KSR proteins in Ras signaling
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Under circumstances of low transfection efficiency, the ectopically expressed mKSR1 helped to reach the optimal scaffold concentration, enhancing signal transduction. To avoid this problem, which is inherent to the plasmid transfection methods, Morrison and coworkers developed an assay to measure the Ras-dependent ERK scaffolding activity of KSR in Xenopus laevis oocytes. The system allows for a careful titration of the amount of KSR expressed in each oocyte an scores their meiotic maturation (germinal vesicle breakdown, or GVBD) as the biological readout [1, 17]. More recently, the availability of immortalized KSR1-deficient cells has greatly facilitated the study of KSR function since variable amounts of mKSR1 proteins (wild-type or mutant) can be reintroduced in these cells by retroviral infection and subsequent sorting of cell populations with different KSR1 expression levels [25-27]. The following sections are a summary of the current molecular knowledge regarding KSR structure and function.
Figure 1. The effective concentration of KSR1 is critical for an optimal scaffolding activity. This is achieved when the scaffold (KSR1, in red) and its ligands (C-Raf, MEK and ERK, in yellow, green and orange, respectively) are in a concentration near the stoichiometry of the reaction (B). Below such levels, a local increase in the effective concentration of KSR1 results in an increased signaling (A). However, an excess of the scaffold (i.e., a concentration higher than the optimal) results in the formation of non-productive complexes that block signaling (C).