Effects of a microbial pigment violacein on the activities of protein kinases

Abstract

Violacein, a purple bacterial pigment, has a variety of biological activities, including cytotoxicity against tumour cells. To investigate the physiological properties of this pigment, we studied its inhibitory effect on protein kinases in vitro. We used phosphorylation experiments to assess the inhibitory effect of violacein on protein kinase activity. Violacein inhibited the classical-type protein kinase C (PKC) and some novel and atypical PKC enzymes. The catalytic subunits of protein kinase A (PKA) and classical-type PKC were strongly inhibited by violacein. Kinetic analysis of PKA inhibition displayed mixed-type inhibition with respect to ATP. Slight inhibition of Ca²⁺/calmodulin-dependent protein kinase was observed while protein tyrosine kinase (Src) was not inhibited by violacein. The inhibition of PKA and PKCs by violacein provided evidence that violacein binds to the catalytic subunit of enzymes. The protein kinase inhibition revealed here may be important to the mechanism of violacein cytotoxicity.

Keywords:

• microbial pigment
• enzyme inhibition
• violacein
• protein kinase A
• protein kinase C

Public Interest Statement

Violacein is a pigment produced by some terrestrial and marine bacteria. The physiological role of violacein is unknown, but it has been reported to play a role in chemical defence against predators. Furthermore, violacein has strong antitumour activity against various types of cancer cells. In this study, we examined the inhibitory properties of violacein on the activity of protein kinases and showed that violacein could inhibit protein kinase C and protein kinase A. The findings of this study will help characterise the molecular mechanisms underlying violacein’s antitumour activity and may suggest possible medical applications of violacein.

Competing interests

The authors declare no competing interest.

Additional information

Funding

This work was supported in part by a Grant-in-Aid (14-051) for research promoting technological seed projects from the Japan Science and Technology Agency.

Notes on contributors

Keiichi Enomoto

Kakushi Hosokawa, Azamjon B. Soliev and Aki Kajihara joined this research project as graduate students at Kochi University of Technology under the supervision of Keiichi Enomoto. Enomoto’s group has focused its attention on marine bacteria that produce bacterial pigments including violacein and prodigiosin. Prior to this study, the authors had isolated pigment-producing marine bacteria, purified and identified bacterial pigments and isolated the gene cluster for violacein synthesis. Further research is being conducted to clarify the regulatory mechanism of violacein biosynthesis in marine bacteria and the molecular mechanism underlying violacein’s antitumour activity. This study was conducted as a research project related to the molecular mechanism of violacein’s physiological properties.