Biomedical Engineering Reference
In-Depth Information
Figure 6.11.
Structures of representative sialic acids.
6.3.11 Energy Source
Information about direct energy sources is essential for elucidation of biomotil-
ity systems. One can effectively obtain this information by analyzing the
inhibitory effects of various drugs on motility. We examined the effects of
various drugs on
M. mobile
gliding and found that the addition of arsenate
reduces gliding speed with a decrease in cellular ATP concentration. However,
ionophores had little or no effect on gliding speed, even if they diminished the
membrane potential. These observations suggest that the gliding mechanism
is coupled to ATP hydrolysis [55].
In a study of the motility of eukaryotic axonemes such as sperm and proto-
zoa, Gibbons et al. [58] constructed a useful system, called the “Triton model”,
where the cell membrane is permeabilized and the intracellular reactions can
be manipulated externally. This system demonstrated sliding between micro-
tubules, ATP as the energy source, dynein as the motor protein, and so on.
Therefore, we constructed this kind of model for
M. mobile
, probed the in-
ternal reactions, and concluded that the gliding mechanism is driven by the
energy of ATP [11]. Treatment with Triton X-100 immediately stopped the
gliding and converted the cells to permeabilized “ghosts”. Surprisingly, when
ATP was added exogenously, 85% of the ghosts were reactivated, gliding at
speeds similar to those of living cells. The reactivation activity and inhibition
by various nucleotides and ATP analogs, as well as their kinetic parameters,
showed that the machinery is driven by the hydrolysis of ATP to ADP plus
phosphate, caused by an unknown ATPase.
P42 is a 356-amino-acid protein encoded downstream of Gli521 (see Figure
6.6). This protein was found to have ATPase activity although the amino acid
sequence does not have any motifs suggesting such activities [59]. The pref-
erences of nucleotides and ATP analogs are similar to those of the unknown
ATPase presumed to be the motor of gliding ghosts. These observations sug-
gest that gliding is driven by the ATPase activity of P42.
