Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 399

Alternatively one may consider the relevance of firmly installed CVC
protein components (hypothesis (ii)). One may assume such a role for pro-
teins occurring at the CV/plasma membrane interface, that is, the pore. In a
first step, a new pore would have to be assembled in order to serve as a nucle-
ation site for a young CV. This hypothesis considers two facts, that is, (i) a
new CVC forms around a small CV as a core part from which subsequently
radial arms grow out. (ii) A new CVC forms in contact with the end of one
of the radial arms of an old CVC ( Allen et al., 1990 ). Again a dual question
emerges: Are there special proteins at the pore and at the tip of a radial arm of
a preexisting CVC, respectively? Remarkably, the radial canals of the old
CVC shrink, while the new ones expand ( Allen et al., 1990 ).
This suggests a steady-state balancing, once the nucleation site for a new
CVC has been established. The pore is known to contain g -tubulin, and pos-
sibly also centrin, as a nucleation center; see Section 5.1 .Justlikeatothersites
of the cell, such components of microtubule-organizing centers can represent
only one point in a chain of molecular components and their stepwise arrange-
ment and eventual modification. In Paramecium , dynamic cell-surface structur-
ing depends on the phosphorylation state of different cortical proteins ( Keryer
et al., 1987; Sperling et al., 1991 ) including centrin ( Klotz et al., 1997 ). In fact,
centrin4 has been found at the pore of Tetrahymena by Stemm-Wolf et al.
(2005) (although in non-dividing Tetrahymena centrin has not been found
at the pore by Elde et al., 2005 ). Generally, centrin is associated with
microtubule-organizing centers ( Levy et al., 1996 ) and is a target for reversible
phosphorylation processes ( Thissen et al., 2009 ). Yet, in the only study the
effect of a general kinase inhibitor on the CVC in Paramecium has been clas-
sified as “partly normal” ( Kaczanowska et al., 1996 ). More work is desirable.
As to regulation of microtubule length at their plus end, several proteins
are to be envisaged. Considering that some kinesin superfamily members can
exert dual functions, it appears rewarding to look for microtubule-associated
motor proteins. For instance, Kif18A restricts the length of kinetochore-
microtubules of the mammalian cytospindle ( Mayr et al., 2007 ), and Kif19A
regulates the length of cytoplasmic microtubules in mammalian cells ( Niwa
et al., 2012 ). If such a mechanism could be found with CVC-associated
microtubules, this would be relevant not only for defining the final size
of CVC arms but also for formation of new ones adjacent to the end of
an old radial arm. The protein kinase NIMA, known to regulate the length
of axonemal microtubules and to occur at the pore of the Tetrahymena CV
( Wloga et al., 2006 ), could also spring into action. Local posttranslational
modifications of tubulin should also be envisaged.