Research Article: Structural Diversity of the Active N-Terminal Kinase Domain of p90 Ribosomal S6 Kinase 2

Date Published: November 30, 2009

Publisher: Public Library of Science

Author(s): Margarita Malakhova, Igor Kurinov, Kangdong Liu, Duo Zheng, Igor D’Angelo, Jung-Hyun Shim, Valerie Steinman, Ann M. Bode, Zigang Dong, Wenqing Xu.

Abstract: The p90 ribosomal protein kinase 2 (RSK2) is a highly expressed Ser/Thr kinase activated by growth factors and is involved in cancer cell proliferation and tumor promoter-induced cell transformation. RSK2 possesses two non-identical kinase domains, and the structure of its N-terminal domain (NTD), which is responsible for phosphorylation of a variety of substrates, is unknown. The crystal structure of the NTD RSK2 was determined at 1.8 Å resolution in complex with AMP-PNP. The N-terminal kinase domain adopted a unique active conformation showing a significant structural diversity of the kinase domain compared to other kinases. The NTD RSK2 possesses a three-stranded βB-sheet inserted in the N-terminal lobe, resulting in displacement of the αC-helix and disruption of the Lys-Glu interaction, classifying the kinase conformation as inactive. The purified protein was phosphorylated at Ser227 in the T-activation loop and exhibited in vitro kinase activity. A key characteristic is the appearance of a new contact between Lys216 (βB-sheet) and the β-phosphate of AMP-PNP. Mutation of this lysine to alanine impaired both NTDs in vitro and full length RSK2 ex vivo activity, emphasizing the importance of this interaction. Even though the N-terminal lobe undergoes structural re-arrangement, it possesses an intact hydrophobic groove formed between the αC-helix, the β4-strand, and the βB-sheet junction, which is occupied by the N-terminal tail. The presence of a unique βB-sheet insert in the N-lobe suggests a different type of activation mechanism for RSK2.

Partial Text: The 90-kDa ribosomal S6 kinase 2 (RSK2) is a serine-threonine kinase, which plays a key role in the mitogen-activated protein (MAP) kinase signaling pathway. RSK2 is activated in response to a variety of stimuli, including insulin, growth factors, neurotransmitters, and chemokines [1]. The RSK2 pathway is a key regulator of cancer cell proliferation [2]–[4]. RSK2 phosphorylates a variety of substrates, including the cAMP-responsible element-binding 2 protein [5], histone H3 [6], the tumor suppressor p53 protein [7], cell cycle proteins Myt1 [8] and Bub1 [9], transcriptional factors including c-Fos [10], [11], activating transcriptional factor-4 [12], and nuclear factor of activated T cell [13].



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