Inhibition of ERK-MAP kinase signaling by RSK during Drosophila development

Abstract

Although p90 ribosomal S6 kinase (RSK) is known as an important downstream effector of the ribosomal protein S6 kinase/extracellular signal-regulated kinase (Ras/ERK) pathway, its endogenous role, and precise molecular function remain unclear. Using gain-of-function and null mutants of RSK, its physiological role was successfully characterized in Drosophila. Surprisingly, RSK-null mutants were viable, but exhibited developmental abnormalities related to an enhanced ERK-dependent cellular differentiation such as ectopic photoreceptor-and veincell formation. Conversely, overexpression of RSK dramatically suppressed the ERK-dependent differentiation, which was further augmented by mutations in the Ras/ ERK pathway. Consistent with these physiological phenotypes, RSK negatively regulated ERK-mediated developmental processes and gene expressions by blocking the nuclear localization of ERK in a kinase activity-independent manner. In addition, we further demonstrated that the RSK-dependent inhibition of ERK nuclear migration is mediated by the physical association between ERK and RSK. Collectively, our study reveals a novel regulatory mechanism of the Ras/ERK pathway by RSK, which negatively regulates ERK activity by acting as a cytoplasmic anchor in Drosophila.

The C-terminal Kinase and ERK-binding Domains of Drosophila S6KII (RSK) Are Required for Phosphorylation of the Protein and Modulation of Circadian Behavior

“…We report that S6KII's circadian function does not require its N-terminal kinase activity, similar to findings reported for Drosophila eye development (15). In contrast, C-terminal kinase activity, previously thought to be responsible only for N-terminal kinase activation, is required for normal circadian behavior.…”

“…Our studies also suggest that ERK binding to and phosphorylation of S6KII threonine 732 (T732) within clock neurons is essential for normal rhythmicity. Whereas S6KII was shown to negatively regulate ERK in the fly eye (15) and at the neuromuscular junction (20), our work indicates that activation of S6KII by ERK is required for modulation of the circadian clock. Further, we show that both ERK binding and C-terminal kinase activity are important for autophosphorylation of S6KII serine 515 (S515) and T732 phosphorylation, whereas phosphorylation at S357, which activates the N-terminal kinase, is not dependent on these activities.…”

“…Even though S6KII is ubiquitously expressed throughout Drosophila development and in all embryonic tissues (15), S6KII null mutant flies are viable (15,19). Interestingly, S6KII null (S6KII ignorant or S6KII ign ) flies exhibit circadian molecular and behavioral phenotypes previously described by our laboratory (17).…”

“…The flies were generously donated by J. Chung (KAIST, Korea) and described in (15). Additional S6KII mutants were generated from a pUAST-myc-S6KII construct obtained from Marc Bourouis (University of Nice, France) that was used to create Bloomington's UAS-S6KII ϩ transgenic line.…”

“…In contrast, the N-terminal kinase is responsible for phosphorylation of other protein substrates (9,13,14). N-terminal kinase activity was thought to be essential for RSK function until a recent study demonstrated an alternative role for RSK in fly eye development as a non-catalytic, scaffolding protein (15).…”