PsaC | PSI-C core subunit of photosystem I

Rubisco quantitation in plant and algal samples using Agrisera anti-PsaC global antibody and PsaC 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 µg 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 in 2%  blocking agent in Tris buffered saline with 0.1% Tween 20 (TBS-T) for 1 hour at room temperature with gentle agitation. The blot is incubated with primary antibody, usually at 1:25 000 to 1:50 000 diluted in 2% blocking agent, for 1 hour at room temperature. 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, at 1:50 000 in 2% blocking agent, for one hour at room temperature. The blot is washed as above and developed with chemiluminescence detection reagents as described by the manufacturer.

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 (5-7) Prochlorococcus sp. extracted with Agrisera Protein Extration Buffer, PEB (AS08 300), (1-4) PsaC protein standard (AS02 042S) were separated on 4-12% NuPage (Invitrogen) LDS-PAGE and blotted 1h to PVDF. Blots were blocked immediately following transfer in 2% blocking reagent in 20 mM Tris, 137 mM sodium chloride pH 7.6 with 0.1% (v/v) Tween-20 (TBS-T) for 1h at room temperature with agitation. Blots were incubated in the primary antibody at a dilution of 1: 50 000 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 1:10 000 in 2% blocking solution for 1h at room temperature with agitation. The blots were washed as above and developed for 5 min with ECL detection reagent in mid picogram range, according the manufacturers instructions. Images of the blots were obtained using a CCD imager (FluorSMax, Bio-Rad) and Quantity One software (Bio-Rad). The native protein is significantly smaller than the tagged quantitation standard (lane 1-4). There is also a consistent cross-reaction with a larger FeS protein.

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

Recommended chemiluminescent detection reagent: AgriseraECLBright