Site-Directed and Time-Resolved Photocrosslinking in Cells Metabolically Labeled with Radioisotopes

A Photo-Crosslinking Approach to Monitoring the Assembly of an LptD Intermediate with LptE in a Living Cell

Elucidating the dynamic behavior of proteins in living cells is extremely important for understanding the physiological roles of biological processes. The site-specific in vivo photo-crosslinking approach using a photoreactive unnatural amino acid enables the analysis of protein interactions with high spatial resolution in vivo. Recently, by improving the photo-crosslinking technique, we developed the "PiXie" method for the analysis of dynamic interactions of newly synthesized proteins. Here, we describe the detailed protocols of the "PiXie" method and its application to the analysis of the assembly processes of the lipopolysaccharide translocon components, a β-barrel outer membrane protein, LptD, and a lipoprotein, LptE.

Job contenders: roles of the β-barrel assembly machinery and the translocation and assembly module in autotransporter secretion

In Gram-negative bacteria, autotransporters secrete effector protein domains that are linked to virulence. Although they were once thought to be simple and autonomous secretion machines, mounting evidence reveals that multiple factors of the bacterial envelope are necessary for autotransporter assembly. Secretion across the outer membrane of their soluble effector "passenger domain" is promoted by the assembly of an outer membrane-spanning "β-barrel domain". Both reactions require BamA, an essential component of the β-barrel assembly machinery (BAM complex) that catalyzes the final reaction step by which outer membrane proteins are integrated into the lipid bilayer. A large amount of data generated in the last decade has shed key insights onto the mechanistic coordination of autotransporter β-barrel domain assembly and passenger domain secretion. These results, together with the recently solved structures of the BAM complex, offer an unprecedented opportunity to discuss a detailed model of autotransporter assembly. Importantly, some autotransporters benefit from the presence of an additional machinery, the translocation and assembly module (TAM), a two-membrane spanning complex, which contains a BamA-homologous subunit. Although it remains unclear how the BAM complex and the TAM cooperate, it is evident that multiple preparatory steps are necessary for efficient autotransporter biogenesis.