Regulation of insulin secretion by SIRT4, a mitochondrial ADP-ribosyltransferase

Sirtuins are homologues of the yeast transcriptional repressor Sir2p and are conserved from bacteria to humans. We report that human SIRT4 is localized to the mitochondria. SIRT4 is a matrix protein and becomes cleaved at amino acid 28 after import into mitochondria. Mass spectrometry analysis of proteins that coimmunoprecipitate with SIRT4 identified insulindegrading enzyme and the ADP/ATP carrier proteins, ANT2 and ANT3. SIRT4 exhibits no histone deacetylase activity but functions as an efficient ADP-ribosyltransferase on histones and bovine serum albumin. SIRT4 is expressed in islets of Langerhans and colocalizes with insulin-expressing beta cells. Depletion of SIRT4 from insulin-producing INS-1E cells results in increased insulin secretion in response to glucose. These observations define a new role for mitochondrial SIRT4 in the regulation of insulin secretion.