Our long-term goal is to understand the structure and function of the PNC and its roles in malignancy. The perinucleolar compartment (PNC) is a multicomponent non-membrane bound nuclear substructure that localizes to the nucleolar periphery. We have found it to be unique to malignant transformation both in vitro and in vivo. The PNC is enriched with RNA newly synthesized by pol III and RNA binding proteins primarily implicated in pol II RNA processing. Our preliminary findings show that the PNC-associated RNAs are in the same complexes with some of the known pol II RNA binding proteins. These novel complexes are not restricted to the PNC since PTB-MRP RNA interactions can also be detected in normal cells without PNCs. However, the levels or the regulations of these complexes may change during malignancy, leading to the nucleation of these complexes in the PNC. These findings lead to our working model in which the PNC is associated with and may represent changes in novel molecular complexes that are involved in the metabolism of newly synthesized pol III RNA during malignancy. The goal of this proposal is to begin test this hypothesis by identifying and initially characterizing these novel RNPs, using RNase MRP RNA (one of the bona fide components of the PNC) as the first example, and by beginning to address the functional association of these RNPs with the PNC using cell biological approaches. As little is understood regarding the post-transcriptional processing and regulation of the pol III small non-coding RNAs, the solution of the novel RNPs is not only important for the understanding of PNC structure and function, but should also reveal a novel mechanism involved in the post-transcriptional regulation of these functional RNAs.