAP40, with a focus on its evolutionarily conserved BΦ (Basic-Hydrophobic) motif, which I hypothesize to be a key regulatory domain. Using a combination of mammalian cell culture and Drosophila models, I systematically dissected HAP40’s role in HTT stabilization, endosomal trafficking, and neurodegeneration. Structural modeling revealed remarkable conservation of the HTT-HAP40 complex across species, and functional studies demonstrated that deletion of the BΦ motif does not impair HAP40’s ability to stabilize HTT. However, the BΦ motif is essential for the proper endosomal localization of HTT and the regulation of late endosomal morphology. Loss of BΦ selectively impaired Rab7+ endosome growth and disrupted HTT targeting to endosomal membranes.</p> <p>Furthermore, in a Drosophila model of HD, co-expression of mutant HTT with HAP40-ΔBΦ exacerbated neurodegeneration relative to full-length HAP40, indicating that the BΦ domain plays a protective role in modulating HTT toxicity. These findings suggest that while HAP40’s BΦ motif is dispensable for HTT binding, it is essential for key aspects of HTT’s subcellular function, particularly in the endolysosomal pathway. Taken together, this work advances our understanding of the HTT-HAP40 interaction and highlights the BΦ motif as a structurally conserved domain with functional significance in neuronal homeostasis and HD pathogenesis.</p>

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation (PhD)

Year dc:date.available

2025

Authors dc:creator

• Solbach, Amanda M
• <p>https://orcid.org/0000-0001-8130-8514</p>

Contributors dc:contributor

• Sheng Zhang
• Andrea Stavoe
• Andrey Tsvetkov

• Neuroscience
• Neurodegeneration
• Structural Biology
• Huntington's disease
• Huntingtin
• HAP40
• Drosophila
• Molecular and Cellular Neuroscience

Repository record dc:identifier

https://digitalcommons.library.tmc.edu/utgsbs_dissertations/1474

OAI identifier oai:identifier

oai:digitalcommons.library.tmc.edu:utgsbs_dissertations-2531