Protein moonlighting or gene sharing refers to the phenomenon by which proteins perform multiple functions in an organism. A big surprise as a result of sequencing the human genome was the discovery that the human genome appears to code for far fewer proteins than what was predicted. Successive research and investigating the biochemistry of proteins that make up the so called proteome revealed that this could be partially explained by alternative splicing of mRNA transcripts and/or the 'reusing' of proteins for other additional purposes or functions. For example, it was known for quite some time that several soluble enzymes can perform a second function, for example, structural proteins found in the lens of the eye called crystallins. Other proteins that showed a similar behavior are proteins that bind DNA or RNA and are involved in the regulation, translation or transcription of genes. However, more precisely "moonlighting" refers to a single protein that has multiple functions not because of gene fusions, splice variants or multiple proteolytic fragments. Many proteins are now known to moonlight. These include receptor- , trans-membrane channel-, chaperone-, ribosomal proteins, the proteins Clf1p, Sug1/Rpt6 and Sug2/Rpt4, and, potentially, and potentially many others. For example, Clf1p is an essential, highly conserved protein found in S. cerevisiae that has been implicated in pre-mRNA splicing. Sug1/Rpt6, a component of the 19S complex, belongs to the six highly conserved ATPases of the AAA class in the 19S proteasome regulatory particle and a molecular target of RIP-1. RIP-1 refers to the cell death protein "Receptor-interacting protein 1" which is a serine/threonine-protein kinase. The term AAA or AAA+ is an abbreviation for 'ATPases Associated with diverse cellular Activities'. These proteins share a common conserved module or motif of approximately 230 amino acid residues and belong to a functionally diverse protein family, the AAA+ superfamily of ring-shaped P-loop RNA nucleoside triphosphatases (NTPases). AAA ATPases are usually associated with various cellular activities where they play important roles including proteolysis, protein folding, membrane trafficking, cytoskeletal regulation, organelle biogenesis, DNA replication, and intracellular motility. In the past, the identification of moonlighting proteins has been done by chance since there is no clear procedure available for the identification of the secondary functions of a protein. However, various methods have been used to determine a protein's multiple function such as the use of co-precipitation methods, including immune-co-precipitation. The use of these methods in combination with mass spectrometry based methods may now allow for the successful identification and characterization of new members of moonlighting proteins. Furthermore, it is thought that moonlighting proteins can switch functions due to a change in cellular localization, expression in a novel cell type, oligomeric state, and cellular concentration of a ligand, substrate, cofactor or product. It appears that protein moonlighting may occur widely in nature. Unfortunately, in the case of a mutation that causes a disease, the existence of moonlighting proteins could make the development of treatments for diseases caused by this mutation more difficult. In addition, currently there is no general straightforward method available which allows for the identification of moonlighting proteins. More research will need to be done.
Constance J. Jeffery; Moonlighting proteins: old proteins learning new tricks. TRENDS in Genetics Vol.19 No.8 August 2003, 415-417.