A Protein ⋅ Protein Interaction Platform Involved in Recruitment of GLD-3 to the FBF ⋅fem-3 mRNA Complex

The Pumilio and FBF (PUF) family of RNA-binding proteins interacts with protein partners to post-transcriptionally regulate mRNAs in eukaryotes. The interaction between PUF family member fem-3 binding factor (FBF) and germline development defective-3 (GLD-3) protein promotes spermatogenesis in Caenorhabditis elegans by increasing expression of the fem-3 mRNA. Defined here in these studies is the molecular basis for this critical interaction. A 10-amino-acid region within GLD-3 is required for FBF binding, while a 7-amino-acid loop in FBF between PUF repeats 7 and 8 is necessary for GLD-3 binding. These short sequences are conserved, as other FBF-binding proteins bear sequences similar to those in GLD-3 and other C. elegans PUF proteins contain sequences similar to those in FBF. The FBF-binding region of GLD-3 forms a ternary complex with FBF on the point mutation element (PME) in the fem-3 3′ untranslated region, and formation of this GLD-3 ⋅ FBF complex does not impact the RNA-binding activity of FBF. These data raise the possibility of alternative models involving the formation of a GLD-3 ⋅ FBF ⋅ RNA complex in the regulation of germline mRNAs.