RbcL | Rubisco positive control/quantitation standard

Rubisco quantitation in plant and algal samples using Agrisera anti-RbcL global antibody and Rubisco protein standard

Methodology: Plant samples are generally ground with liquid nitrogen in a mortar and pestle. The resulting powder is transferred to a plastic tube. Algal samples can be either concentrated by centrifugation or, preferably, by filtration onto glass fiber filters. Solubilization is performed in Agrisera protein extraction buffer (PEB, AS08 300) containing 0.1mg/mL PefaBloc SC (AEBSF) protease inhibitor (Roche). Disruption is most optimally obtained through flash freezing of the sample in liquid nitrogen alternated with thawing by sonication with a microtip. This process can be repeated depending on the toughness of the sample. The sample is adjusted to 50 mM dithiothreitol and heated to 70°C for 5 minutes. Samples are cooled and centrifuged briefly prior to electrophoresis.

Optimal quantitation is achieved using moderate sample loads per gel lane, generally 0.5 to 2.5 ug total protein, depending on the abundance of the target protein.

Electrophoresis and Immunoblotting: Once solubilized, the proteins can be separated electrophoretically in a number of systems. We obtain optimal results with the Invitrogen NuPAGE gel system using Bis-Tris 4-12% gradient gels. Proteins are separated in MES SDS running buffer according to the manufacturer’s recommendations at 200 V for 35 minutes. The gels are transferred to PVDF in the same apparatus, the SureLock XCell blot module, for 60 minutes at 30 V for a single gel or 80 minutes for a pair. Following transfer the blots are blocked with non-fat dry milk up to 10 % in TBS-T, for 1 h/RT with gentle agitation. The blot is incubated with primary antibody, usually at 1:25 000 to 1:50 000, for 1 h/RT (if extreme femtogram detection reagents are used) or in lower primary antibody dilution for less sensitivie reagents (mid picogram and lower).

For quantitation a relatively high primary antibody: target protein ratio gives more reliable results than immunoblots at low ratios of primary antibody:target protein.

The blot is washed extensively in TBS-T (twice briefly, once for 15 minutes and three times for five minutes). The blot is incubated with secondary antibody, for example goat anti-rabbit IgG horse radish peroxidase conjugated, AS09 602 (Agrisera), for 1h/RT. The blot is washed as above and developed with ECL detection reagents.

Quantitation: When quantitated standards are included on the blot, the samples can be quantitated using the available software. Excellent quantitation can be obtained with images captured on the Bio-Rad Fluor-S-Max or equivalent instrument using Bio-Rad QuantityOne software. The contour tool is used to select the area for quantitation and the values are background subtracted to give an adjusted volume in counts for each standard and sample. Using above protocol linear standard curves are generated over 1-1.5 orders of magnitude range in target load. It is important to note that immunodetections usually show a strongly sigmoidal signal to load response curve, with a region of trace detection of low loads, a pseudolinear range and a region of saturated response with high loads. For immunoquantitation it is critical that the target proteins in the samples and the standard curve fall within the pseudolinear range. Our total detection range using this protocol spans over 2 orders of magnitude, but the quantifiable range is narrower.

References:

MacKenzie et al  (2005). Large reallocations of carbon, nitrogen and photosynthetic reductant among phycobilisomes, photosystems and Rubisco during light acclimation in Synechococcus elongatus are constrained in cells under low environmental inorganic carbon. Arch of Microbiol. 183: 190 - 202.
Bouchard et al. (2006) UVB effects on the photosystem II-D1 protein of phytoplankton and natural phytoplankton communities. Photochem and Photobiol 82: 936-951. 
Morash et al. (2007) Macromolecular dynamics of the photosynthetic system over a seasonal developmental progression in Spartina alterniflora. Can J. of Bot. 85: 476-483(8)

Application example

2 µg of total protein from (1-7) Prochlorococcus sp. extracted with Protein Extration Buffer, PEB (AS08 300), (8-11) Rubisco protein standard (AS01 017S) were separated on  4-12% NuPage (Invitrogen) LDS-PAGE and blotted 1h to PVDF. Blots were blocked immediately following transfer for 1h/RT with agitation. Blots were incubated in the primary antibody at a dilution of 1: 20 000 for 1h at room temperature with agitation. The antibody solution was decanted and the blot was rinsed briefly twice, then washed once for 15 min and 3 times for 5 min in TBS-T at room temperature with agitation. Blots were incubated in secondary antibody (anti-rabbit IgG horse radish peroxidase conjugated) diluted to 1:50 000 for 1h/RT with agitation. The blots were washed as above and developed for 5 min with ECL detection reagent according the manufacturers instructions. Images of the blots were obtained using a CCD imager (FluorSMax, Bio-Rad) and Quantity One software (Bio-Rad). There is a low level of a known RbcL cleavage product below the main band in some of the Prochlorococcus samples.

Recommended secondary antibodies: goat anti-rabbit HRP conjugated, goat anti-rabbit ALP conjugated

Recommended chemiluminescent detection reagent: AgriseraECLBright