Product Uses Include
The Rho binding domain (RBD) of the human Rhotekin protein has been expressed as a GST-fusion protein in E. coli. This protein binds binds specifically to GTP-bound, and not GDP-bound, RhoA, RhoB and RhoC proteins. The domain can therefore be used to specifically precipitate active, GTP-bound Rho proteins as well as to specifically block the activity of these proteins in vitro and in vivo.
The GST-Rhotekin-RBD contains residues 7-89 of Rhotekin. This region includes the sequences required for the high affinity interaction with GTP-Rho.
The protein is supplied in a glutathione agarose bound format and is shipped lyophilized. The beads are colored for ease of use. This product is used in our RhoA pulldown activation assay Biochem Kit™ (Cat. # BK036). The GST-Rhotekin-RBD is also available as a free protein (Cat. # RT01).
Figure 1: GST-Rhotekin-RBD protein purity determination. A 10 µg sample of PAK01 was
Protein purity is determined by scanning densitometry of Coomassie Blue stained protein on a 12% SDS polyacrylamide gel. GST-Rhotekin-RBD protein is ~85% pure (see Figure 2).
Figure 2: GST-Rhotekin-RBD protein purity determination. A 20 µg sample of RT02 was separated by electrophoresis in a 12% SDS-PAGE system and stained with Coomassie Blue. The GST-Rhotekin-RBD protein runs at approximately 35 kDa.
The Rhotekin-RBD protein specifically recognizes and binds to the active, GTP-bound, form of Rho protein. It has a much lower affinity for the inactive, GDP-bound, form of Rho. Biological activity of Rhotekin-RBD protein is therefore determined by its selectivity for GTP-Rho protein. The assay for monitoring the GTP-bound form of Rho is a protein pulldown assay using RT02 beads and a human platelet extract loaded with GTPγS (Cat. # BS01)
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Question 1: How much of the beads should I use for my pull-down experiments?
Answer 1: Rhotekin-RBD protein GST beads (Cat. # RT02) will bind to Rho-GDP with a much lower affinity than Rho-GTP. If too many Rhotekin-RBD beads are added to the pull-down assay, there will be significant binding to inactive (GDP-bound) RhoA. The result of this will be an underestimation of RhoA activation. For this reason, we highly recommend performing a bead titration to determine optimal conditions for any given Rho activation or inactivation assay. Once optimal conditions have been established, bead titrations should no longer be necessary. We recommend 25, 50 and 100 μg bead titrations.
Question 2: How can I test whether the beads are working properly?
Answer 2: A standard biological assay for Rhotekin-RBD protein GST beads consists of a Rho protein pull-down from cells loaded with either GTPγS (Cat. # BS01) or GDP. Here are guidelines to follow (see RT02 datasheet for more details):
Positive Cellular Protein Control:
Total cell lysate (300 – 800 μg) should be loaded with GTPγS as a positive control for the pull-down assay. The following reaction details how to load endogenous RhoA with the nonhydrolysable GTP analog (GTPγS). This is an excellent substrate for Rhotekin-RBD beads and should result in a strong positive signal in a pull-down assay.
a) Perform GTP loading on 300 – 800 μg of cell lysate (0.5 mg/ml protein concentration) by adding 1/10th volume of Loading Buffer.
b) Immediately add 1/100th volume of GTPγS (200 μM final concentration). Under these conditions 5 - 10% of the RhoA protein will load with non-hydrolysable GTPγS and will be “pulled down” with the Rhotekin-RBD beads in the assay.
c) Incubate the control sample at 30°C for 15 min with gentle rotation.
d) Stop the reaction by transferring the tube to 4°C and adding 1/10th volume of STOP Buffer.
e) Use this sample immediately in a pull-down assay.
Negative Cellular Protein Control:
This reaction should be performed in an identical manner to the Positive Control reaction except that 1/100th volume of GDP (1 mM final concentration) should be added to the reaction in place of the GTPγS. Loading endogenous RhoA with GDP will inactivate RhoA and this complex will bind very poorly to Rhotekin-RBD beads.
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