Component of the 60-80S U3 small nucleolar ribonucleoprotein (U3 snoRNP). Required for the early cleavages during pre-18S ribosomal RNA processing (By similarity). Component of a heterotrimeric complex containing IMP3, IMP4 and MPHOSPH10. Interacts with IMP3 and IMP4 (By similarity). Fibrillar region of the nucleolus. After dissolution of the nucleolus in early M phase becomes associated with chromosomes through metaphase and anaphase. In telophase localized to small cellular prenucleolar bodies that not always contain fibrillarin. The reassociation with nucleolus is preceeded by the arrival of fibrillarin. Phosphorylated in M (mitotic) phase. Belongs to the MPP10 family.