Regulation of nucleo-cytoplasmic shuttling of human annexin A2 - A proposed mechanism

Studies have long been focused on the functions of annexin A2 in the cytoplasm. However, the involvement of annexin A2 in DNA replication as a part of primer recognition protein complex and the presence of nuclear export signal (NES) suggest that annexin A2 is also functional in the nucleus, and its localization in the nucleus is under regulation by interaction with other nuclear factors through its N-terminus. During the study of the mechanism of annexin A2 sequestering in the nucleus and the regulation of its export from the nucleus, in this study, we show that endogenous annexin A2 is present in both the cytoplasm and the nucleus in HeLa, PC-3 and DU-145 cells. While exogenously expressed annexin A2 is excluded from nuclei of annexin A2-null LNCaP cells in a CRM1 (Chromosome Maintenance Region 1) mediated nuclear export, endogenous annexin A2 in HeLa, PC-3 and DU-145 cell lines does not undergo the CRM1 mediated nuclear export. While investigating the mechanism of the nuclear retention of annexin A2, we found that an anti-annexin A2 antibody that recognizes the C-terminus of annexin A2 (D1/274.5) cannot recognize nuclear annexin A2, suggesting that the domain recognized by this antibody may be masked in the nuclei. In order to find out the role of annexin A2 C-terminus in the nuclear retention of annexin A2, we transiently transfected green fluorescence protein (GFP)-fused N-terminal 29 amino acids of annexin A2 to LNCaP, PC-3 and DU-145 cells, and determined that the C-terminus is not required for the nuclear retention of annexin A2. Based on the finding described above, we propose a model for nuclear retention of annexin A2 where the regulation sites reside in the N-terminus and are adjacent to the NES, and upon modification, the NES is exposed and annexin A2 is exported from the nucleus.