This family consists of various Pre RNA processing ribonucleoproteins. The function of the aligned region is unknown however it may be a common RNA or snoRNA or Nop1p binding domain. Nop5p (Nop58p) Swiss:Q12499 from yeast is the protein component of a ribonucleoprotein required for pre-18s rRNA processing and is suggested to function with Nop1p in a snoRNA complex [1]. Nop56p Swiss:O00567 and Nop5p interact with Nop1p and are required for ribosome biogenesis [2]. Prp31p Swiss:p49704 is required for pre-mRNA splicing in S. cerevisiae [3]. Fibrillarin, or Nop, is the catalytic subunit responsible for the methyl transfer reaction of the site-specific 2'-O-methylation of ribosomal and spliceosomal RNA [4].

1: Nop5p is a small nucleolar ribonucleoprotein component required for pre- 18 S rRNA processing in yeast. Wu P, Brockenbrough JS, Metcalfe AC, Chen S, Aris JP; J Biol Chem 1998;273:16453-16463. PMID:9632712

2: Nucleolar KKE/D repeat proteins Nop56p and Nop58p interact with Nop1p and are required for ribosome biogenesis. Gautier T, Berges T, Tollervey D, Hurt E; Mol Cell Biol 1997;17:7088-7098. PMID:9372940

3: The PRP31 gene encodes a novel protein required for pre-mRNA splicing in Saccharomyces cerevisiae. Weidenhammer EM, Singh M, Ruiz-Noriega M, Woolford JL Jr; Nucleic Acids Res. 1996;24:1164-1170. PMID:8604353

4: Structural basis for substrate placement by an archaeal box C/D ribonucleoprotein particle. Xue S, Wang R, Yang F, Terns RM, Terns MP, Zhang X, Maxwell ES, Li H; Mol Cell. 2010;39:939-949. PMID:20864039