The NDUFS4 subunit of complex I of the mammalian respiratory chain has a fully conserved carboxy-terminus with a canonical RVSTK phosphorylation site. Immunochemical analysis with specific antibodies shows that the serine in this site of the protein is natively present in complex I in both the phosphorylated and non-phosphorylated state. Two-dimensional IEF/SDS-PAGE electrophoresis, (32)P labelling and immunodetection show that "in vitro" PKA phosphorylates the serine in the C-terminus of the NDUFS4 subunit in isolated bovine complex I. (32)P labelling and TLC phosphoaminoacid mapping show that PKA phosphorylates serine and threonine residues in the purified heterologous human NDUFS4 protein.
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