THE ROLE OF BINDER OF RHO GTPASE (BORG) AND SEPTINS IN MYOTONIC DYSTROPHY KINASE-RELATED CDC42-BINDING KINASE (MRCK) REGULATED ACTOMYOSIN DYNAMICS AND CELL MIGRATION

Abstract

Cell motility is an important process during normal embryonic development but its dysregulation may result in certain diseases. The Rho GTPases have been identified to be important regulators of cellular motility via their control over actin dynamics. Although highly homologous to each other, activation of the three major Rho GTPase family members, RhoA, Rac1 and Cdc42 leads to very different phenotypes in mammalian cells. RhoA activation is associated with cell stabilisation and adhesion via the formation of strongly anchored actin stress fibres. Rac1 activation is associated with cell spreading and motility via cell protrusion and the induction of nascent adhesion turnover. Cdc42 activation is also associated with cell protrusion and motility, but with different characteristics from those induced by Rac1. In this study, a little known Cdc42 effector, Binder of Rho GTPase 5 (BORG5) and its downstream septin filaments were characterised in HeLa cells. These proteins are found to share close links with another Cdc42 effector, Myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK) in regulating two pools of actomyosin, the lamellar bundles (LMB) and the sub-nuclear bundles (SNB). Apart from their discrete intracellular localisation, these two pools of MRCK/BORG5-regulated actomyosin are biochemically and physically distinct from each other. The LMB contains non-muscle Myosin 2A (MYO2A) and are thin, dynamic filaments close to the cell periphery. The SNB contains disassembled components from the LMB as well as from the RhoA/RhoA-binding kinase (ROK)-dependent stress fibres like MYO2B. Although derived from stress fibres, the SNB have very different properties possibly due to MRCK modification and the involvement of BORG5 and septins in this transition. The LMB have been previously shown to be involved in cell protrusion. In this study, the SNB are found to be physically linked to the nucleus and shown to coordinate the protrusions of the leading edge to nuclear/cell body translocation during cell migration. The SNB may fulfil this role by acting as a track to guide nuclear migration. As the SNB is derived from both Cdc42/MRCK/BORG5 and RhoA/ROK activity, it therefore represents a coordination of Cdc42 and RhoA signalling for directional migration. The formation of the SNB may be a mechanism whereby signals from both Cdc42 and RhoA are integrated and utilised in guiding the nucleus during cell migration. These studies also reflect the intricacy of Cdc42 as a master regulator in several aspects of cellular motility ranging from the leading edge to the cell body.