Evidence for the carboxyl methylation of nuclear lamin-B in the pancreaticbeta cell

Lamins are intermediate filament proteins that constitute the main componen ts of the lamina underlying the inner-nuclear membrane and serve to organiz e chromatin, Lamins (e.g., lamin-B) undergo posttranslational modifications (e.g., isoprenylation and methylation) at their C-terminal cysteine. Such modifications are thought to render optimal association of lamins with the nuclear envelop, Herein, we examined whether nuclear lamin-B undergoes carb oxyl methylation in islet beta cells. A 65- to 70-kDa protein was carboxyl methylated in intact rat islets and clonal beta (HIT or INS) cells or in ho mogenates which could be immunoprecipitated using lamin-B antiserum. Incuba tion of purified HIT cell-nuclear fraction with [H-3]S-adenosyl methionine yielded a single carboxyl methylated protein peak (ca. 65-70 kDa); this pro tein was immunologically identified as lamin-B, Several methylation inhibit ors, including acetyl farnesyl cysteine, a competitive inhibitor of protein prenyl cysteine methylation, inhibited the carboxyl methylation of lamin-B , indicating that the carboxyl-methylated amino acid is cysteine, These fin dings, together with our recent observations demonstrating that inhibition of protein isoprenylation causes apoptotic death of the pancreatic beta cel l, raise an interesting possibility that inhibition of C-terminal cysteine modifications of lamin-B might result in disruption of nuclear assembly, le ading to further propagation of apoptotic signals, including DNA fragmentat ion and chromatin condensation. (C) 2000 Academic Press.