Two-dimensional gel analysis of nuclear proteins during muscle differentiation in vitro. II. Changes in protein phosphorylation

Declining histone phosphorylation during myogenesis revealed by in vivo and in vitro labeling

To investigate histone phosphate levels during myogenesis, proliferation (d 1), pre-fusion postmitotic (d 2) and myotube (d 3) stage cultured chicken myoblasts were phosphorylated in vivo with [32P]orthophosphate or in vitro by incubating isolated nuclei with 32P-gamma-ATP. Incorporation of radioactive phosphate into histone was assessed by SDS and acid/urea/Triton-X-100 (AUT) gel electrophoresis and radioautography. During proliferation, in vivo labeling with [32P]orthophosphate revealed that all histones except H2b were phosphorylated in the following order of decreasing modification: H1 a greater than H2a greater than H1 b greater than H3 greater than H4. In pre-fusion post-mitotic cells phosphorylation of histones H1 a, H3 and H4 declined, whereas all histones exhibited significantly decreased modification at the myotube stage. It is unlikely that these changes resulted from decreased specific radioactivity of intracellular inorganic phosphate pools, since uptake of [32P]orthophosphate by myotubes increased six-fold, compared with proliferating cells. Isolated nuclei incubated with 32P-gamma-ATP displayed similar decreases during myogenesis; however, 1 a, H1 b and H3 were the only histones modified by in vitro phosphorylation.

Comparative studies of nuclear phosphoproteins from four different muscle tissues and liver

1. Nuclei isolated from red and white skeletal muscle, heart, uterus and liver of rabbit were incubated with [lambda-32P]ATP and the incorporation of 32P into nuclear protein was examined. 2. Liver nuclear proteins were phosphorylated to a much greater extent than those of muscle. Particularly low levels of phosphorylation were observed in white greater extent than those of muscle. Particularly low levels of phosphorylation were observed in white muscle nuclei. 3. Most of the phosphoproteins were common to all the tissues but significant quantitative differences and a limited number of qualitative differences were evident, particularly between liver and the muscle tissues. 4. Many phosphoproteins detected were minor components of the total nuclear protein.