Actin in young and senescent fibroblasts.

Double labelling and the isolation of peptides specific to muscle actin were used to determine the amount of actin in young and senescent fibroblasts from chicken embryo. A 20-residue peptide produced from the C-terminus of muscle actin was found in amounts that indicated 6.9% of the protein of young fibroblasts and 11.5% of the protein of senescent fibroblasts to be actin. Two other peptides of muscle actin were present in very small amounts. This indicates that the actin of both young and senescent fibroblasts is homologous to, but not identical with, muscle actin. Increased actin content of senescent fibroblasts may be related to their loss of proliferative capacity and low cell density at confluence.