Activation of the tumour suppressor kinase LKB1 by the STE20-like pseudokinase STRAD

Abstract

The LKB1 gene encodes a serine/threonine kinase mutated in Peutz±Jeghers cancer syndrome. Despite several proposed models for LKB1 function in development and in tumour suppression, the detailed molecular action of LKB1 remains unde®ned. Here, we report the identi®cation and characterization of an LKB1-speci®c adaptor protein and substrate, STRAD (STe20 Related ADaptor). STRAD consists of a STE20-like kinase domain, but lacks several residues that are indispensable for intrinsic catalytic activity. Endogenous LKB1 and STRAD form a complex in which STRAD activates LKB1, resulting in phosphorylation of both partners. STRAD determines the subcellular localization of wild-type, but not mutant LKB1, translocating it from nucleus to cytoplasm. One LKB1 mutation previously identi®ed in a Peutz±Jeghers family that does not compromise its kinase activity is shown here to interfere with LKB1 binding to STRAD, and hence with STRAD-dependent regulation. Removal of endogenous STRAD by siRNA abrogates the LKB1-induced G 1 arrest. Our results imply that STRAD plays a key role in regulating the tumour suppressor activities of LKB1.

Gene Ontology: Pitfalls, Biases, and Remedies

“…A NOT annotation may also be assigned to a protein that does not have an activity typical of its homologs, for instance the STRADA pseudokinase (UniProtKB:Q7RTN6); STRADA adopts a closed conformation typical of active protein kinases and binds substrates, promoting a conformational change in the substrate, which is then phosphorylated by a "true" protein kinase, STK11 [ 26 ]. In this case, the "NOT" annotation is created to alert the user to the fact that although the sequence suggests that the protein has a certain activity, experimental evidence shows otherwise.…”

LKB1; linking cell structure and tumor suppression

“…In mammalian cells LKB1 forms a complex with STRAD (a pseudokinase that shares homology with the STE20 family of kinases) and MO25, both of which are required for enzymatic activity (Hawley et al, 2003;Boudeau et al, 2004). When overexpressed alone, LKB1 is localized primarily in the nucleus, however, when expressed with STRAD and MO25, it is found largely in the cytosol (Tiainen et al, 2002;Baas et al, 2003). Immunohistochemical studies show that endogenous levels of LKB1 are predominantly cytoplasmic in the pancreas and liver in vivo (Hezel AF and Bardeesy N, unpublished data).…”

“…When complexed with STRAD and MO25, LKB1 shows constitutive activation in vitro (that is, for LKB1 to be catalytically active, it does not require phosphorylation by an upstream kinase). STRAD interaction and cytosolic localization appear to be required for the capacity of LKB1 to induce G1 growth arrest in an LKB1-deficient cancer cell line (G361 melanoma cells) as an LKB1 point mutation identified in a PJS kindred that abrogates this interaction, but does not compromise kinase activity, fails to promote G1 arrest (Baas et al, 2003).…”

Host cell manipulation by the human pathogen Toxoplasma gondii

“…Recently, proteins containing a degenerated catalytic center were shown to modulate the activity of their paralogous active enzymes [77]. This regulation can take different forms through modulation of the active enzyme by direct protein-protein interactions with its inactive homologue or by substrate sequestration by the inactive enzyme [78][79][80][81]. This phenomenon is mainly observed in proteins involved in signaling pathways.…”

“…Approximately 10% of kinase proteins characterized from eukaryotes are predicted to have an inactive catalytic center [82]. Some of these pseudokinases play a role in the activation or inhibition of other kinases, or in the assembly of kinase in complex [78][79][80][81][82]. While ROP2 subfamily proteins contain a predicted kinase domain, sequence analysis by El Hajj et al [41] shows that ROP2, 4, 5, 7, 8, 11, 2L3 and 2L6 proteins lack important residues usually essential for the catalytic activity.…”