CDK1-mediated phosphorylation of GORASP1 (GRASP65) enables GORASP1 to recruit PLK1 (Preisinger et al. 2005). PLK1 phosphorylates GORASP1 on serine residue S189 (Sengupta and Linstedt 2010). This serine residue is near the GORASP1 region involved in GORASP1 dimerization and oligomerization, a process underlying the stacking of cis-Golgi cisternae (Wang et al. 2003). The phosphorylation of S189 by PLK1 impairs Golgi cisternae stacking (tethering), contributing to Golgi unlinking and fragmentation in mitosis, probably by preventing formation of GORASP1 dimers and oligomers (Sutterlin et al. 2001, Sengupta and Linstedt, 2010). Two other potential phosphorylation sites that match PLK1 substrate consensus sequence exist in GORASP1, but their functional significance has not yet been examined (Sengupta and Linstedt, 2010).

FIRRM (also known as Apolo1, FINGL1-interacting regulator of recombination and mitosis, or FLIP) colocalizes with PLK1 to kinetochores during prometaphase. The N-terminus of FIRRM binds to the polo-box domain (PBD) of PLK1 (Xu et al. 2021).

PP1gamma (PPP1CC), a kinetochore component, dephosphorylates PLK1 at threonine residue T210 in a time-dependent manner. PPP1CC-mediated dephosphorylation of p-T210-PLK1 is attenuated by binding of FIRMM (also known as Apolo1 or FLIP) to PPP1CC (Xu et al. 2021).

PLK1 phosphorylates BORA on serine residue S497 and threonine residue T501 that both lie in the DSGYNT degron recognized by beta-TrCP F-box proteins (Seki et al. 2008).

PLK1 phosphorylates FOXM1 on serine residues S730 and S739 (S715 and S724 in FOXM1B isoform) in the C-terminal transactivation domain (TAD). PLK1-mediated phosphorylation of FOXM1 upregulates FOXM1 transcriptional activity and is crucial for FOXM1 function at G2/M transition (Fu et al. 2008).

The kinase activity of PLK1 is required for cell cycle progression as PLK1 phosphorylates and regulates a number of cellular proteins during mitosis. Centrosomic AURKA (Aurora A kinase), catalytically activated through AJUBA facilitated autophosphorylation on threonine residue T288 at G2/M transition (Hirota et al. 2003), activates PLK1 on centrosomes by phosphorylating threonine residue T210 of PLK1, critical for PLK1 activity (Jang et al. 2002), in the presence of BORA (Macurek et al. 2008, Seki et al. 2008). Once activated, PLK1 phosphorylates BORA and targets it for ubiquitination mediated degradation by SCF-beta-TrCP ubiquitin ligases. Degradation of BORA is thought to allow PLK1 to interact with other substrates (Seki, Coppinger, Du et al. 2008, Seki et al. 2008).

The interaction of PLK1 with OPTN (optineurin) provides a negative-feedback mechanism for regulation of PLK1 activity. Phosphorylated PLK1 binds and phosphorylates OPTN associated with the Golgi membrane GTPase RAB8, promoting dissociation of OPTN from Golgi and translocation of OPTN to the nucleus. Phosphorylated OPTN facilitates the mitotic phosphorylation of the myosin phosphatase subunit PPP1R12A (MYPT1) and myosin phosphatase activation (Kachaner et al. 2012). The myosin phosphatase complex dephosphorylates threonine residue T210 of PLK1 and inactivates PLK1 (Yamashiro et al. 2008).

NEK6 and NEK7 are activated during mitosis by another NIMA family kinase, NEK9 (Belham et al. 2003, Richards et al. 2009), which is activated by CDK1- and PLK1-mediated phosphorylation (Roig et al. 2002, Bertran et al. 2011).

At mitotic entry, Plk1 phosphorylates and activates Cdc25C phosphatase, whereas it phosphorylates and down-regulates Wee1A (Watanabe et al. 2004). Plk1 also phosphorylates and inhibits Myt1 activity (Sagata 2005). Cyclin B1-bound Cdc2, which is the target of Cdc25C, Wee1A, and Myt1, functions in a feedback loop and phosphorylates the latter components (Cdc25C, Wee1A, Myt1). The Cdc2- dependent phosphorylation provides docking sites for the polo-box domain of Plk1, thus promoting the Plk1-dependent regulation of these components and, as a result, activation of Cdc2-Cyclin B1.

PLK1 phosphorylates and activates the transcription factor FOXM1 which stimulates the expression of a number of genes needed for G2/M transition, including PLK1, thereby creating a positive feedback loop (Laoukili et al. 2005, Fu et al. 2008, Sadasivam et al. 2012, Chen et al. 2013).

The pericentriolar stacks of Golgi cisternae undergo extensive fragmentation and reorganization in mitosis.

In mammalian cells, Golgi apparatus consists of stacked cisternae that are connected by tubules to form a ribbon-like structure in the perinuclear region, in vicinity of the centrosome. Reorganization of the Golgi apparatus during cell division allows both daughter cells to inherit this organelle, and may play additional roles in the organization of the mitotic spindle.

First changes in the structure of the Golgi apparatus likely start in G2 and are subtle, involving unlinking of the Golgi ribbon into separate stacks. These changes are required for the entry of mammalian cells into mitosis (Sutterlin et al. 2002). This initial unlinking of the Golgi ribbon depends on GRASP proteins and on CTBP1 (BARS) protein, which induces the cleavage of the tubular membranes connecting the stacks (Hidalgo Carcedo et al. 2004, Colanzi et al. 2007), but the exact mechanism is not known. Activation of MEK1/2 also contributes to unlinking of the Golgi ribbon in G2 (Feinstein and Linstedt 2007).

From prophase to metaphase, Golgi cisternae undergo extensive fragmentation that is a consequence of unstacking of Golgi cisternae and cessation of transport through Golgi. At least three mitotic kinases, CDK1, PLK1 and MEK1, regulate these changes. CDK1 in complex with cyclin B phosphorylates GOLGA2 (GM130) and GORASP1 (GRASP65), constituents of a cis-Golgi membrane complex (Lowe et al. 1998, Preisinger et al. 2005). Phosphorylation of GOLGA2 prevents binding of USO1 (p115), a protein localizing to the membrane of ER (endoplasmic reticulum) to Golgi transport vesicles and cis-Golgi, thereby impairing fusion of these vesicles with cis-Golgi cisternae and stopping ER to Golgi transport (Lowe et al. 1998, Seeman et al. 2000, Moyer et al. 2001). Phosphorylation of GORASP1 by CDK1 enables further phosphorylation of GORASP1 by PLK1 (Sutterlin et al. 2001, Preisinger et al. 2005). Phosphorylation of GORASP1 by CDK1 and PLK1 impairs stacking of Golgi cisternae by interfering with formation of GORASP1 trans-oligomers that would normally link the Golgi cisternae together (Wang et al. 2003, Wang et al. 2005, Sengupta and Linstedt 2010).

In the median Golgi, GORASP2 (GRASP55), a protein that forms a complex with BLFZ1 (Golgin-45) and RAB2A GTPase and contributes to cisternae stacking and Golgi trafficking (Short et al. 2001), is also phosphorylated in mitosis. Phosphorylation of GORASP2 by MEK1/2-activated MAPK1 (ERK2) and/or MAPK3-3 (ERK1b in human, Erk1c in rat) contributes to Golgi unlinking in G2 and fragmentation of Golgi cisternae in mitotic prophase (Acharya et al. 1998, Jesch et al. 2001, Colanzi et al. 2003, Shaul and Seger 2006, Duran et al. 2008, Feinstein and Linstedt 2007, Feinstein and Linstedt 2008, Xiang and Wang 2010).

The phosphorylation of Emi1, which is required for its degradation in mitosis, appears to involve both Plk1 and Cdk1.

ROCK I (alternatively called ROK ?) and ROCK II (also known as Rho kinase or ROK ?) were originally isolated as RhoA-GTP interacting proteins. The kinase domains of ROCK I and ROCK II are 92% identical, and so far there is no evidence that they phosphorylate different substrates. RhoA, RhoB, and RhoC associate with and activate ROCK but other GTP-binding proteins can be inhibitors, e.g. RhoE, Rad and Gem. PDK1 kinase promotes ROCK I activity not through phosphorylation but by blocking RhoE association. PLK1 can phosphorylate ROCK II and this enhances the effect of RhoA. Arachidonic acid can activate ROCK independently of Rho.

ROCK I (alternatively called ROK ?) and ROCK II (also known as Rho kinase or ROK ?) were originally isolated as RhoA-GTP interacting proteins. The kinase domains of ROCK I and ROCK II are 92% identical, and so far there is no evidence that they phosphorylate different substrates. RhoA, RhoB, and RhoC associate with and activate ROCK but other GTP-binding proteins can be inhibitors, e.g. RhoE, Rad and Gem. PDK1 kinase promotes ROCK I activity not through phosphorylation but by blocking RhoE association. PLK1 can phosphorylate ROCK II and this enhances the effect of RhoA. Arachidonic acid can activate ROCK independently of Rho.