The role of SMN in box C/D snoRNP biogenesis

Abstract

Box C/D small nucleolar ribonucleic protein complexes (snoRNPs) either direct 2’-O-methylation of the pre-ribosomal RNA (pre-rRNA) or function as chaperones in pre-rRNA processing. It has been demonstrated that box C/D snoRNP biogenesis is a highly intricate and co-ordinated process that is mediated by a large, dynamic complex, known as the pre-snoRNP. This complex contains the box C/D snoRNA and a number of common core proteins, which become stably associated with the snoRNA, as well as factors linked to assembly, 3’ RNA processing and transport. Interestingly, the product of the gene linked to the neurodegenerative disorder spinal muscular atrophy, the survival of motor neurons protein (SMN), interacts with the box C/D snoRNP common core protein fibrillarin and is vital for the cellular localisation of both fibrillarin and box C/D snoRNA. The SMN protein operates with Gemins2-8 and UNRIP in what is termed the SMN complex, which has known functions in the assembly of the spliceosomal small nuclear RNPs (snRNPs). As the SMN protein interacts with fibrillarin and is also required for the cellular localisation of both fibrillarin and box C/D snoRNA this study set out to investigate the association of fibrillarin with the box C/D snoRNPs and the role of the SMN complex in this process. In this study it was revealed that the C terminal domain of fibrillarin is essential for cellular localisation and interactions with the box C/D snoRNP assembly factors, which suggests that this domain may mediate fibrillarin association with the box C/D snoRNPs. Also in this study the SMN protein was shown to interact with the box C/D snoRNP assembly factors NUFIP and BCD1, which are stable components of the pre-snoRNP. Analysis of the interaction of SMN with the pre-snoRNP, however, indicates that if SMN does associate with the pre-snoRNP then it is only a transient interaction. Further analysis revealed that as well as the SMN protein, Gemin2, 5, 6 and 7 are essential for the localisation of box C/D snoRNA and that Gemin5 is also required for the accumulation of box C/D snoRNA. This study strengthens the case that the SMN complex is involved in box C/D snoRNP biogenesis, with the data suggesting that it functions as a transport factor rather than in the association of fibrillarin with the box C/D snoRNPs. Analysis of the snRNP transport factor, snurportin1, revealed that it also interacts with numerous components of the pre-snoRNP; however, does not interact with SMN.