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#protein

10 public questions tagged with this topic.

Cytoskeleton

Which protein links intermediate filaments to actin filaments and microtubules?

Plectin is a giant cytolinker of the plakin family that simultaneously recognizes actin, microtubules and intermediate filaments, integrating cytoskeletal networks into a cohesive mechanical continuum. Its structure includes an N-terminal actin-binding domain composed of two calponin-homology motifs, a central 200-nm coiled-coil rod dimerization domain, and a C-terminal repeat domain that binds vimentin, keratin, desmin and glial fibrillary acidic protein with distinct isoform specificity. Alternative splicing of first exons targets isoforms to hemidesmosomes via integrin beta4, to focal adhesions, desmosomes, mitochondria and nuclear envelope. At these sites plectin recruits microtubule-associated proteins and directly contacts tubulin, coordinating plus-end dynamics with intermediate filament anchorage. Deletion produces skin blistering with muscular dystrophy due to failure of stress transfer. In contrast spectrin forms tetramers capping actin at membranes, fimbrin bundles actin in microvilli, filamin crosslinks actin orthogonally, none containing high-affinity intermediate filament repeats. Thus plectin alone provides universal bridging activity essential for epithelial and muscle integrity.

Ref: Alberts Ch 16; Fuchs & Cleveland, Science 1998 plakin plectin crosslinks IF, actin, microtubules.

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Cytoskeleton

Which protein is responsible for anchoring actin filaments to the Z-disc in muscle cells?

Organization of sarcomere depends on anchoring thin filaments at Z disc providing mechanical integrity during repetitive contraction. Z disc 100 nanometer thick electron dense structure contains alpha actinin antiparallel dimer crosslinking antiparallel barbed ends from adjacent sarcomeres into orthogonal lattice spaced 20 nanometer, plus titin Z repeats, telethonin, myotilin, FATZ. Nebulin huge filamentous protein 600 to 900 kilodalton with 185 repeats of 35 residue nebulin motif SDXXYK each binding one actin monomer, plus N terminus interacting with tropomodulin capping pointed end and C terminal SH3 domain binding myopalladin CapZ inside Z disc embedding. By acting as molecular ruler nebulin dictates thin filament length and stabilizes filament against cofilin severing and depolymerization, loss causing nemaline myopathy with shorter thin filaments. Spectrin provides membrane skeleton in erythrocytes, dystrophin links actin via cysteine rich domain to dystroglycan complex at costameres for lateral force transmission, not Z disc anchoring. Titin provides elasticity and scaffold but nebulin specialized for actin Z disc anchorage and length specification.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Nebulin Anchoring Actin at Z-Disc.

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Cytoskeleton

In skeletal muscle contraction, which protein regulates Ca2+ reuptake into the sarcoplasmic reticulum?

Relaxation requires rapid calcium clearance to restore resting state permitting cross bridge detachment and tropomyosin return to blocking. Sarcoplasmic reticulum calcium ATPase SERCA, family of P type ATPases with ten transmembrane helices and three cytoplasmic domains N P A, couples ATP hydrolysis to transport of two calcium ions per cycle from cytosol to SR lumen against concentration gradient 10,000 fold. In skeletal fast twitch SERCA1a constitutes 60 percent SR protein, in cardiac SERCA2a regulated by phospholamban PLN small inhibitor that reduces calcium affinity when dephosphorylated, phosphorylation by PKA relieves inhibition accelerating relaxation during sympathetic stimulation. Sarcolipin SNL similar regulator in skeletal muscle. Cytosolic calcium drops from peak 10 micromolar to 100 nanomolar within 20 milliseconds enabling troponin to revert. Myosin generates force not calcium reuptake, calmodulin sensor activating MLCK, tropomyosin blocks access. SERCA inhibition by thapsigargin or cyclopiazonic acid depletes SR stores, raises cytosol, causing contracture and ER stress, demonstrating central role for Ca2+ ATPase in reuptake and muscle relaxation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: SERCA Ca2+-ATPase and Muscle Relaxation.

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Protein Sorting and Trafficking

Which protein is required for peroxisomal membrane biogenesis?

De novo biogenesis and growth peroxisomal membrane depends targeted insertion peroxisomal membrane proteins PMPs specialized machinery distinct matrix pathway. PMPs including docking Pex14 Pex13 fission factors Pex11 isoforms transporters ABCD1-3 synthesized free cytosolic ribosomes containing hydrophobic transmembrane domains aggregation prone requiring chaperoning. Cytosolic receptor chaperone Pex19 farnesylated C-terminal CAAX box binds PMPs hydrophobic groove preventing aggregation maintaining solubility delivering peroxisomal membrane docking platform. Targeting essential factors Pex3 42 kDa integral membrane short lumenal N-terminus docking anchor Pex19-PMP complex, and Pex16 integral membrane two transmembrane helices recruits Pex3 ER-derived pre-peroxisomal vesicles stabilizes growth. Current model Pex3 Pex16 first insert ER via Sec61 segregate subdomain forming pre-peroxisomal vesicles budding Pex19 dependent mature functional peroxisomes importing matrix enzymes. Pex5 Pex7 soluble matrix import receptors PTS1 PTS2 luminal proteins not membrane biogenesis, Tom20 Tom22 mitochondrial outer receptors presequences, Hsp70 Hsp90 general cytosolic folding chaperones unrelated specific PMP insertion. Human cells lacking Pex3 or Pex16 absence detectable membranes microscopy PMPs mislocalized mitochondria rapidly degraded clinical Zellweger illustrating foundational role membrane formation identity maintenance and disease.

Ref: Distel et al., Annu Rev Biochem: Pex3 and Pex16 essential for peroxisomal membrane biogenesis.

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Cell Cycle Regulation

Which protein is the catalytic subunit in cell cycle regulation?

Cell cycle progression driven by oscillating cyclin-dependent kinase activities where temporal regulation derived from cyclin synthesis and degradation while catalytic function resides within CDK subunit. Cyclins contain cyclin box fold binding hydrophobic patch on CDK but lack enzymatic residues for phosphate transfer. CDKs share bilobal kinase architecture with ATP binding pocket between N and C lobes, PSTAIRE helix aligning catalytic residues and activation T-loop blocking substrate binding when unphosphorylated. Cyclin binding rotates PSTAIRE helix inward positioning Glu51 to coordinate Lys33 for ATP orientation and moves T-loop aside exposing substrate binding groove. Full activation additionally requires phosphorylation at conserved threonine 160 in CDK2 or 161 in CDK1 by CAK complex CDK7-Cyclin H-MAT1 stabilizing T-loop interaction with substrate peptide through arginine pocket, and removal of inhibitory phosphorylations at Thr14 Tyr15 added by Wee1 Myt1 kinases via Cdc25 phosphatases. Active CDK then transfers gamma phosphate of ATP to serine-threonine-proline motifs on hundreds of targets, powering cell cycle transitions. Hence catalytic subunit capable of phosphotransfer is CDK, cyclins provide allosteric activation, localization and substrate specificity.

Ref: Morgan, Cyclin-Dependent Kinases Structure and Activation, Annu Rev Cell Dev Biol 1997; Alberts et al., Chapter 17, CDK as Catalytic Subunit.

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UV-VIS spectroscopy

Which fluorophore is intrinsic to protein?

Intrinsic protein fluorescence originates from aromatic side chains capable of absorbing UV and emitting without external dye. Tryptophan dominates because indole nucleus exhibits relatively high quantum yield near 0.2, excitation maximum near 280 nm, emission 308-355 nm highly sensitive to environment polarity. Phenylalanine quantum yield extremely low near 0.02, tyrosine emission often quenched via resonance energy transfer to tryptophan when both present. Extrinsic fluorophores like green fluorescent protein chromophore requires autocatalytic cyclization, DAPI and FAD are added ligands or redox cofactors. Hence tryptophan remains natural probe for folding, quenching and binding studies.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Cell Communication

Which is NOT an extracellular matrix protein? (June 2020)

Keratin, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Atoms and Molecules

The monomers of proteins are:

Amino acids is the scientifically accurate answer to this question. Within the study of Atoms and Molecules, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of Amino acids directly address what is being asked. Among the other options, Nucleotides, Fatty acids, and Monosaccharides do not correctly answer this question because they either refer to different concepts, describe properties of other molecules or processes, or represent common misconceptions about this topic.

Ref: Campbell Biology, Urry et al., 12th Ed.

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Ethylene

CTR1 is a:

The right choice is B: MAPKKK. In Ethylene, that matches how the process or concept actually works — the other choices mix up related ideas or use the wrong mechanism. Skip these: A) Receptor; C) Transcription factor; D) Ion channel. A quick check: if an option needs energy, pumps, or the opposite direction of movement, ask whether that really applies.

Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.

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