Identification of ubiquitin conjugated Protein Phosphatase Inhibitor 1 from postmortem bovine muscle

Previously, we characterised the presence of ubiquitin and some ubiquitinated proteins (conjugates), which were considered as ubiquitin conjugates in the sarcoplasmic fraction of postmortem skeletal and cardiac muscles using SDS–PAGE and Western blot. In this study, an ubiquitin conjugate around 27 kDa was subjected to identification. The N-terminal amino acid sequence analysis generated by Edman degradation resulted in a 13 amino acid sequence (KIQFTVPLLEPHL), which is similar to that of Protein Phosphatase Inhibitor 1. Based on this result, Western blot analysis was performed by using a polyclonal antibody for Protein Phosphatase Inhibitor 1 and the conjugate was recognised and reacted clearly. Thus, the ubiquitinated protein around 27 kDa is identified as Protein Phosphatase Inhibitor 1.