The Muir lab is using novel techniques to study molecular recognition in prokaryotic and eukaryotic signal transduction. One of these techniques is a general approach to investigating protein activity called expressed protein ligation, which allows synthetic peptides and recombinant proteins to be chemoselectively linked together. This is done by introducing bits of 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 (for the synthetic building blocks) or biosynthetically (for the recombinant pieces), 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 site-specifically into proteins. Dr. Muir is currently using expressed protein ligation to study R-SMAD regulation in TGF-â signaling and, together with Rockefeller’s Roderick MacKinnon, potassium channel selectivity.

Because expressed protein ligation is strictly an in vitro technique, Dr. Muir has been looking at different, chemistry-driven approaches in order to study protein function in vivo. In the past few years the lab has developed two ways to do this, both of which exploit protein splicing. In protein splicing, 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 techniques is a system that allows protein trans-splicing to occur only in the presence of a small cell-permeable molecule; 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 control over protein function that’s difficult to achieve using standard genetic approaches, and Dr. Muir has shown that it works well in various cultured mammalian cell lines and in animals. Moreover, the Muir lab has recently developed an alternate version that can be controlled with light, rather than with a small molecule. This version of the technology has the potential to provide not just temporal but also spatial control over protein function.

The second technique the lab developed is for protein semisynthesis 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, expanding the genetic code for cell biological studies.

Another area of research in the Muir lab focuses on quorum sensing in Staphylococcus aureus. Dr. Muir is studying a class of the bacterium’s secreted peptides that has the ability to either 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 to account for how they can have opposite effects on virulence in different S. aureus strains. This discovery made it possible for them to design peptide analogues that inhibit only the virulence expression. These molecules are powerful tools for studying this quorum-sensing circuit both in vitro and in vivo; both molecules continue to be studied by the Muir lab and could potentially be used as compounds for the development of novel therapeutics.

CAREER

Dr. Muir received his B.S. in chemistry in 1989 and his Ph.D. in organic chemistry in 1993, both from the University of Edinburgh. After studying bioorganic chemistry as a postdoc and then as a senior research associate at the Scripps Research Institute, he joined Rockefeller in 1996 as assistant professor. He was named associate professor in 2000, professor in 2002 and Richard E. Salomon Family Professor in 2005. He is also director of the Pels Family Center for Biochemistry and Structural Biology at Rockefeller.

Dr. Muir received the Irving Sigal Young Investigator Award in 2005, the Leonidas Zervas Award from the European Peptide Society in 2002 and a Burroughs Wellcome Fund New Investigator Award in 2000. He was deemed an Alfred P. Sloan Research Fellow in 1999, and a Pew Scholar in the Biomedical Sciences in 1997.