[Structural changes in actin filaments during binding with phosphofructokinase (F-protein), detected using an optical...

It has been previously shown by us that phosphofructokinase (F-protein) binds to rabbit skeletal muscle F-actin and reconstituted thin filaments forming ordered bundles. Upon low molar ratios of phosphofructokinase to actin in the complex bundles the enzyme molecules are arranged between actin filaments regularly in the form of crossbridges. The increase of phosphofructokinase: actin ratio up to equimolar one and more leads to the filling of the space between actin filaments with phosphofructokinase-molecules. In these bundles the inclined cross striation with axial repeat of about 7.0 nm is clearly seen. Optical diffraction analysis of the micrographs revealed new features in the structure of such complexes in comparison with F-actin and reconstituted thin filaments. Optical diffraction patterns from F-actin and reconstituted thin filaments exhibit the first (35.5 nm) and sixth (5.9 nm) layer lines typical of F-actin helix structure. On the optical diffraction patterns from the complex bundles the meridional reflection of about 7.2nm is present, that is not observed on the optical diffraction patterns from F-actin alone. This reflection is characteristic of optical diffraction- and X-ray patterns of myosin filaments and is the sixth order of axial period of their structure (42.9 nm/6). The structural changes occurring upon binding is supposed to be important for the mutual regulation of functional activity of enzymes and actin-containing filaments in muscle.