ULTIMATE COMPUTING - Quantum Consciousness Studies

92 Cytoskeleton/Cytocomputer
Disassembly or separation of tubulin dimer subunits from MT is induced
when terminal dimers bind GDP. When GTP is hydrolyzed, a phosphate group is
lost and guanosine triphosphate (GTP) becomes guanosine diphosphate (GDP).
GTP bound dimers at open ends of MT (GTP “caps”) are stable: they stay
assembled. However, when open ends of MT contain subunits binding GDP, they
tend to release and shorten the MT cylinder. MT whose distal ends have exposed
GDP tubulin, and are not anchored by structures like MTOC, shrink and
depolymerize. Generally, free MT polymers (those not anchored by MTOC) are
“chimeric” with stretches of unhydrolyzed GTP at the ends (GTP caps) and GDP
in the interior. Growing polymers have “protective” GTP subunit caps at their
end; shrinking polymers lose their GTP caps and expose GDP subunits at the end,
causing disassembly.
Microtubules thus appear to exist in two populations: the majority growing at
an appreciable rate and the minority shrinking very rapidly and providing new
subunits for growth. This concept has been called “dynamic instability” with the
rapid shrinkage of MT referred to as “microtubule catastrophes” (Kirschner and
Mitchison, 1986). The faster the growth rate, the larger the GTP cap and the lower
the probability of the cap disappearing and the microtubule depolymerizing.
When the cap disappears and GDP subunits are exposed, the polymer enters a
rapid depolymerizing (“catastrophic”) phase. MT contain thirteen parallel
protofilaments and it is unclear how many exposed GTP or GDP containing
subunits at MT terminals are required for stability or instability. An essential
factor is the rate of GTP hydrolysis which results in GDP tubulin and induces
disassembly. The utilization of GTP hydrolysis energy by MT and the
cytoskeleton is a significant portion of biological energy consumption, yet
remains unappreciated. Theories of protein conformational state regulation such
as solitons and coherent excitations which could account for the useful
consumption of hydrolysis energy will be described in Chapter 6. Solitons,
coherent lattice vibrations, and cooperative resonance present possibilities for
collective, and possibly intelligent effects within cells.
Japanese investigators Horio and Hotani (1986) have directly observed
growing and shrinking MT using dark field microscopic video. They observed
that both ends of a microtubule can exist in either the growing or the shortening
phase and alternate quite frequently between the two phases in an apparently
random manner. Further, growing and shortening ends can coexist on a single
microtubule: treadmilling. One end may continue to grow simultaneously with
shortening at the other end. The two ends of any given microtubule have
remarkably different characteristics. One “active” end (presumably the beta, plus
end) grows faster, alternates in phase between growing and shrinking more
frequently, and fluctuates in length to a greater extent than the inactive end.
Microtubule associated proteins (“MAPs”) suppress the phase conversion and
stabilize microtubules in the growing phase.
There are many apparent mechanisms for stabilizing polymerized
microtubules: they may be capped or bound to various structures. Binding at the
proximal end to MTOCs stabilizes MT and prevents their depolymerizing at the
opposite end. MT need to be stabilized at merely one end to predominate within
cells because, even if they depolymerize, another will reform in the same place
and orientation (DeBrabander, 1985). Thus MTOC provide cells with a means of
selective retention of a subclass of specifically oriented MT.
MTOC oriented in a specific direction can determine cell polarity, including
extension of cells in embryological growth, formation of probing appendages
called filopodia in locomotory cells and growth of nerve processes. Signals at the
cell periphery apparently cause an asymmetry of the microtubule cytoskeleton