THE STRUCTURAL-CHANGES IN ACTIN-FILAMENTS ON THE BINDING TO PHOSPHOFRUCTOKINASE (F-PROTEIN) AS REVEALED BY OPTICAL DIFFRACTION

It has been previously shown by us that phosphofructokinase (F-protein ) binds to rabbit skeletal muscle F-actin and reconstituted thin filam ents forming ordered bundles. Upon low molar ratios of phosphofructoki nase to actin in the complex bundles the enzyme molecules are arranged between actin filaments regularly In the form of crossbridges. The in crease of phosphofructokinase: actin ratio up to equimolar one and mor e leads to the filling of the space between actin filaments with phosp hofructokinase-molecules, in these bundles the inclined cross striatio n with axial repeat of about 7,0 nm is clearly seen. Optical diffracti on analysis of the micrographs revealed new features in the structure of such complexes in comparison with F-actin and reconstituted thin fi laments. Optical diffraction patterns from F-actin and reconstituted t hin filaments exhibit the first (35,5 nm) and sixth (5,9 nm) layer lin es typical of F-actin helix structure. On the optical diffraction patt erns 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 diffr action- 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 occuring upon binding is supposed to be important for the mutual regu lation of functional activity of enzymes and actin-containing filament s in muscle.