Fig 1: STARPA reveals differential GRK expression in commonly used cell lines. (a) STARPA was used to determine relative GRK expression in HEK293, HeLa, HepG2, Jurkat, K562, MCF-7, Molm-13, U2OS, and U-251 MG cells. Lysates of different cell lines together with the validated STARPA standards (Figure 4c,d) were loaded onto polyacrylamide gels. Western blot analysis using anti-HA antibody confirmed the STARPA standards. Immunoblotting using anti-GRK antibodies for each isoform allowed the determination of endogenously expressed GRK levels in the cell lines. Shown are representative images of three independent blots from identical lysates. (b) Close ups of the STARPA standards from the immunoblot presented in (a) displaying the cross-reactivity of the anti-GRK2 and anti-GRK6 antibody towards GRK3 and GRK5, respectively. Below, the quantification of three independent blots ± SD. The means of the obtained signals are indicated and were used to determine the respective cross-reactivity coefficient of the antibodies. (c) STARPA blots were quantified according to each GRK isoform-specific antibody, and the relative signal compared to the respective signal in the standards is shown as mean + SD of three independent blots from identical lysates. The calculated relative expression levels of GRK2 and GRK6 before (striped) and after (solid) cross-reactivity correction are presented.