Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 89

populations of MT appear suggests a strong evolutionary constraint on over-
all spindle architecture. Interestingly, k-fiber, spindle, and astral MTs each
appear to possess unique features and regulation. The identification of MAPs
that are specific to certain MT populations (discussed in Section 4 ) supports
the idea that these are truly functionally distinct; however, an important line
of investigation will be to understand how crosstalk among populations
contributes to spindle function.
2.4. Microtubule dynamics and organization in the spindle
The dynamic properties of MTs have been studied extensively, and a variety
of biochemical conditions and protein factors have been shown to modulate
parameters of MT growth, pause, catastrophe, shrinkage, and rescue ( Al-
Bassam and Chang, 2011; Amos, 2004, 2011; Amos and Schlieper, 2005;
Desai and Mitchison, 1997; Drummond, 2011 ). One question is whether
the densely packed spindle MTs are similarly modulated. Techniques mark-
ing a subset of MTs in the spindle, such as fluorescence recovery after pho-
tobleaching (FRAP), have established that MTs are very dynamic with
recovery half-times in the tens of seconds ( Wadsworth and Salmon,
1986 ). Imaging individual MTs in the spindle is extremely difficult, but
two dynamic properties have been well characterized: MT growth within
the spindle and their continuous transport toward the spindle pole, known
as polewardMT flux ( Ganem et al., 2005; Sawin andMitchison, 1991, 1994;
Tirnauer et al., 2004 ). The identification and fluorescent labeling of proteins
such as EB1 that remains associated with the growing MT plus-end have
allowed the tracking of individual MTs and established that growth rates
of MTs both within and outside of the spindle are similar at metaphase
( Tirnauer et al., 2004 ). In addition, plus-end tracking provides information
about where MTs are polymerizing in the spindle and their polarity
( Fig. 3.3 A). Although dynamic instability largely accounts for the high rate
of MT turnover in the spindle, the use of FRAP, photoactivation, and, espe-
cially, fluorescent speckle microscopy (FSM) to generate fiduciary marks has
provided measurements of MT flux ( Fig. 3.3 B), which also promotes turn-
over through the net assembly at MT plus-ends and disassembly at MT
minus-ends. However, the extent to which these movements are represen-
tative of all spindle MTs or particular MT populations remains unclear
( Maddox et al., 2003; Sawin and Mitchison, 1991, 1994; Wadsworth and
Salmon, 1986; Waterman-Storer et al., 1999 ).