ULTIMATE COMPUTING - Quantum Consciousness Studies

Models of Cytoskeletal Computing 161
of the filaments appears to be controlled by the microtubule system, analogous to
its function in cell movement and cell shape. DeBrabander (1977) observes that in
these phenomena MT act beyond their capacity as skeletal support elements and
perform as signal transducers from the cell center to the periphery, and vice versa.
Interactive signaling in the cytoskeleton fits with observations that local events at
one site in the cell often lead to a global cellular rearrangement. DeBrabander
raises several MT features which could favor signaling: MT intrinsic polarity, the
existence of “centrifugal and centripetal microtubules,” the conformational
propagation mechanism proposed by Atema (1973), directional organelle
movement and topography, and transport of ions such as calcium along
microtubules.
DeBrabander and his associates (1986) have also innovated new techniques in
the examination of cytoskeletal structure and dynamics. These include “Nanovid
Microscopy,” a nanometer scale video microscopy in which transport of labeled
particles along individual MT may be visualized, and a method of labeling
individual tubulin subunits within MT which are either tyrosinated or glutamated
(Geuens, Gundersen, Nuydens, Cornelissen, Bulinski, DeBrabander, 1987). As
described in Chapter 5, polymerized MT may be detyrosinated in the cytoplasm
subsequent to DNA directed genetic input. Tyrosine, the terminal amino acid in
tubulin, may be removed to expose glutamate as the new terminal amino acid.
Thus all tubulins within polymerized MT are either tyrosinated, or glutamated. A
variety of cytoplasmic factors determine whether or not a particular tubulin is
tyrosinated. The precise significance or function of tyrosination/glutamation is
unknown but could serve as a convenient programming or memory function.
DeBrabander and colleagues developed a double labeling technique in which
immunogold particles bind to tubulin subunits. Large immunogold particles (10
nanometers) identify glutamated tubulin and smaller particles (5 nanometers) bind
to tyrosinated tubulin. Patterns of tyrosinated/glutamated tubulin within MT show
heterogenous distribution, and suggest the potential for a coding mechanism.
MAP attachment, tubulin conformation, calcium binding and other factors could
be coupled to MT function via such patterns. Although predisposition to
detyrosination may be genetically linked to tubulin isozymes, the actual patterns
of tubulin detyrosination are determined by “real time” cytoplasmic factors.
DeBrabander and colleagues have provided the first direct evidence of modifiable
patterns of tubulin variability in intact microtubules. Consequently, MT appear
capable of signal processing in addition to signal transduction.