The study investigates the role of HuR, a nuclear-cytoplasmic shuttling protein, in the stability of mRNAs containing AU-rich elements (AREs). HuR, a member of the Elav family of RNA binding proteins, has been implicated in the rapid degradation of mRNAs from proto-oncogenes, cytokines, and lymphokines through its affinity for ARE sequences. Overexpression of HuR in mouse L929 cells was found to enhance the stability of β-globin reporter mRNAs containing both class I and class II AREs, suggesting an in vivo role for HuR in mRNA decay. The increase in mRNA stability parallels the level of HuR overexpression. Overexpression of HuR deletion mutants lacking RNA recognition motif 3 (RRM 3) did not exhibit a stabilizing effect, indicating that RRM 3 is crucial for HuR function. Immunofluorescence staining of HeLa and L929 cells using anti-HuR antibodies revealed that both endogenous and overexpressed HuR proteins are localized in the nucleus. However, the formation of heterokaryons between HeLa and L929 cells demonstrated that HuR shuttles between the nucleus and cytoplasm. The study concludes that HuR may initially bind to ARE-containing mRNAs in the nucleus and provide protection during their export to the cytoplasm, where they are degraded.The study investigates the role of HuR, a nuclear-cytoplasmic shuttling protein, in the stability of mRNAs containing AU-rich elements (AREs). HuR, a member of the Elav family of RNA binding proteins, has been implicated in the rapid degradation of mRNAs from proto-oncogenes, cytokines, and lymphokines through its affinity for ARE sequences. Overexpression of HuR in mouse L929 cells was found to enhance the stability of β-globin reporter mRNAs containing both class I and class II AREs, suggesting an in vivo role for HuR in mRNA decay. The increase in mRNA stability parallels the level of HuR overexpression. Overexpression of HuR deletion mutants lacking RNA recognition motif 3 (RRM 3) did not exhibit a stabilizing effect, indicating that RRM 3 is crucial for HuR function. Immunofluorescence staining of HeLa and L929 cells using anti-HuR antibodies revealed that both endogenous and overexpressed HuR proteins are localized in the nucleus. However, the formation of heterokaryons between HeLa and L929 cells demonstrated that HuR shuttles between the nucleus and cytoplasm. The study concludes that HuR may initially bind to ARE-containing mRNAs in the nucleus and provide protection during their export to the cytoplasm, where they are degraded.