Control Systems in the Living World - Building the Most Complex Structure on Earth - page 27

their structure is known as selective stabilization ( Kirschner and Mitchison, 1986;
Mitchison and Kirschner, 1985 ).
Positioning of organelles in the cell is species-specific, and it is maintained under
varying internal and external environments. This implies that some kind of informa-
tion is responsible for the strict determination of the positioning of organelles in the
cell. In fission yeast, the position of mitochondria depends on microtubule dynam-
ics not on motor proteins ( Pon, 2011 ), and even the morphology of mitochondria
and the Golgi apparatus depends on microtubules ( Höög, 2003 ). The position of the
Golgi apparatus in plant cells is determined by the organization of the actin cytoskel-
eton ( Akkerman et al., 2011 ). It seems that microtubules somehow search and find
the appropriate site for transporting and settling molecules, supramolecular com-
ponents, and organelles. For example, actin filaments are responsible for the myo-
sin-based transport of membrane organelles and the dynamics of these filaments is
essential for their transport ( Semenova et al., 2008 ) and for determining their destina-
tion. Microtubules are closely associated with the transport of peroxisomes and the
depolymerization of microtubules causes peroxysomes to accumulate in the middle
of the cell, preventing their transport ( Rapp et al., 1996 ), whereas the actin cytoskel-
eton may be involved in determining peroxysome size, shape, number, and clustering
( Schollenberger et al., 2010 ). In yeasts, the actin cytoskeleton regulates the partition-
ing of organelles and their movement to the bud ( Catlett and Weisman, 2000 ). In the
case of melanosomes, pigment granules where melanin is synthesized and stored
( Wasmeier et al., 2008 ), their dispersion is related to the extension of microtubules
that, via kinesins, move melanosomes away from the nucleus throughout the cell; and
the reverse occurs when the microtubules shrink ( Ikeda et al., 2011 ). Recent evidence
shows that dispersion and aggregation of melanosomes is related to the increase
and decrease (respectively) in the number of microtubules nucleated at the centro-
some ( Lomakin et al., 2011 ). This is proved by the fact that experimental inhibition
of microtubule growth prevents the aggregation of melanosomes in the pericentriolar
region ( Lomakin et al., 2009 ). During pigment aggregation, it is observed that grow-
ing (plus) ends of microtubules capture melanosomes ( Lomakin et al., 2009 ).
Microtubules are involved in exocytosis and in the transport of hormones, neu-
rotransmitters, and neurotrophins through vesicles from the Golgi apparatus to the
actin cytoskeleton at the site of cell membranes where these substances are released
( Park and Loh, 2008 ) ( Figure 1.15 ).
An examination of the mechanism of regulation of the vital functions of the cell
and particular organelles may help shed some light on the presently dim picture of the
control system in unicellulars. Adequate observational evidence shows all the stages
of the cell cycle, both in unicellulars and multicellulars, are under the control and
regulation of the cytoskeleton (see below and Chapter 2, section “Regulation of the
Length of Microtubules - Key to Transport of Maternal Determinants in the Oocyte”).
Cytoskeleton Controls the Formation of the Eyespot in Unicellulars
Over a century ago, Russian botanist Andrei Famintzin (1835-1918) observed that
green algae from the shores of the Neva River in St. Petersburg contained a yellow