Research Article: The Chromosomal Passenger Protein Birc5b Organizes Microfilaments and Germ Plasm in the Zebrafish Embryo

Date Published: April 18, 2013

Publisher: Public Library of Science

Author(s): Sreelaja Nair, Florence Marlow, Elliott Abrams, Lee Kapp, Mary C. Mullins, Francisco Pelegri, Elizabeth R. Gavis

Abstract: Microtubule-microfilament interactions are important for cytokinesis and subcellular localization of proteins and mRNAs. In the early zebrafish embryo, astral microtubule-microfilament interactions also facilitate a stereotypic segregation pattern of germ plasm ribonucleoparticles (GP RNPs), which is critical for their eventual selective inheritance by germ cells. The precise mechanisms and molecular mediators for both cytoskeletal interactions and GP RNPs segregation are the focus of intense research. Here, we report the molecular identification of a zebrafish maternal-effect mutation motley as Birc5b, a homolog of the mammalian Chromosomal Passenger Complex (CPC) component Survivin. The meiosis and mitosis defects in motley/birc5b mutant embryos are consistent with failed CPC function, and additional defects in astral microtubule remodeling contribute to failures in the initiation of cytokinesis furrow ingression. Unexpectedly, the motley/birc5b mutation also disrupts cortical microfilaments and GP RNP aggregation during early cell divisions. Birc5b localizes to the tips of astral microtubules along with polymerizing cortical F-actin and the GP RNPs. Mutant Birc5b co-localizes with cortical F-actin and GP RNPs, but fails to associate with astral microtubule tips, leading to disorganized microfilaments and GP RNP aggregation defects. Thus, maternal Birc5b localizes to astral microtubule tips and associates with cortical F-actin and GP RNPs, potentially linking the two cytoskeletons to mediate microtubule-microfilament reorganization and GP RNP aggregation during early embryonic cell cycles in zebrafish. In addition to the known mitotic function of CPC components, our analyses reveal a non-canonical role for an evolutionarily conserved CPC protein in microfilament reorganization and germ plasm aggregation.

Partial Text: A fundamental feature of cell biology is cytoskeletal cross-talk between microtubule and microfilament networks. One key cellular process dependent on these interactions is the positioning of the contractile ring during cytokinesis. Two major groups of microtubules are involved in contractile ring positioning: the center of the mitotic spindle, which resolves into the antiparallel central spindle microtubules, and the poles of the mitotic spindle, which generates astral microtubules. Several studies indicate that central spindle and astral microtubules redundantly stimulate furrowing at the equatorial cortex [1]. Both sets of microtubules must ultimately communicate with cortical microfilaments that form the contractile ring, and the precise mechanism of this communication is an area of intense research. Candidate mediators include the Chromosomal Passenger Complex (CPC), which localizes with chromosomes during metaphase and transitions cortically to the prospective site of membrane ingression during telophase [2], [3]. Loss of CPC function affect two distinct yet related cellular events: chromosomes tend to lag during metaphase, resulting in chromosome segregation errors, and cleavage furrows fail to maintain ingression resulting in cytokinesis failures [4], [5], [6], [7], [8]. Lagging chromosomes can secondarily cause cytokinesis failures during telophase, but analysis of point mutations in CPC proteins reveal independent roles for components of this complex in the initiation of cytokinesis as well [9], [10], [11], in agreement with the localization of the CPC to the early equatorial cortex.

The Chromosomal Passenger Complex (CPC) consisting of AurB, INCENP (Inner Centromere Protein), Borealin/Dasra and Survivin/Bir1/BIRC5 has been ascribed a number of roles during cell division, including chromosome bi-orientation and cytokinesis [2], [3]. Survivin is a member of the Baculoviral Inhibitor of Apoptosis Repeat Containing (BIRC) protein family and contains a single CX2CX16HX6C BIR domain [26]. The BIR domain is an evolutionarily conserved Zn finger fold present in Inhibitor of Apoptosis (IAP) proteins from Baculoviruses to humans [27], [28], [29]. Survivin/BIRC5 is unique amongst CPC and BIRC proteins as it is thought to be directly involved in both cytokinesis and cell survival, though its exclusivity to each process is debated [30].