Transition of β-actinin isoforms during development of chicken skeletal muscle

We examined by means of the immunoblotting technique the transition of β-actinin isoforms during the development of the chicken from 5 day embryo to adult. As an antigen, β-actinin was prepared from adult chicken breast muscle (pectoralis major) and polyclonal antibody was obtained by injecting undenatured β-actinin into a rabbit. Immunoblotting examination of breast muscle at several stages of development (except 5 day embryo, in which the whole body minus the head and limbs was examined) showed that the species of β-actinin subunits change during development: 1) βI is already present in 5 day embryo, whereas βII appears only after 9 days. 2) In 5 day embryo, we found, instead of βII, a new subunit (designated βIII) that cross-reacts with the antibody, has the apparent molecular weight of 30,000 daltons and has a slightly alkaline isoelectric point compared with βI. The content of βIII gradually decreased and βIII completely disappeared a week after hatching. Such a type of transition of the isoforms in β-actinin subunits is similar to that observed in other muscle proteins. The transition of β-actinin isoforms may correlate to the organization of an I-Z-I brush, especially to the length determination of thin filaments, because the developmental stage at which βIII disappears coincides with that at which the length of thin filaments is strictly determined.