The Role of TACC3 in Regulating Microtubule Dynamics During Herpesvirus Infections

Furey, Colleen Patricia

doi:https://doi.org/10.21985/n2-1n8w-dr25

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The Role of TACC3 in Regulating Microtubule Dynamics During Herpesvirus Infections

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The microtubule (MT) network and associated regulatory proteins play a critical role during viral infection from facilitating viral particle transport towards the nucleus upon entry to later mediating virion assembly and egress. Many of the precise mechanisms by which viruses commandeer the host MT network to propagate infection remain poorly defined. The MT network is largely controlled by a group of proteins known as plus-end tracking proteins or +TIPs which directly or indirectly bind growing MT plus-ends and regulate their dynamic instability. Among the +TIPs, the end-binding (EB) proteins have long been considered the master regulators of the MT network due to their ability to directly bind MT plus-ends and recruit a diverse array of other +TIP partners there. As mediators of MT dynamics and stability, +TIPs can be exploited by viruses to manipulate MT behavior. Indeed, previous work from our lab demonstrated that herpes simplex virus type 1 (HSV-1) hijacks a +TIP complex including EB1 at the cell periphery to initiate viral particle transport on dynamic tyrosinated MTs during early infection in primary normal human dermal fibroblasts (NHDFs). In examining whether HSV-1 utilizes similar mechanisms during infections in other cell types, I found that HSV-1 adopts a different approach to infect the neuronal SK-N-SH cell line, with HSV-1 particles trafficking upon stable de-tyrosinated MTs in an EB-independent manner to the nucleus. To explore the potential contribution of an EB-independent +TIP to HSV-1 infection in SK-N-SHs, I began studying transforming acidic coiled-coil protein 3 (TACC3), an autonomous tip-tracker primarily characterized as a mitotic protein that promotes MT spindle elongation by recruiting the MT polymerase chTOG to spindle plus-ends. Through my studies, I found that TACC3 plays a role not only in facilitating HSV-1 infection in both NHDFs and SK-N-SHs, but more generally in controlling interphase MT dynamics. In this work, I show that altering TACC3 levels disrupts the nuclear-cytoplasmic localization of chTOG and thus the balance between dynamic and stable MTs in the cell. Loss of TACC3 results in nuclear sequestration of chTOG, reduced MT growth in the cytoplasm, and accumulation of post-translationally modified stable MTs. In NHDFs, the loss of EB1 and dynamic MTs at the periphery of TACC3-depleted cells inhibits HSV-1 particle transport to the nucleus and subsequently blocks infection. In SK-N-SHs the upregulation of de-tyrosinated MTs in TACC3-depleted cells results in impaired HSV-1 particle transport to the nucleus which I found is due to the biasing of MT-dependent transport towards outward-directing kinesin motors. Together, these findings highlight the previously unappreciated role for TACC3 in regulating interphase MTs and in turn facilitating transport of cellular and viral cargo. To expand upon these findings, I next examined the role of TACC3 in a different herpesvirus infection, human cytomegalovirus (HCMV). Compared to HSV-1, HCMV has a protracted infectious cycle and is characterized by the formation of a viral assembly compartment (AC) which consists of rearranged host secretory machinery and requires precise coordination by the MT network. I found that HCMV specifically upregulates TACC3 late in infection to maintain high levels of chTOG and dynamic MTs in the cytoplasm, which is necessary for recruitment of secretory machinery to the AC and subsequent virion egress. The work presented here provides evidence towards the centrality of TACC3 in MT network regulation. This work also presents an example of a virus targeting TACC3 to promote infection, laying a foundation for future investigations of TACC3 in the context of other viral infections.

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