Loading controls

In biology there are no universal controls. One has to determine a priori what does not change in one's test conditions and use that as a control. The choice of appropriate house-keeping protein would depend upon the particular experiment and it needs to be confirmed. House-keeping proteins cannot be used for normalization if there is a suspicion that experimental conditions are affecting their expression. 

Loading control guide

Other possible internal controls:
• Total protein (express fmol target protein/µg total protein)
• Dry mass
• Total chlorophyll (express fmol target protein/µg chlorophyll)
• Cells (express fmol target protein/cell)
• Total nitrogen (express fmol target protein/fmol N)
• Experiment specific ratios of particular proteins (ex. fmol PsbA/fmol RbcL)          

For short term experiments, Rubisco protein makes a good loading control as its turnover is slow. However, Rubisco level do vary with nutrient levels, particularly nitrogen.

Special notes:
FtsZ (AS07 217) can be used as a loading control in cyanobacteria. 
GAPC1/2 use of this protein as a loading control should be supported with specific experimental data. 
RbcL or RA (Rubisco activase) OEC223AtpB (for Chlamydomonas reinhardtii, Rasala et al. 2013) or PsbO (OEE1) or PsbP (OEE2) for Chlamydomonas reinhardtii.  
PsbO (OEE1) or PsbP (OEE2) are not so suitable for higher plants as accumulation of these proteins might drop to 12.5-25 % of the WT level in mutants defective for PSII core (Schult et al. 2007). 

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