Universal Stress Protein (UspA)
The crystal structure of Haemophilus influenzae UspA reveals a dimer with a tertiary a/b fold similar to that of the Methanococcus jannaschi MJ0577 protein, a protein whose crystals structure revealed a novel ATP binding motif. UspA differs significantly from the MJ0577 structure in several details, including the triphosphate binding loop of the ATP binding motif; UspA shows no ATP binding activity. Within the universal stress protein family that is delineated by sequence similarity, UspA is the only member which has been correlated with a cellular activity ("stress endurance"), and MJ0577 is the only member which has been assigned a biochemical activity (ATP binding). UspA has a similar fold to the MJ0577 protein, but does not bind ATP. This suggests that members of this protein family will segregate into two groups, based on whether or not they bind ATP. By implication, one subset of the universal stress proteins presumably have an ATP-dependent function, while another subset function in ATP-independent activities.