Interaction of talin with actin: sensitive modulation of filament crosslinking activity.

Talin is an adhesion plaque protein believed important in linking actin filaments to the plasma membrane. The nature of a direct talin-actin interaction, however, is complex and has remained unclear. We have systematically characterized the effects of pH, ionic strength, temperature, and protein molar ratio on the interaction between highly purified talin and actin. The ability of talin to increase viscosity of F-actin at 25 degrees C and low ionic strength increased with decreasing pH from 7.3 to 6.4 and increasing molar ratio of talin to actin. At pH 6.4 and low ionic strength, talin could extensively crosslink actin filaments into ordered bundles as shown by negative staining and could cosediment with F-actin at molar ratios as high as one talin to two actin monomers. Talin crosslinked prepolymerized actin filaments to a similar extent as actin filaments polymerized in its presence. The 190-kDa calpain-generated proteolytic fragment of talin bound poorly to actin under conditions favorable for intact talin, but was able to crosslink actin filaments at a lower pH. Increasing the ionic strength within a relatively narrow range significantly decreased ability of talin to bind to actin, regardless of pH. The effects of pH and ionic strength on the talin-actin interaction were rapid and reversible. Low-shear-viscosity studies revealed a strong temperature dependence in the talin-actin interaction with significant crosslinking activity at physiological-like ionic conditions and temperature (37 degrees C). Our results consistently demonstrated that talin crosslinks actin filaments and that this direct interaction is highly sensitive to, and dependent upon, ionic conditions and temperature.