Comparative electron microscopic study on projectin and titin binding to F-actin.

Using the system of F-actin paracrystals, we have obtained electron microscopic evidence that projectin from synchronous flight muscles of Locusta migratoria binds to actin filaments in the same fashion as skeletal titin. Control actin paracrystals formed in the presence of Mg(2+) ions have great width and length and blunted ends. The addition of either projectin or titin results in disruption of compact ordered packing of F-actin in paracrystals and leads to the formation of loose filament bundles with smaller diameters and tapered ends. It is also accompanied with the appearance of individual actin filaments in considerable amounts. The effect becomes more pronounced with the increase in concentrations of added projectin or titin. Possible physiological implications of projectin-actin interactions are discussed.

Study of F-actin interaction with planar and liposomal bilayer phospholipid membranes.

Interaction of the cytoskeletal protein F-actin with planar bilayer lipid membrane (BLM) induced formation of single ionic channels in both NaCl and KCl bathing solutions. We also recorded noiselike high-currentjumps with a mean conductivity of approximately 160 pS, which might represent the simultaneous opening and closing of several channels of lower conductivity. The ratio of cation to anion permeabilities (Pc/Pa) of the BLM with many channels in KCl was 26 +/- 2. Freeze-fracture electron microscopy revealed fibrillar-like structures on the hydrophobic surfaces of liposomal membranes. We also observed some structural features giving evidence for the penetration of F-actin fibers through an artificial phospholipid membrane. We suggest that the F-actin/lipids complexes can transmit electric signals in synaptic and other intercellular contacts.

Ca(2+)-dependent effects of C-protein on the actin-activated ATPase of phosphorylated and dephosphorylated skeletal muscle myosin.

The effects of C-protein on actin-activated myosin ATPase depending on Ca(2+)-level and LC2-phosphorylation were studied. Column-purified myosin and non-regulated actin were used. At ionic strength of 0.06 C-protein inhibits actomyosin ATPase activity both in the presence and in the absence of calcium, more effective in the case of dephosphorylated myosin. For this myosin, at mu = 0.12 C-protein activates actomyosin ATPase at pCa4, but slightly inhibits at pCa8. No such effects have been observed in the case of phosphorylated myosin. The possibility of coordinative action of LC2-chains and C-protein in regulatory mechanism of skeletal muscle contraction is discussed.

Effect of C-protein and LC-light chains on actomyosin ATPase at various ionic strength and calcium levels.

Interrelation between the effects of C-protein and LC2-light chains on actin-activated ATPase activity of skeletal muscle myosin has been investigated at various ionic strength (0.06-0.14) and free calcium levels (10(-4) M, 10(-8) M). The ATPase activity of AM reconstituted with column-purified myosin or partly-purified myosin and non-regulated actin exhibits a pronounced dependence on ionic strength with maximum at I = 0.1. C-protein impurities (5 per cent) usually present in Minit can inhibit AM ATPase at every ionic strength assayed, without changing the character of this dependence. Actin-activated ATPase of the above myosins shows calcium sensitivity at every ionic strength studied. The partial removal of LC2 from Mcol results in a decrease of AM ATPase and in a disappearance of its calcium sensitivity. C-protein added to Mcol in a molar ratio of 1:1 inhibits considerably AM ATPase, reduces its sensitivity to ionic strength and abolishes its calcium sensitivity.