Title: Murine Sirt3 protein isoforms have variable half-lives Authors
|Yang, Yongjie -|
|Chen, Ke Yun -|
|Tong, Qiang -|
Submitted to: Islets
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 27, 2011
Publication Date: November 15, 2011
Citation: Yang, Y., Chen, K., Tong, Q. 2011. Murine Sirt3 protein isoforms have variable half-lives. Islets. 488:46-51. Interpretive Summary: SIRT3 is an important protein modification enzyme (deacetylase) that regulates the function of many metabolic enzymes. It was recently found that three variants can be expressed from the mouse SIRT3 gene. In this study, we characterized the stability of these three SIRT3 isoforms. We found that the shortest M3 isoform, but not the longer M1 and M2 isoforms, has a short half-life. The M3 proteins are ubiquitinated and degraded by the proteasomes. We further found that the longer M1 and M2 proteins but not M3, interact with chaperon protein HSP60, which may protect the longer isoforms from degradation. This offers a possible explanation of the difference of the half-lives of these protein isoforms.
Technical Abstract: Sirt3 is a NAD+-dependent protein deacetylase mainly localized in mitochondria. Recent studies indicate that the murine Sirt3 gene expresses different transcript variants resulting in three possible Sirt3 protein isoforms with variable lengths at the N-terminus: M1 (aa 1-334), M2 (aa 15-334), and M3 (aa 78-334). In this study, we stably expressed these variants in several cell lines. We found that Sirt3 M1 or M2 can be stably expressed with predominant mitochondrial localization. However, stable expression of Sirt3 M3 protein was consistently at very low levels. Fast proteasomal degradation contributes to the low expression of Sirt3 M3 protein, as proteasome inhibitor treatment increased Sirt3 M3 protein levels in these cells. Sirt3 M3 protein is ubiquitinated and the E3 ubiquitin ligase subunit Skp2 is involved in Sirt3 M3 protein degradation. Additionally, we found Sirt3 M3 protein can be produced from Sirt3 transcripts encoding longer M1 and M2 isoforms. To explore the mechanism underlying the instability of Sirt3 M3 protein, we found that Sirt3 M1 and M2 proteins, but not M3, specifically interact with HSP60. This suggests that heat shock proteins might play a role in the maintenance of Sirt3 protein stability.