Phosphorylation of Drosophila Brahma on CDK-phosphorylation sites is important for cell cycle regulation and differentiation.

The SWI/SNF ATP-dependent chromatin-remodeling complex is an important evolutionarily conserved regulator of cell cycle progression. It associates with the Retinoblastoma (pRb)/HDAC/E2F/DP transcription complex to modulate cell cycle-dependent gene expression. The key catalytic component of the SWI/SNF complex in mammals is the ATPase subunit, Brahma (BRM) or BRG1. BRG1 was previously shown to be phosphorylated by the G1-S phase cell cycle regulatory kinase Cyclin E/CDK2 in vitro, which was associated with the bypass of G1 arrest conferred by BRG1 expression. However, it is unknown whether direct Cyclin E/CDK2-mediated phosphorylation of BRM/BRG1 is important for G1-S phase cell cycle progression and proliferation in vivo. Herein, we demonstrate for the first time the importance of CDK-mediated phosphorylation of Brm in cell proliferation and differentiation in vivo using the Drosophila melanogaster model organism. Expression of a CDK-site phospho-mimic mutant of Brm, brm-ASP (all the potential CDK sites are mutated from Ser/Thr to Asp), which acts genetically as a brm loss-of-function allele, dominantly accelerates progression into the S phase, and bypasses a Retinoblastoma-induced developmental G1 phase arrest in the wing epithelium. Conversely, expression of a CDK-site phospho-blocking mutation of Brm, brm-ALA, acts genetically as a brm gain-of-function mutation, and in a Brm complex compromised background reduces S phase cells. Expression of the brm phospho-mutants also affected differentiation and Decapentaplegic (BMP/TGFß) signaling in the wing epithelium. Altogether our results show that CDK-mediated phosphorylation of Brm is important in G1-S phase regulation and differentiation in vivo. ABBREVIATIONS: A-P: Anterior-Posterior; BAF: BRG1-associated factor; BMP: Bone Morphogenetic Protein; Brg1: Brahma-Related Gene 1; Brm: Brahma; BSA: Bovine Serum Albumin; CDK: Cyclin dependent kinase dpp: decapentaplegic; EdU: 5-Ethynyl 2'-DeoxyUridine; EGFR: Epidermal Growth Factor Receptor; en: engrailed; GFP: Green Fluorescent Protein; GST: Glutathione-S-Transferase; HDAC: Histone DeACetylase; JNK: c-Jun N-terminal Kinase; Mad: Mothers Against Dpp; MAPK: Mitogen Activated Protein Kinase; MB:: Myelin Basic Protein; nub: nubbin; pH3: phosphorylated Histone H3; PBS: Phosphate Buffered Saline; PBT: PBS Triton; PFA: ParaFormAldehydep; Rb: Retinoblastoma protein; PCV: Posterior Cross-Vein; Snr1: Snf5-Related 1; SWI/SNF: SWitch/Sucrose Non-Fermentable; TGFß: Transforming Growth Factor ß; TUNEL: TdT-mediated dUTP Nick End Labelling; Wg: Wingless; ZNC: Zone of Non-Proliferating Cells.

Cyclin-dependent kinase-mediated phosphorylation of breast cancer metastasis suppressor 1 (BRMS1) affects cell migration.

Expression of Breast Cancer Metastasis Suppressor 1 (BRMS1) reduces the incidence of metastasis in many human cancers, without affecting tumorigenesis. BRMS1 carries out this function through several mechanisms, including regulation of gene expression by binding to the mSin3/histone deacetylase (HDAC) transcriptional repressor complex. In the present study, we show that BRMS1 is a novel substrate of Cyclin-Dependent Kinase 2 (CDK2) that is phosphorylated on serine 237 (S237). Although CDKs are known to regulate cell cycle progression, the mutation of BRMS1 on serine 237 did not affect cell cycle progression and proliferation of MDA-MB-231 breast cancer cells; however, their migration was affected. Phosphorylation of BRMS1 does not affect its association with the mSin3/HDAC transcriptional repressor complex or its transcriptional repressor activity. The serine 237 phosphorylation site is immediately proximal to a C-terminal nuclear localization sequence that plays an important role in BRMS1-mediated metastasis suppression but phosphorylation does not control BRMS1 subcellular localization. Our studies demonstrate that CDK-mediated phosphorylation of BRMS1 regulates the migration of tumor cells.

Mechanisms of generating polyubiquitin chains of different topology.

Ubiquitination is an important post-translational process involving attachment of the ubiquitin molecule to lysine residue/s on a substrate protein or on another ubiquitin molecule, leading to the formation of protein mono-, multi- or polyubiquitination. Protein ubiquitination requires a cascade of three enzymes, where the interplay between different ubiquitin-conjugating and ubiquitin-ligase enzymes generates diverse ubiquitinated proteins topologies. Structurally diverse ubiquitin conjugates are recognized by specific proteins with ubiquitin-binding domains (UBDs) to target the substrate proteins of different pathways. The mechanism/s for generating the different ubiquitinated proteins topologies is not well understood. Here, we will discuss our current understanding of the mechanisms underpinning the generation of mono- or polyubiquitinated substrates. In addition, we will discuss how linkage-specific polyubiquitin chains through lysines-11, -48 or -63 are formed to target proteins to different fates by binding specific UBD proteins.

Protein monoubiquitination and polyubiquitination generate structural diversity to control distinct biological processes.

Ubiquitination involves the attachment of ubiquitin (Ub) to lysine residues on substrate proteins or itself, which can result in protein monoubiquitination or polyubiquitination. Polyubiquitination through different lysines (seven) or the N-terminus of Ub can generate different protein-Ub structures. These include monoubiquitinated proteins, polyubiqutinated proteins with homotypic chains through a particular lysine on Ub or mixed polyubiquitin chains generated by polymerization through different Ub lysines. The ability of the ubiquitination pathway to generate different protein-Ub structures provides versatility of this pathway to target proteins to different fates. Protein ubiquitination is catalyzed by Ub-conjugating and Ub-ligase enzymes, with different combinations of these enzymes specifying the type of Ub modification on protein substrates. How Ub-conjugating and Ub-ligase enzymes generate this structural diversity is not clearly understood. In the current review, we discuss mechanisms utilized by the Ub-conjugating and Ub-ligase enzymes to generate structural diversity during protein ubiquitination, with a focus on recent mechanistic insights into protein monoubiquitination and polyubiquitination.