The isolated La-module of LARP1 mediates 3’ poly(A) protection and mRNA stabilization, dependent on its intrinsic PAM2 binding to PABPC1
Sandy Mattijssen, Guennadi Kozlov, Sergei Gaidamakov, Amitabh Ranjan, Bruno D. Fonseca, Kalle Gehring, Richard J Maraia
Abstract
The protein domain arrangement known as the La-module, comprised of a La motif (LaM) followed by a linker and RNA recognition motif (RRM), is found in seven La-related proteins: LARP1, LARP1B, LARP3 (La protein), LARP4, LARP4B, LARP6, and LARP7 in humans. Several LARPs have been characterized for their distinct activity in a specific aspect of RNA metabolism. The La-modules vary among the LARPs in linker length and RRM subtype. The La-modules of La protein and LARP7 bind and protect nuclear RNAs with UUU-3ʹ tails from degradation by 3ʹ exonucleases. LARP4 is an mRNA poly(A) stabilization factor that binds poly(A) and the cytoplasmic poly(A)-binding protein PABPC1 (also known as PABP). LARP1 exhibits poly(A) length protection and mRNA stabilization similar to LARP4. Here, we show that these LARP1 activities are mediated by its La-module and dependent on a PAM2 motif that binds PABP. The isolated La-module of LARP1 is sufficient for PABP-dependent poly(A) length protection and mRNA stabilization in HEK293 cells. A point mutation in the PAM2 motif in the La-module impairs mRNA stabilization and PABP binding in vivo but does not impair oligo(A) RNA binding by the purified recombinant La-module in vitro. We characterize the unusual PAM2 sequence of LARP1 and show it may differentially affect stable and unstable mRNAs. The unique LARP1 La-module can function as an autonomous factor to confer poly(A) protection and stabilization to heterologous mRNAs.