Tom Muir

The Muir lab is using novel methods to study molecular recognition in prokaryotic and eukaryotic signaling processes. One of these techniques is a general approach to investigating protein activity called expressed protein ligation, which allows synthetic peptides (or other molecules) and recombinant proteins to be chemoselectively and regioselectively linked together. This is done by introducing molecular "sticky ends," which act like Velcro, at the complementary ends of the pieces, causing them to react when mixed together in water. Introduction of these sticky ends can either be achieved chemically or biosynthetically, and it is possible to append the synthetic molecule at either end of the recombinant protein, or even insert the synthetic cassette right into the middle. This technology opens up proteins to the tools of organic chemistry by allowing researchers to incorporate unnatural amino acids, posttranslational modifications and isotopic probes into proteins at specific sites. The major focus of Dr. Muir's current research is in the broad area of epigenetics where his group is applying in vitro protein chemistry methods to study how histone modifications control the local structure and function of chromatin.

Because expressed protein ligation is an in vitro technique, the Muir laboratory simultaneously pursues complementary approaches for studying protein function in vivo. The group has developed two ways to do this, both of which exploit protein splicing, in which an intervening sequence — termed an intein — catalyzes its removal from a host protein, the extein. In trans-splicing, the intein is split into two pieces and splicing occurs only upon reconstitution of these fragments.

The first of the methods is a platform technology that allows protein trans-splicing to occur only in the presence of a user specified stimulus such as a small cell-permeable molecule or red light; this "conditional protein trans-splicing" method provides a means to trigger posttranslational synthesis of a target protein from two fragments, thereby controlling that protein's function. Conditional protein trans-splicing provides a level of temporal and spatial control over protein function that is impossible to achieve using standard genetic approaches, and Dr. Muir's laboratory has shown that the methods works well in various cultured mammalian cell lines and in living animals.

The second technique developed by the lab allows for protein semisynthesis to be performed inside living cells. Using a different type of protein trans-splicing with peptide-transduction domain technology, this method allows a targeted cellular protein to be specifically ligated to an artificial probe delivered into the cell, effectively expanding the genetic code for cell biological studies. Currently we are developing second-generation versions of this technology specifically geared towards applications in the epigenetics area.

Another active area of research in the Muir lab focuses on quorum sensing in the pathogen Staphylococcus aureus. The Muir laboratory studies a class of the secreted peptides produced by the bacterium that have the ability to activate or inhibit expression of virulence, depending on which strain of S. aureus the peptides encounter. Dr. Muir and collaborators at New York University Medical Center have found that the peptides contain an unusual thiolactone structure, and further studies have suggested a mechanism that accounts for the changes in virulence. This discovery made it possible for them to design peptide analogues that globally inhibit virulence expression. These molecules are powerful tools for studying this quorum-sensing circuit both in vitro and in vivo. The Muir laboratory is currently studying the biosynthesis of these peptides and their mechanism of secretion. The group is also interested in how these peptides are recognized by their cognate cell surface receptors and how binding translates to receptor activation.