Loading and unloading: orchestrating centrosome duplication and spindle assembly by Ran/Crm1

Abstract

The cell cycle is an intricate process of DNA replication and cell division that concludes with the formation of two genetically equivalent daughter cells. In this progression, the centrosome is duplicated once and only once during the G1/S transition to produce the bipolar spindle and ensure proper chromosome segregation. The presence of multiple centrosomes in cancer cells suggests that this process is mis-regulated during carcinogenesis. This suggests that certain factors exist that license the progression of centrosome duplication and serve to inhibit further duplications during a single cell cycle. Recent studies suggest that the Ran/Crm1 complex not only regulates nucleocytoplasmic transport but is also independently involved in mitotic spindle assembly. Factors that are capable of interacting with Ran/Crm1 through their nuclear export sequences, such as cyclins/cdks, p53 and Brca1/2, may potentially function as centrosome checkpoints akin to the G1/S and G2/M checkpoints of the cell cycle. Our recent findings indicate that nucleophosmin, a protein whose trafficking is mediated by the Ran/Crm1 network, is one of such checkpoint factors for maintaining proper centrosome duplication. We propose that Ran/Crm1 may act as a 'loading dock' to coordinate various checkpoint factors in regulating the fidelity of centrosome duplication during cell cycle progression, and the disruption of these processes may lead to genomic instability and an acceleration of oncogenesis.

Figure 1

The Ran/Crm1 network: Nucleocytoplasmic transport and mitotic spindle assembly. The small GTPase, Ran, shuttles between an inactive GDP and an active GTP-bound state through interaction with RanBP1 and RCC1 respectively. In its GTP-bound state, Ran can interact with importin receptors (a and b) to promote the cytoplasmic to nuclear transport of proteins containing nuclear localization signals (NLS). The transport of certain NLS-containing proteins such as NuMA and TPX2 can promote microtubule nucleation. Ran-GTP can also interact with the nuclear export receptor, Crm1 that binds to proteins containing nuclear export signals (NES). The hepatitis B viral oncoprotein HBx, interacts with and inactivates Crm1 through its NES, leading to centrosome over-duplication and multipolar spindles. Other NES-containing substrates that bind Crm1, such as nucleophosmin (NPM), may have tumor suppressive effects and function as licensing factors to regulate centrosome duplication during the cell cycle.

Figure 2

Ran/Crm1 functions as a loading dock to coordinate cell cycle checkpoints and centrosome duplication. The centrosome duplication cycle is shown through the various stages of the cell cycle. The Ran/Crm1 network functions as a loading dock to spatially and temporally coordinate NES-containing factors that ensure the fidelity of the centrosome duplication process. The Ran/Crm1 complex interacts with various NES-containing proteins, such as NPM, p53, BRCAs, etc., in a cell cycle dependent manner to inhibit centriole splitting. At the G₁/S transition of a normal cell, such a process may be inactivated by phosphorylation of the substrates, such as NPM, thereby preventing their bindings to Ran/Crm1. Disruption of this coordination may lead to genomic instability and carcinogenesis.