ULTIMATE COMPUTING - Quantum Consciousness Studies

Cytoskeleton/Cytocomputer 91
Figure 5.8: Effects of MT disassembly drug nocodazole in PtK2 cells. MT
visualized with peroxidase-antiperoxidase (PAP) method. Upper left: organized
MT radiate from dense MTOC region near nucleus. Upper right: cells have been
treated with nocodazole (2 x 10 -5 M, 4 hours) and MT depolymerized. Lower left:
cells washed after nocodazole, 5 minutes later MT growing from MTOCs. Lower
right: cells 20 minutes after wash have reorganized MT system. With permission
from DeBrabander, Geuens, Nuydens, Willebrords and DeMey (1981).
MT assembly also requires the presence of magnesium ion and a low
concentration of calcium ion. Calcium is an important messenger system within
many forms of cytoplasm. Waves of calcium can be caused by membrane and
cytoskeletal activities, and can induce conformational changes in proteins and
dissolve cytoplasmic gel, converting it to a more aqueous state: “sol.” Tubulin
dimers loosely bind 16 calcium ions per dimer (Hayashi and Matsumara 1975); an
excess of calcium, however, causes MT disassembly. In the presence of abundant
zinc ions, tubulin polymerizes into flat expansive sheets rather than cylinders.
The dimer subunits alpha/beta tubulin are all arranged in the same direction.
The beta subunit protrudes at the “plus” end and contains the exchangeable GTP
binding site. “Cc” for assembly is lower at the plus end than it is at the minus end.
At steady state in the presence of hydrolyzable GTP, a net incorporation of dimer
subunits is seen at the plus end, and a net loss of dimer subunits is seen at the
minus end. Subunits thus move from the plus end to the minus end in MT—a
phenomenon called “treadmilling.” In MT anchored at one end to organizing
centers, treadmilling is a net movement of subunits within the MT lattice, and
may correspond to the slow 1 millimeter per day component of axoplasmic
transport. MT thus appear to be continually growing, perhaps twisting, at a rate of
about 10 nanometers per second. Labile MT can polymerize their way through the
cytoplasm, adding GTP-tubulin at the beta plus end, and dumping GDP tubulin at
the alpha minus end. These types of MT can thus behave like mobile tractors,
cytoskeletal caterpillars (Figure 5.10).