The in vitro motility assay is valuable for fundamental studies of actomyosin function and has recently been
combined with nanostructuring techniques for the development of nanotechnological applications. However, the
limited understanding of the interaction mechanisms between myosin motor fragments (heavy meromyosin, HMM)
and artificial surfaces hampers the development as well as the interpretation of fundamental studies. Here we elucidate
the HMM−surface interaction mechanisms for a range of negatively charged surfaces (silanized glass and SiO2), which
is relevant both to nanotechnology and fundamental studies. The results show that the HMM-propelled actin filament
sliding speed (after a single injection of HMM, 120 μg/mL) increased with the contact angle of the surfaces (in the
range of 20−80°). However, quartz crystal microbalance (QCM) studies suggested a reduction in the adsorption of
HMM (with coupled water) under these conditions. This result and actin filament binding data, together with previous
measurements of the HMM density (Sundberg, M.; Balaz, M.; Bunk, R.; Rosengren-Holmberg, J. P.; Montelius, L.;
Nicholls, I. A.; Omling, P.; Tågerud, S.; Månsson, A. Langmuir
2006, 22, 7302−7312. Balaz, M.; Sundberg, M.;
Persson, M.; Kvassman, J.; Månsson, A. Biochemistry
2007, 46, 7233−7251), are consistent with (1) an HMM
monolayer and (2) different HMM configurations at different contact angles of the surface. More specifically, the QCM
and in vitro motility assay data are consistent with a model where the molecules are adsorbed either via their flexible
C-terminal tail part (HMMC) or via their positively charged N-terminal motor domain (HMMN) without other surface
contact points. Measurements of ζ potentials suggest that an increased contact angle is correlated with a reduced
negative charge of the surfaces. As a consequence, the HMMC configuration would be the dominant configuration
at high contact angles but would be supplemented with electrostatically adsorbed HMM molecules (HMMN configuration)
at low contact angles. This would explain the higher initial HMM adsorption (from probability arguments) under the
latter conditions. Furthermore, because the HMMN mode would have no actin binding it would also account for the
lower sliding velocity at low contact angles. The results are compared to previous studies of the microtubule−kinesin
system and are also discussed in relation to fundamental studies of actomyosin and nanotechnological developments
and applications.