Beta-actinin: a capping protein at the pointed end of thin filaments in skeletal muscle.
We examined the function of beta-actinin as a pointed end capping protein of thin filaments in skeletal muscle. An improvement in preparing beta-actinin yielded purified beta-actinin which retained its activity for more than a week. Two-dimensional gel electrophoresis showed that the two subunits, beta I and beta II, of beta-actinin are, respectively, split into two to three components (isoforms) with different isoelectric points. Polyclonal antibody was raised by injecting such purified and undenatured chicken breast muscle beta-actinin composed of several components into a rabbit. Immuno-gold labeling examination with electron microscopy of an F-actin-beta-actinin complex decorated with HMM showed that 85% of bound gold particles was on the pointed end of actin filaments, while the remaining 15% was on the barbed end. This suggests that in beta-actinin preparation pointed end and barbed end capping proteins inevitably coexist. Immunofluorescence and immunoelectron microscopy directly showed that beta-actinin is located at the pointed end of thin filaments in myofibrils; it was also suggested that a capping protein having common antigenic determinants to beta-actinin is located at Z-line. Thus, the physiological function of beta-actinin as a pointed end capping protein was examined as follows: When beta-actinin was dissociated from the pointed end of thin filaments in an I-Z-I brush by using a high salt solution, thin filaments could be disassembled at the pointed ends at concentrations of exogenous actin lower than a critical value. At a physiological ionic strength, these salt-washed thin filaments gradually shortened at a constant rate of about 45 nm/h. Both the association and dissociation of monomeric actin at the pointed end were suppressed by the rebinding of exogenous beta-actinin. The main physiological role of beta-actinin is therefore to stabilize thin filaments in the sarcomere by preventing addition and removal of actin monomers at the pointed filament end.
Funatsu
T
T
Department of Physics, School of Science and Engineering, Waseda University, Tokyo.
Asami
Y
Y
Ishiwata
S
S
D008841:Actin Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D000185:Actinin / Q000378:metabolism*; D000199:Actins / Q000378:metabolism; D000818:Animals; D002645:Chickens; D003599:Cytoskeleton / Q000378:metabolism*; D007700:Kinetics; D009132:Muscles / Q000378:metabolism*; D009210:Myofibrils / Q000378:metabolism* / Q000648:ultrastructure; D009218:Myosins / Q000378:metabolism; D011817:Rabbits; D014783:Viscosity
D000199:Actins; D000185:Actinin; D009218:Myosins