Evidence for the carboxyl methylation of nuclear lamin-B in the pancreatic beta cell.

Lamins are intermediate filament proteins that constitute the main components of the lamina underlying the inner-nuclear membrane and serve to organize 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 carboxyl 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 homogenates which could be immunoprecipitated using lamin-B antiserum. Incubation of purified HIT cell-nuclear fraction with [(3)H]S-adenosyl methionine yielded a single carboxyl methylated protein peak (ca. 65-70 kDa); this protein was immunologically identified as lamin-B. Several methylation inhibitors, 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 findings, together with our recent observations demonstrating that inhibition of protein isoprenylation causes apoptotic death of the pancreatic beta cell, raise an interesting possibility that inhibition of C-terminal cysteine modifications of lamin-B might result in disruption of nuclear assembly, leading to further propagation of apoptotic signals, including DNA fragmentation and chromatin condensation.