Even though a great deal has been learned about moonlighting proteins in the past few decades, many questions still remain and are the topics of current study. Some of the key questions include the following.
How Many More Proteins Moonlight?
The MoonProt Database contains 270 entries (Mani et al. 2015; moonlighting- proteins.org), but more and more moonlighting proteins are found every year. Their diverse characteristics and widespread presence in the evolutionary tree suggest that moonlighting could be found in many more proteins. This raises the question “do most proteins moonlight?”
How Can We Identify Additional Proteins That Moonlight and all the Moonlighting Functions of Proteins?
In many cases, the multiple functions of a moonlighting protein were discovered by serendipity. There is currently no good method of amino acid sequence analysis or structural analysis that can predict which of the millions of amino acid sequences that are available from genome projects, or the over 100,000 protein structures in the Protein Data Bank, correspond to moonlighting proteins. Sequence or structural homology is often not enough to identify moonlighting proteins, in part because sequence homologs of moonlighting proteins can share one, both, or neither function.