PT - JOURNAL ARTICLE AU - Kelly M. Hennessey AU - Germain C.M. Alas AU - Ilse Rogiers AU - Renyu Li AU - Ethan A. Merritt AU - Alexander R. Paredez TI - Nek8445, a protein kinase required for microtubule regulation and cytokinesis in <em>Giardia lamblia</em> AID - 10.1101/719005 DP - 2019 Jan 01 TA - bioRxiv PG - 719005 4099 - http://biorxiv.org/content/early/2019/07/30/719005.short 4100 - http://biorxiv.org/content/early/2019/07/30/719005.full AB - Giardia has 189 Nek kinases whereas humans have only 11. We previously identified Nek8445 as a potential drug target due to susceptibility to a specific class of kinase inhibitors and essential function in parasite proliferation and attachment. Giardia has a complex tubulin cytoskeleton that includes eight flagella and several unique microtubule arrays that are utilized for parasite attachment and facilitation of rapid mitosis and cytokinesis. The need to regulate these structures may explain the parallel expansion in the number of Nek family kinases. Here we use live and fixed cell imaging to uncover the role of Nek8445 in regulating Giardia cell division. We demonstrate that Nek8445 localization is cell cycle regulated and this kinase has a role in regulating overall microtubule organization. Nek8445 depletion results in short flagella, aberrant ventral disc organization, loss of the axostyle (funis), defective flagella exit and altered cell shape. The flagella exit defect is specific to the caudal flagella, which exit from the posterior of the cell, and this defect correlates with rounding of the cell posterior and loss of the axostyle. This correlation implicates a role for the axostyle in establishing Giardia’s cell shape and guiding flagella docking. Axostyle biogenesis can now be included in the list of functions regulated by Nek family kinases.