Segmental flexibility of the C1q subcomponent of human complement and its possible role in the immune response

Fluorescence polarization techniques were used to study the rotational dynamics of the C1q subcomponent of human complement. C1q was covalently labeled with dansyl (DNS) chloride. Digestion of either C1q-DNS4.0 or C1q-DNS1.8 conjugates with pepsin showed that about 75% of the DNS probes were attached to the C1q globular heads and that the remainder were on the collagen-like stalk (peptic fragment). C1q-DNS conjugates readily agglutinated IgG-coated latex beads and combined with C1r2C1s2 to form hemolytically active 16 S C1-DNS. Both C1q-DNS and C1-DNS samples displayed steady-state rotational correlation time and fluorescence lifetime transitions near 48 degrees C. Hydrodynamic studies showed that C1q formed soluble aggregates near the transition temperature. In contrast, stalk samples with a DNS probe apparently attached to the large central fibril showed no thermal transitions or aggregation even when heated above 50 degrees C. Nanosecond fluorescence depolarization measurements detected restricted flexible motions of the C1q heads with an associated rotational correlation time, phi s, of about 25 ns. The C1q anisotropy decay was dominated, however, by a long component, phi L, of perhaps 1000 ns. Except for probe wiggle, the stalk-DNS anisotropy profile was essentially flat. The rapid rotations associated with phi s could represent restricted twisting motions of the arm-head segments or wobbling motions of the heads themselves. Such motions may facilitate binding of the C1q heads to immune complexes. Straightforward diffusion calculations indicated that phi L could represent either global tumbling of the entire C1q molecule or wagging motions of the individual arm-head segments, as suggested by electron micrographs. Upon binding of the C1q heads to an activator, some of the C1q segments may be held in a slightly more open or more closed conformation, which in turn may trigger activation of the C1 proenzymes. In conclusion, we suggest a plausible triggering mechanism for C1 activation that is compatible with the flexible properties of its subcomponents.