  The cytoskeleton protein actin assembles into large bundles when supporting
stresses in the cell, but grows into a fine branched network to induce cell
motion. Such self-organization processes are studied in artificial networks of
cytoskeleton proteins with thick actin bundles and large motor protein
aggregates to enable optical observation. The effect of motor aggregate size on
the cytoskeleton mechanical properties is studied here in networks comprised of
much smaller motor assemblies. Large motor protein clusters are known to
increase the stiffness of actin based networks by introducing tension and
additional cross-linking cites. We find that these effects are universal to
actin gels regardless of actin bundle thickness and motor aggregate size and
are relevant, therefore, to a wide range of cytoskeleton based cellular
processes. In contrast, motor induced active fluctuations depend significantly
on motor assembly size, featuring unique non-Gaussian statistics at high
concentrations of small assemblies.
