Recent Glimpses of the Elusive Spindle Matrix

Abstract

KEY WORDS Spindle formation, Spindle matrix, Microtubules, MitosisThe authors' work on the spindle matrix is supported by NSF grant MCB0090877. ABSTRACTA stationary spindle matrix has been proposed on theoretical grounds to help mediate force production at the mitotic spindle. Direct molecular evidence for the existence of such a matrix has the potential to profoundly influence our view of the molecular mechanisms leading to chromosome segregation during mitosis. Three recent papers suggest that the spindle matrix may be more than a theoretical idea.

Megator, an Essential Coiled-Coil Protein that Localizes to the Putative Spindle Matrix during Mitosis inDrosophila

“…A spindle matrix has been hypothesized to provide a stationary substrate that anchors molecules during force production and microtubule sliding (Pickett-Heaps et al, 1997). Such a matrix could also be envisioned to have the added properties of helping to organize and stabilize the microtubule spindle (Johansen and Johansen, 2002). Previously, we demonstrated using RNAi assays in S2 cells that depletion of Chromator protein leads to abnormal spindle morphology and that chromosomes are scattered in the spindle, indicating defective spindle function in the absence of Chromator (Rath et al, 2004).…”

“…Studies using preparations spanning the evolutionary spectrum from lower eukaryotes to vertebrates have provided new and intriguing evidence that a spindle matrix may be a general feature of mitosis (Bloom, 2002;Johansen and Johansen, 2002;Kapoor and Mitchison, 2001). Here we show that at least three proteins, Megator, Chromator, and Skeletor, from two different cellular compartments reorganize to form a putative spindle matrix during mitosis in Drosophila.…”

“…The involvement of a spindle matrix that can act as a stationary substrate to stabilize the spindle during force production, and microtubule sliding has long been proposed (Pickett-Heaps et al, 1982;; however, direct evidence for its existence has remained elusive (Scholey et al, 2001;Wells, 2001;Bloom, 2002;Kapoor and Compton, 2002;Johansen and Johansen, 2002). Recently, a putative spindle matrix protein, Skeletor, was identified in Drosophila (Walker et al, 2000).…”

EAST interacts with Megator and localizes to the putative spindle matrix during mitosis in Drosophila

“…The features of this structure are compatible with those of a spindle matrix which on theoretical grounds has been proposed to provide a stationary substrate that anchors molecules during force production and microtubule sliding [Pickett-Heaps et al, 1982Johansen and Johansen, 2002]. Two of these proteins, Skeletor and Chromator, are localized to chromosomes during interphase [Walker et al, 2000;Rath et al, 2004] whereas the third, Megator (Bx34 antigen), occupies the intranuclear space surrounding the chromosomes in addition to being localized to the nuclear rim [Zimowska et al, 1997;Qi et al, 2004].…”

“…In its simplest formulation a spindle matrix is hypothesized to provide a more or less stationary substrate that provides a backbone or strut for motor molecules to interact with during force generation and microtubule sliding [Pickett-Heaps et al, 1997;Johansen and Johansen, 2002]. Thus, a prediction of the spindle matrix hypothesis is that if such a scaffold were interfered with in a way that it could not properly anchor motor proteins, the dynamic behavior of spindle components such as motors would be affected leading to abnormal chromosome alignment and segregation .…”