REVIEW Playing polo during mitosis: PLK1 takes the lead G Combes 1,2 , I Alharbi 1,2 , LG Braga 1,2 and S Elowe 1,2,3 Polo-like kinase 1 (PLK1), the prototypical member of the polo-like family of serine/threonine kinases, is a pivotal regulator of mitosis and cytokinesis in eukaryotes. Many layers of regulation have evolved to target PLK1 to different subcellular structures and to its various mitotic substrates in line with its numerous functions during mitosis. Collective work is starting to illuminate an important set of substrates for PLK1: the mitotic kinases that together ensure the fidelity of the cell division process. Amongst these, recent developments argue that PLK1 regulates the activity of the histone kinases Aurora B and Haspin to define centromere identity, of MPS1 to initiate spindle checkpoint signaling, and of BUB1 and its pseudokinase paralog BUBR1 to coordinate spindle checkpoint activation and inactivation. Here, we review the recent work describing the regulation of these kinases by PLK1. We highlight common themes throughout and argue that a major mitotic function of PLK1 is as a master regulator of these key kinases. Oncogene advance online publication, 24 April 2017; doi:10.1038/onc.2017.113 INTRODUCTION Cell cycle progression is regulated by post-translational modifica- tions, with reversible phosphorylation being arguably the most the dynamic of these. Protein kinases and phosphatases collectively balance the level of phosphorylation of key substrates to ensure an accurate and timely transition of the cell cycle. 1,2 Often, kinases or phosphatases are themselves regulated by phosphorylation, resulting in complex regulatory networks with multiple feedback loops. 3 In mitosis, a set of highly conserved kinases is responsible for the overall control of these regulatory networks including the polo-like kinases (PLKs). 4–6 Since its discovery almost 30 years ago in Drosophila, the polo-like kinase (PLK) family has risen to the ranks as a driving force in cell cycle progression in both meiosis and mitosis. PLKs are well conserved between species, with at least one PLK family member present from fungi to humans, but are conspicuously absent in plants and a number of protozoan parasites. 4,6 Five PLKs have been discovered in humans; they appear to be for the most part functionally distinct with little or no overlap in substrates. 7 In all organisms that do have PLKs, the single family member (for example cdc5 in S.cerevisiae) or multiple family members (at least PLK1 and PLK4 in humans) assist in the execution of cell division. PLKs consist of an N-terminal Ser/Thr kinase domain and the characteristic two polo box regions in the C terminus. The exceptions in humans are PLK4 which has a cryptic polo box region that includes 2 polo boxes and a third bona fide polo box, and PLK5 which lacks a functional kinase domain. 7 The two polo boxes of PLK proteins fold together to form a functional polo box domain (PBD), that binds principally to phosphorylated peptides but may also associate with partners in a phosphoinde- pendent manner. 8–10 In humans, the most pleiotropic of the PLKs is surely PLK1, which directs a remarkable number of functions across the cell cycle, and several excellent recent reviews have provided in-depth perspectives on the role of PLK1 in the cell cycle and beyond. 6,7,11–13 Most recently, PLK1 has been shown to have a role in maintaining an active spindle assembly checkpoint (SAC) response. The SAC is a signaling cascade that ascertains the integrity of the attachments between the mitotic spindle and chromatids at kinetochores, subsequently delaying the transition to anaphase allowing time for correction of attachment errors. 14,15 PLK1 performs its functions coordinately and in conjunction with other major mitotic kinases, several of which have been shown to be themselves substrates of PLK1, and through mechanisms that often involve positive and negative feedbacks. Here, we highlight recent advances in the relationship between PLK1 and the kinases critical for mitotic progression and the SAC, namely Haspin, Aurora B, Monopolar Spindle 1 (MPS1), budding uninhibited by benzimi- dazole1 (BUB1), and its pseudokinase paralog BUB1-related 1 (BUBR1), and argue that the common role of PLK1 is to ensure their timely and efficient activation and functioning. Much of the emergent evidence is largely due to the development of a number of PLK inhibitors, and we highlight the contribution of these where relevant. PLK1 AND CENTROMERE ESTABLISHMENT The breadth of PLK1 substrates identified in mitosis suggests that several centromere and kinetochore pools of PLK1 must exist, in addition to a centrosome-associated population. Recently, the bulk pool of PLK1 has been proposed to be at the centromere and inner kinetochore; 16 indeed PLK1 functions at chromatin and the inner centromere in a manner distinct from its roles in chromosome attachment at outer kinetochore. 16 Somewhat surprisingly, centromere-localized PLK1 greatly contributes to ensuring accurate chromosome alignment at metaphase and preventing lagging chromosomes at anaphase, 16 although it is clear that outer kinetochore PLK1 substrates also contribute to these functions. 17 PLK1 promotes catalytic activation of at least two kinases, Aurora B and Haspin in early mitosis. These kinases, together with another PLK1-regulated kinase, BUB1, function to establish the centromere as defined by recruitment of the 1 Program in Molecular and Cellular biology, Faculty of Medicine, Université Laval, Quebec City, Quebec, Canada; 2 Axe of Reproduction, Mother and Youth Health, CHU de Québec Research Centre, Quebec City, Quebec, Canada and 3 Department of Pediatrics, Faculty of Medicine, Université Laval, Quebec City, Quebec, Canada. Correspondence: Dr S Elowe, Département de Pédiatrie, Université Laval, 2705, boulevard Laurier, T3-67, Quebec City, Quebec G1V 4G2, Canada. E-mail: Sabine.Elowe@crchuq.ulaval.ca Received 30 January 2017; revised 16 March 2017; accepted 18 March 2017 Oncogene (2017), 1 – 9 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved 0950-9232/17 www.nature.com/onc