A Novel Bipartite Centrosome Coordinates the Apicomplexan Cell Cycle

Abstract

Apicomplexan parasites can change fundamental features of cell division during their life cycles, suspending cytokinesis when needed and changing proliferative scale in different hosts and tissues. The structural and molecular basis for this remarkable cell cycle flexibility is not fully understood, although the centrosome serves a key role in determining when and how much replication will occur. Here we describe the discovery of multiple replicating core complexes with distinct protein composition and function in the centrosome of Toxoplasma gondii. An outer core complex distal from the nucleus contains the TgCentrin1/TgSfi1 protein pair, along with the cartwheel protein TgSas-6 and a novel Aurora-related kinase, while an inner core closely aligned with the unique spindle pole (centrocone) holds distant orthologs of the CEP250/C-Nap protein family. This outer/inner spatial relationship of centrosome cores is maintained throughout the cell cycle. When in metaphase, the duplicated cores align to opposite sides of the kinetochores in a linear array. As parasites transition into S phase, the cores sequentially duplicate, outer core first and inner core second, ensuring that each daughter parasite inherits one copy of each type of centrosome core. A key serine/threonine kinase distantly related to the MAPK family is localized to the centrosome, where it restricts core duplication to once per cycle and ensures the proper formation of new daughter parasites. Genetic analysis of the outer core in a temperature-sensitive mutant demonstrated this core functions primarily in cytokinesis. An inhibition of ts-TgSfi1 function at high temperature caused the loss of outer cores and a severe block to budding, while at the same time the inner core amplified along with the unique spindle pole, indicating the inner core and spindle pole are independent and co-regulated. The discovery of a novel bipartite organization in the parasite centrosome that segregates the functions of karyokinesis and cytokinesis provides an explanation for how cell cycle flexibility is achieved in apicomplexan life cycles.

Lytic Cycle of Toxoplasma gondii: 15 Years Later

“…This has drawn the attention to a pivotal role for the centrosome in coordinating the mitotic, karyokinetic and cytokinetic events (34, 48, 49, 139). The centrosome resides near the spindle pole embedded in the nuclear envelope.…”

Apicomplexan cell cycle flexibility: centrosome controls the clutch

“…However, the mechanistic coordination of these two events remains largely unclear. In an elegant recent study, Suvorova and colleagues systematically searched for orthologs of known centrosomal proteins in Toxoplasma and used these reagents to evaluate the phenotypes of previously isolated temperature sensitive cell division cycle mutants [4]. Localization studies with the candidate centrosomal proteins led to the surprising discovery of two different core structures making up the parasite's centrosome.…”

“…Since the physical connection of inner core and the centrocone (spindle pole) remains intact in a TgSfi1-deficient background, this finding suggests a mitosis organizing function for the inner core, but, moreover, illustrates that the cores can function independently, and that it is possible to uncouple budding from mitosis. In a reciprocal experiment, Chen and Gubbels showed that in a TgCep250-deficient inner core line, the budding cycle persists but mitosis is disrupted [4]. These observations suggest that the different cores likely associate with different functions of the centrosome, i.e.…”

“…Indeed, a candidate of pericentriolar matrix (PCM) component, TgMAPK-L1, transiently surrounds the centrosome during S-phase, and vanishes during division. With a TgMAPK-L1 deficient parasite line, it was shown that TgMAPK-L1 limits centrosome core duplication to only once per cell cycle, and promotes the connection of the nuclear centrocone and the daughter basal complex [4]. Furthermore, a previously reported kinase, TgNek1-2, that plays a role in centrosome splitting, showed similar localization pattern to TgMAPK-L1 but resides towards the centrosome [3].…”

Specialising the parasite nucleus: Pores, lamins, chromatin, and diversity

“…gondii also possesses CenH3, suggesting that conventional kinetochores are a conserved feature in apicomplexans [84] (Fig 2). TgCenH3 associates with centromeres, which cluster together at the centrocone, a unique, specialised spindle pole body that constitutively associates with the nuclear envelope throughout the cell cycle [84–86]. …”