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#calmodulin

3 public questions tagged with this topic.

Cytoskeleton

Which protein regulates myosin activity in smooth muscle contraction?

Smooth muscle lacks troponin placing regulation directly on myosin thick filament via calcium sensing protein calmodulin small acidic protein essential for many signaling pathways. Calmodulin is 17 kDa dumbbell protein with four EF hand calcium binding loops two in each lobe. Apo calmodulin inactive at low calcium. Calcium rise induced by cholinergic stimulation and IP3 mediated sarcoplasmic release saturates calmodulin to Ca4 calmodulin exposing methionine rich hydrophobic grooves that wrap around autoinhibitory sequence of myosin light chain kinase MLCK displacing it from substrate binding site. Active MLCK phosphorylates serine 19 of 20 kDa myosin regulatory light chain on each heavy chain head increasing myosin ATPase Vmax dramatically up to hundred fold. Phosphorylated myosin unfolds from autoinhibited folded 10S to extended 6S conformation capable of assembling into side polar filaments and interacting with actin. Dephosphorylation by myosin light chain phosphatase MYPT1 containing regulatory targeting subunit returns myosin to inactive state. Thus calmodulin converts calcium signal into covalent myosin modification.

Ref: Webster et al., J Physiol; Kamm & Stull Physiol Rev – Calmodulin regulation of smooth muscle myosin via MLCK.

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Protein-Protein Interaction

Which tag in TAP binds calmodulin?

Tandem Affinity Purification tag is engineered for sequential, highly specific purification of native complexes under mild conditions. Typical TAP contains Protein A repeats that bind IgG and calmodulin-binding peptide that binds calmodulin resin in calcium-dependent manner, separated by TEV protease site. Calmodulin-binding peptide is the module that interacts with calmodulin-coated beads in second affinity step. ProtA, GST, and His bind IgG, glutathione, and nickel respectively, not calmodulin. This dual orthogonal purification markedly enhances specificity and preserves physiological interactions for downstream mass spectrometry identification with reduced background contaminants.

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 Signaling GPCR Pathways - I

Calmodulin binds Ca2+ using how many binding sites?

4, 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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