  Myosin-II's rod-like tail drives filament assembly with a head arrangement
that should generate equal and opposite contractile forces on actin--if one
assumes that the filament is a symmetric bipole. Self-assembled myosin
filaments are shown here to be asymmetric in physiological buffer based on
cross-correlated images from both atomic force microscopy (AFM) and total
internal reflection fluorescence (TIRF). Quantitative cross-correlation of
these orthogonal methods produces structural information unavailable to either
method alone in showing that fluorescence intensity along the filament length
is proportional to height. This implies that myosin heads form a shell around
the filament axis, consistent with F-actin binding. A motor density of ~50 -
100 heads/micron is further estimated but with an average of 32% more motors on
one half of any given filament compared to the other, regardless of length. A
purely entropic pyramidal lattice model is developed that qualitatively
captures this lack of length dependence and the distribution of filament
asymmetries. Such strongly asymmetric bipoles are likely to produce an
imbalanced contractile force in cells and in actin-myosin gels, and thereby
contribute to motility as well as cytoskeletal tension.
