Bacterially synthesized vertebrate calmodulin is a specific substrate for ubiquitination

Area of Science:

• Molecular Biology
• Biochemistry

Background:

• Calmodulin is a vital calcium-binding protein.
• Post-translational modifications regulate calmodulin function.
• Ubiquitination targets proteins for degradation.

Purpose of the Study:

• To investigate the ubiquitination site and mechanism of bacterially expressed calmodulin.
• To compare ubiquitination of bacterial calmodulin with Dictyostelium calmodulin.
• To explore the role of post-translational modifications in protecting calmodulin.

Main Methods:

• Selective conjugation of calmodulin with ubiquitin using reticulocyte extracts.
• Limited proteolytic digestion to identify the ubiquitination site.
• Enzyme kinetics (ATP hydrolysis) to determine enzyme affinity (Km).

Main Results:

• Lysine 115 was identified as the primary ubiquitination site on bacterial calmodulin, matching Dictyostelium calmodulin.
• Similar enzyme affinities (Km values) were observed for both calmodulin types.
• Bacterial calmodulin, unlike Dictyostelium calmodulin, formed multi-ubiquitin chains.

Conclusions:

• Post-translational methylation of lysine 115 likely protects calmodulin from ubiquitination and degradation.
• Acetylation of the N-terminus may also protect calmodulin from ubiquitination.