Summary
In our studies of human platelets we have detected the presence of the molecular motors
kinesin and dynein. Dynein is present at a concentration (0.8 μg/g tissue) that is
approximately 1/3 the concentration reported for neuronal tissue. Immunofluorescence
microscopy of resting platelets shows that, while platelet microtubules are arranged
in coiled hoops forming the marginal band in the cortical region of the platelet,
dynein is distributed in a pattern of punctate staining throughout the cytoplasm of
the platelets. Fractionation of unactivated platelets shows that dynein partitions
to the soluble fraction. Stimulation of platelets with thrombin, ADP or epinephrine
causes a partial translocation of dynein from the soluble fraction to the particulate
fraction with thrombin being the most efficient agent at promoting this shift. Dynein
intermediate chain recovered in the soluble fraction of disrupted platelets following
activation displays a transient, time-dependent phosphorylation. In contrast, dynein
intermediate chain recovered in the particulate fraction shows decreased phosphorylation.
These results indicate that human platelets contain a complex microtubule-based system
of motor proteins that is an integral part of the physiological changes occurring
during platelet activation.
