Identification of nuclear/nucleolar localization signal in Aplysia learning associated protein of slug with a molecular mass of 18 kDa homologous protein.

We isolated a learning associated protein of slug with a molecular mass of 18 kDa (LAPS18) homologue from the expressed sequence tag database of Aplysia kurodai and named it Aplysia LAPS18-like protein (ApLLP). ApLLP encodes 120 amino acids and has 57% identity with LAPS18. To examine the subcellular expression pattern of ApLLP we constructed an EGFP-tagged ApLLP fusion protein and overexpressed it in both Aplysia neurons and COS-7 cells. In contrast to the previous findings, which showed that LAPS18 is secreted by COS-7 cells, ApLLP-EGFP was localized to the nucleus, and most of it to nucleoli. Analysis of deletion mutants of ApLLP-EGFP showed that the N-terminal and the C-terminal nucleolar and nucleus localization signal sequences are important for localization to the nucleus and the nucleoli.