Proteins can also direct the rearrangement of their own polypeptide backbones. For instance, proteins called inteins facilitate a process known as protein splicing. Inteins interrupt the amino acid sequence, and probably the function, of other proteins. Examples include an intein in yeast that interrupts an ATPase, one in a mycobacteria that interrupts the RecA protein (which is involved in DNA repair and recombination), and one in a pyrococcus species that interrupts a DNA polymerase.
Inteins promote their own excision from their target protein as well as the ligation of the flanking protein segments, which are called exteins. It is possible that some inteins play a role in regulating gene expression, but it is also possible that they are vestiges of ancient control mechanisms or simply molecular parasites.
Inteins are analogous to introns in an RNA transcript. Introns interrupt a gene in DNA. The introns are excised after transcription from the surrounding RNA sequences. These flanking sequences, called exons, are then spliced together. Unlike protein splicing, RNA splicing is usually aided by other proteins and RNA molecules.
While it is unclear if protein splicing has a regulatory role, a class of proteins evolutionarily related to inteins, the hedgehog proteins, are involved in embryonic patterning and segmentation. Hedgehog proteins promote their own internal cleavage, coupled to the addition of cholesterol to the C-terminal end of the N-terminal fragment, which is probably important for anchoring the hedgehog protein in the cell membrane.
Kenneth V. Mills
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