Ribosomal protein pNO40 mediates nucleolar sequestration of SR family splicing factors and its overexpression impairs mRNA metabolism.

The nucleolus acts as a key stress sensor and responds to changes in cellular growth rate and metabolic activity. In addition to its major role as the site of ribosome biogenesis, high-throughput proteomic analyses of purified nucleoli have highlighted the multi-functional nature of these organelles, and several SR family splicing factors, including SRSF1 and SRSF2, have been detected in human nucleolar proteome analysis. Here we provide evidence that pNO40, a 60s ribosomal protein associated with nucleoli, acts as a mediator for recruitment of SR family splicing factors into nucleoli. As a nucleolar protein, pNO40 was originally identified by yeast two-hybrid analysis as interacting with pnn, an SR-like protein involved in pre-mRNA splicing. To explore its functional interaction with pnn, we characterized the interplay between pNO40 and SR family proteins and demonstrated that pNO40 plays a role in recruiting SR splicing factors into the nucleoli. The targeting of pNO40 to the nucleoli is dependent on its extreme-carboxyl-terminus nuclear localization signals while the sequence at the amino-terminus of pNO40 enables its interaction with pnn. Nucleolar association of SR proteins results in defects in mRNA metabolism leading to global nuclear accumulation of poly(A)+ RNA and splicing defects. Animal studies confirmed aberrant mRNA splicing in transgenic muscles overexpressing pNO40 which displayed histological features of muscular dystrophy. Thus it appears that by pNO40 overexpression, we created mimics of nucleolar association of SR proteins occurring in the presence of transcription inhibitors which induce nucleolar segregation and redistribute SR proteins to the periphery of the nucleolar region. We therefore provide an extra-ribosomal function for pNO40 and, based on our data, it is conceivable that pNO40 may function as a general recruiter for nucleolar association of SR proteins and regulation of its expression may be crucial in cellular homeostasis.