Citrulline-modified phage display: a novel high-throughput discovery approach for the identification of citrulline-containing ligands. 
Phage display is a high-throughput technology used to identify ligands for a given target.  A drawback of the approach is the absence of PTMs in phage-displayed peptides.  The applicability of phage display could be broadened considerably by the implementation of PTMs in this system.  The aim of this study was to investigate the possible application of citrullination, a PTM of an arginine into a citrulline amino acid, in filamentous (M13) and lytic (T7) phage display.  After in vitro citrullination of T7 and M13 phages, citrullination was confirmed and the infectivity of both citrullinated and non-citrullinated phage was compared by titer determination.  We demonstrated the successful in vitro citrullination of T7 and M13 phage-displayed peptides.  This in vitro modification did not affect the viability or infectivity of the T7 virions, a necessary prerequisite for the implementation of this approach in T7 phage display.  For M13 phage, however, the infecting phage titer decreased five-fold upon citrullination, limiting the use of this modification in M13 phage display.  In conclusion, in vitro citrullination can be applied in T7 phage display giving rise to a high-throughput and sensitive approach to identify citrulline-containing ligands by the use of the strengths of phage display technology.