Control Systems in the Living World - Building the Most Complex Structure on Earth - page 31

The cytoskeleton is not unique to eukaryotic unicellulars. Bacteria also contain
all three structural and functional homologues of eukaryotic cytoskeletal structures
(microtubules, intermediate filaments, and actin filaments) ( Muñoz-Espín et al., 2009 ),
as well as the protein-building blocks of these polymers, tubulins, intermediate fila-
ment dimers, and globular actins. In prokaryotes like E. coli FtsZ, the prokaryote tubu-
lin homologue is responsible for forming the Z ring, a constriction at the middle of
the dividing cell. Depolymerization of the FtsZ ring is believed to provide the force
necessary to complete cytokinesis ( Errington et al., 2003; Nanninga, 2001 ) and the for-
mation of septum by PBP3 (penicillin-binding protein) ( Vollmer and Höltje, 2001 ). In
the fission yeast, microtubules are the main factors determining organization of actin
filaments and formation of the cytokinetic ring ( Chang and Martin, 2009 ).
The cytoskeleton modifies its structure in response to the binding of integrins,
hormones, growth factors, etc., to their respective cell-membrane receptors. This
leads to changes in the cell shape and the relative position of cell organelles. The
changes also extend up to the cell nucleus. The resulting phosphorylation of the
fibrillar elements of the cytoskeleton and nuclear matrix induces the reorganization
of the fibrillar network, which leads to the exposure and sequestration of specific
regions of the chromosome-only enabling expression of specific genes ( Ben-Ze'ev,
1991; Puck et al., 1990 ). This implies that the cytoskeleton may also be involved in
the process of cell differentiation in mammals. In support of this, there is empiri-
cal evidence that the administration of different agents (insulin and IGF-I) in mam-
mal cells induces different changes in the structure of cytoskeleton and in different
results of their administration, implying activation of different signal transduction
pathways ( Berfield et al., 1997 ).
How does a unicellular coordinate, in space and time, the formation of its orga-
nelles and complex processes such as cell reproduction? How does a unicellular so
finely tune its locomotion course in the direction of light or nutrients?
The above evidence shows clearly that all the processes of cell division, including
duplication of chromosomes, their bidirectional separation and their placement to the
to-be “daughter cells,” directed cell locomotion, etc., are not determined either by
the nucleus or by chromosomes. This short review of the regulation of all the stages
of cell reproduction, as well as formation and precise asymmetric localization of the
eyespot, shows that the key element in controlling all of the above is the cytoskel-
eton: all the evidence leads to the cytoskeleton and MTOCs.
There is ample evidence that the cytoskeleton regulates all the above vital func-
tions of the cell, gene expression, DNA replication, chromosome segregation, cytoki-
nesis, and assembly of the organelles in daughter cells. But what the cytoskeleton
does cannot help us to understand how it does it; what we need is not a description
but an explanation of these functions of the cytoskeleton. We need to know whether
cytoskeletal structures are endowed with the capacity to receive data from the inter-
nal and external environments, and, by processing them, to make decisions and send
instructions to organelles or be directly involved in executing these decisions. Again,
the evidence at hand strongly suggests that microtubules may be endowed with such
computational capabilities, which are very much unknown.