1. Insulin-like growth factor (IGF2)-mRNA-binding proteins. A family of proteins that bind differentially to the various mRNAs with different 5′ untranslated regions that are generated from the IGF2 gene, and repress translation. IMP1 (577 aa) has six RNA-binding modules, IMP2 (599 aa) and IMP3 (579 aa) have similar binding domains but the expression of the IMPs seems to be temporally and spatially regulated during development.
2. Intramembrane protease-3 (IMP3; signal peptide peptidase-like-2A), a presenilin-like membrane protease.
3. U3 small nucleolar ribonucleoprotein (U3 snoRNPs; IMP3, 184 aa; IMP4, 291 aa), required for the early cleavages during pre-18S ribosomal RNA processing.
4. Inositol monophosphate.