Preliminary structural studies on MPN423 expressed from an orthologous ORFan of Mycoplasma pneumoniae

ORFans are orphan open reading frames. The numbers of ORFans are steadfastly increasing despite of the genome database increment. Characterizing ORFans is essential to fully understanding the diversity of the structure and function of proteins in nature. In this study, MPN423 from Mycoplasma pneumoniae has been cloned, expressed, purified, and crystallized. MPN423 is an orthologous ORFan whose only known homologue in the whole genome database is MG296 from M. genitalium. X-ray diffraction data were collected to 2.7 Å from the crystal of a selenomethionine substitute MPN423. The crystal belongs to the primitive monoclinic space group P2inf1/inf, with unit-cell parameters of a = 50.5 Å, b = 89.2 Å, c = 50.6 Å, and β = 102.9°. A preliminary electron density map shows five α-helical segments per MPN423 molecule. A full structure determination is under way to provide helpful information to general questions about orthologous ORFan products.