Functional Assays of Membrane-Bound Proteins with SAMDI-TOF Mass Spectrometry

Green proteorhodopsin reconstituted into nanoscale phospholipid bilayers (nanodiscs) as photoactive monomers

Over 4000 putative proteorhodopsins (PRs) have been identified throughout the oceans and seas of the Earth. The first of these eubacterial rhodopsins was discovered in 2000 and has expanded the family of microbial proton pumps to all three domains of life. With photophysical properties similar to those of bacteriorhodopsin, an archaeal proton pump, PRs are also generating interest for their potential use in various photonic applications. We perform here the first reconstitution of the minimal photoactive PR structure into nanoscale phospholipid bilayers (nanodiscs) to better understand how protein-protein and protein-lipid interactions influence the photophysical properties of PR. Spectral (steady-state and time-resolved UV-visible spectroscopy) and physical (size-exclusion chromatography and electron microscopy) characterization of these complexes confirms the preparation of a photoactive PR monomer within nanodiscs. Specifically, when embedded within a nanodisc, monomeric PR exhibits a titratable pK(a) (6.5-7.1) and photocycle lifetime (∼100-200 ms) that are comparable to the detergent-solubilized protein. These ndPRs also produce a photoactive blue-shifted absorbance, centered at 377 or 416 nm, that indicates that protein-protein interactions from a PR oligomer are required for a fast photocycle. Moreover, we demonstrate how these model membrane systems allow modulation of the PR photocycle by variation of the discoidal diameter (i.e., 10 or 12 nm), bilayer thickness (i.e., 23 or 26.5 Å), and degree of saturation of the lipid acyl chain. Nanodiscs also offer a highly stable environment of relevance to potential device applications.

Chemical strategies for generating protein biochips

Protein biochips are at the heart of many medical and bioanalytical applications. Increasing interest has been focused on surface activation and subsequent functionalization strategies for immobilizing these biomolecules. Different approaches using covalent and noncovalent chemistry are reviewed; particular emphasis is placed on the chemical specificity of protein attachment and on retention of protein function. Strategies for creating protein patterns (as opposed to protein arrays) are also outlined. An outlook on promising and challenging future directions for protein biochip research and applications is also offered.

Mass spectrometry of self-assembled monolayers: a new tool for molecular surface science

Most reactions can be performed in solution and on a surface, yet the challenges faced in applying known reactions or in developing entirely new reactions for modifying surfaces remain formidable. The products of many reactions performed in solution can be characterized in minutes, and even products having complex structures can be characterized in hours. When performed on surfaces, even the most basic reactions require a substantial effort--requiring several weeks--to characterize the yields and structures of the products. This contrast stems from the lack of convenient analytical tools that provide rapid information on the structures of molecules attached to a surface. This review describes recent work that has established mass spectrometry as a powerful method for developing and characterizing a broad range of chemical reactions of molecules attached to self-assembled monolayers of alkanethiolates on gold. The SAMDI-TOF mass spectrometry technique will enable a next generation of applications of molecularly defined surfaces to problems in chemistry and biology.