Table 1.

Calculated and measured masses for lactose permease

E. coli lactose permease	Calculated mass,‡ Da	Measured mass, Da	Difference (%)
Native (+6 His tag)*	47,354	47,357	(<0.01)
“Red” (+10 His tag)†	60,458	60,457	(<0.01)

^*Native lactose permease sequence with a six-His tag and N-formylation of Met1.  

^†Permease construct with cytochrome b₅₆₂ in the middle cytoplasmic loop, a 10-His tag at the C terminus and N-formylation of Met1.  

^‡Average masses are used based on natural isotopic abundance.