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#phosphorylation

31 public questions tagged with this topic.

Membrane Transportation

The regulatory (R) domain of CFTR is phosphorylated by:

Regulatory R domain of CFTR spanning approximately residues 590 to 850 is an intrinsically disordered segment bridging NBD1 and TMD2 containing numerous consensus PKA and PKC phosphorylation sites. Phosphorylation is central to activation. Elevated intracellular cAMP activates protein kinase A which phosphorylates serines including Ser660, Ser737, Ser795, Ser813 and Ser768, neutralizing inhibitory interactions between R domain and NBDs and between NBD1-NBD2 interface. This conformational unblocking permits ATP binding to NBDs, dimer formation, and pore opening. Protein kinase C, Ca2+/calmodulin kinase and Src kinases provide modulatory phosphorylation but PKA is obligatory; non-phosphorylated CFTR remains closed even with ATP present. Dephosphorylation by PP2A and PP2C closes channel. Disease variants lacking PKA sites or with R domain deletions show reduced open probability. Potentiator ivacaftor enhances gating after phosphorylation, illustrating therapeutic relevance of understanding kinase regulation of this atypical ABC channel in epithelial fluid secretion and electrolyte homeostasis. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Hwang & Sheppard, Trends Pharmacol Sci 2009, CFTR R domain regulation; Riordan, Annu Rev Biochem 2008.

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Cytoskeleton

How does phosphorylation affect intermediate filaments?

Intermediate filaments are apolar polymers of tetrameric coiled-coil dimers that lack intrinsic nucleotide turnover, so dynamic regulation relies on post-translational modification. Phosphorylation within the N-terminal head domain by mitotic kinases such as Cdk1, Plk1, Aurora B, as well as PKA, PKC and MAP kinases, introduces dense negative charge that disrupts head-to-rod electrostatic interactions essential for filament elongation. The result shifts equilibrium toward soluble tetramers and monomers, enabling mitotic disassembly of vimentin networks and nuclear lamina breakdown at prometaphase, which is required for envelope rupture and chromosome access. Upon mitotic exit phosphatases PP1 and PP2A remove phosphates, promoting rapid reassembly into 10-nm ropes anchored at desmosomes and hemidesmosomes to restore tensile resilience. Phosphorylation also modulates binding to plectin and 14-3-3 adaptors during cell migration. Because assembly requires no ATP or GTP, kinase-phosphatase balance provides primary control over local turnover, solubility, and interaction with signaling scaffolds under stress. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Alberts Ch 16 Cytoskeleton; Lodish Ch 18; IF phosphorylation controls disassembly via head domain serine.

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Protein Sorting and Trafficking

What happens if a mutation prevents phosphorylation of Mannose-6-Phosphate (M6P)?

Targeting soluble acid hydrolases to lysosomes hinges on carbohydrate tag mannose-6-phosphate discovered by fractionation. In cis-Golgi, UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase alpha2 beta2 gamma2 recognizes structural patch including lysine residues clustered in tertiary structure on hydrolases like cathepsin B and adds GlcNAc-phosphate to terminal mannose of high-mannose N-glycans. Uncovering in TGN by NAGPA removes GlcNAc exposing M6P monoester capable of binding M6P receptors. M6P receptors divert enzymes from default secretory flow into clathrin-AP1 and GGA vesicles destined to late endosomes where acid releases enzyme. If phosphorylation fails due to loss-of-function mutation in GNPTAB encoding alpha beta subunits, as in mucolipidosis II I-cell disease and milder III pseudo-Hurler, hydrolases lack tag cannot bind receptors therefore traverse default secretory conduit to plasma membrane and secreted with high extracellular activity detectable in serum. Lysosomes deficient in over 50 hydrolases accumulating undigestible glycosaminoglycans and sphingolipids forming inclusion bodies pathognomonic. Glycosylation otherwise normal, ER-Golgi transport persists, but lysosomal delivery selectively lost causing severe multisystem disease.

Ref: Alberts et al., MBC: I-cell disease defective GlcNAc-phosphotransferase causes lysosomal enzyme secretion.

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Protein Sorting and Trafficking

In which Golgi compartment does phosphorylation of lysosomal enzymes occur?

Soluble acid hydrolases acquire mannose-6-phosphate marker for lysosomal delivery through selective two-step modification restricted to Golgi apparatus, discovered by Kornfeld laboratory. Newly synthesized hydrolases enter ER, receive high-mannose N-glycans, fold with calnexin cycle, then travel to cis-Golgi network. There UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase alpha2 beta2 gamma2 hexamer encoded by GNPTAB and GNPTG recognizes conformational patch on hydrolases and transfers GlcNAc-1-phosphate to C6 hydroxyl of select mannose residues forming phosphodiester intermediate. Enzyme resides in cis-Golgi ensuring early tagging before further trimming. In trans-Golgi network, uncovering enzyme N-acetylglucosamine-1-phosphodiester alpha-N-acetylglucosaminidase encoded by NAGPA hydrolyzes masking GlcNAc exposing M6P monoester. Only then two P-type lectin receptors 300 kDa cation-independent and 46 kDa cation-dependent bind slightly acidic pH packaging cargo into AP1 and GGA clathrin vesicles to late endosomes where acid releases cargo for lysosomal delivery. Failure cis-phosphorylation causes mucolipidosis II I-cell disease with secreted hydrolases and lysosomal storage phenotype. ER never adds M6P, so phosphorylation definitive marker.

Ref: Lodish et al., MCB, Chapter 14 Figure 14-21 Formation of M6P residues in cis-Golgi by phosphotransferase.

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Cell Cycle Regulation

What happens if the Rb protein is mutated and cannot be phosphorylated?

Retinoblastoma protein Rb functions as central gatekeeper integrating mitogenic signaling with E2F transcription program controlling G1/S restriction point. In quiescent or early G1 cells, Rb exists in hypophosphorylated state, binding E2F1, E2F2, E2F3 through pocket domains A and B, simultaneously recruiting histone deacetylases, SWI/SNF chromatin remodelers, and Polycomb complexes to repress promoters of cyclin E, cyclin A, thymidine kinase, dihydrofolate reductase, and DNA polymerase subunits. Mitogens induce cyclin D-CDK4/6 synthesis downstream of Ras-MAPK and PI3K pathways, which initiates progressive phosphorylation of Rb at serine 780, 795, 807, 811. Subsequent cyclin E-CDK2 mediated hyperphosphorylation fully displaces Rb, liberating E2F to activate S-phase genes. Mutation eliminating CDK phosphorylation sites locks Rb in constantly bound conformation, maintaining E2F repression despite abundant cyclin-CDK activity. Consequently, cells fail to transcribe nucleotide biosynthesis enzymes and replication factors, arresting before S-phase commitment, illustrating how non-phosphorylatable Rb creates dominant negative barrier to proliferation. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Weinberg, Biology of Cancer, 2nd ed., Chapter 8: Rb Pathway. NCBI Bookshelf: Cell Cycle Control.

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Cell Cycle Regulation

In normal conditions, phosphorylation of Rb by Cyclin D-CDK4 leads to:

Retinoblastoma tumor suppressor Rb forms central brake of G1 progression. Hypophosphorylated form present in quiescent G0 and early G1 binds E2F transcription factor family E2F1-3 together with DP1/2 dimer partners and recruits chromatin repressors histone deacetylase HDAC1/2, Suv39h1 H3K9 methyltransferase, BRG1/BRM SWI/SNF subunits and DNMT1 to promoters, maintaining hypoacetylated closed chromatin silencing S-phase genes. Cyclin D-CDK4/6 complexes activated by mitogens phosphorylate Rb at C-terminal residues such as Ser780 Ser795 partially weakening binding to LXCXE motif proteins and permitting initial transcription of Cyclin E. Cyclin E-CDK2 then adds phosphorylations at additional pocket sites including Ser612 causing conformational shift that promotes dissociation through Pin1 prolyl isomerase action. Hyperphosphorylated Rb fully releases E2F, allowing transcription of genes required for nucleotide synthesis, replication licensing and S phase progression, thus permitting entry into S phase. Phosphorylation persists through S G2 M phases until PP1 phosphatase dephosphorylates Rb in late mitosis restoring repressive complex for next G1. Loss of Rb pathway via mutation or viral oncoprotein E1A binding results in constitutive E2F activity driving unchecked proliferation hallmark of many cancers.

Ref: Weinberg, Retinoblastoma Gene and Cell Cycle Control, Cell 1995; Giacinti & Giordano, RB Regulatory Pathway: Mechanisms of Rb Phosphorylation, Oncogene 2006.

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Cell Cycle Regulation

Which kinase phosphorylates Chk1 in response to replication stress?

Replication stress exposes extended single-stranded DNA regions coated by replication protein A, forming a key signaling platform. ATR kinase, recruited via ATRIP-interacting domain to RPA-ssDNA, is the principal sensor of this structure. At stalled forks, factors TopBP1, Claspin, Timeless-Tipin and Rad9-Rad1-Hus1 clamp augment ATR activation through direct binding and allosteric stimulation. Once activated, ATR directly phosphorylates checkpoint kinase Chk1 on serine residues 317 and 345 within its C-terminal regulatory domain. This modification relieves autoinhibition, enabling Chk1 catalytic activity through autophosphorylation at Ser296. Activated Chk1 diffuses away from chromatin to phosphorylate effectors: Cdc25A phosphatase marked for SCF-betaTrCP mediated degradation, Cdc25C generating 14-3-3 binding and cytoplasmic sequestration, and downstream targets like Wee1, Rad51 and polymerase eta. Consequences include inhibition of CDK2-Cyclin E and CDK1-Cyclin B, suppression of late origin firing, stabilization of replisome and arrest in intra-S and G2/M phases. This ATR-Chk1 axis provides time for fork repair and prevents entry into mitosis with under-replicated genomes preserving stability. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Cimprich & Cortez, ATR role in replication checkpoint, Nat Rev Mol Cell Biol 2008; NCBI Bookshelf, Replication Stress Response.

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