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The plant symbiotic kinase CCaMK integrates calcium binding and calmodulin-dependent regulation. Its activation can connect nuclear calcium signaling to symbiotic developmental programs.

Myosins convert a nucleotide-driven actin-binding cycle into movement, force or anchoring. Their different cellular roles arise from variations in motor kinetics, directionality, lever-arm geometry, oligomerization and cargo binding. A class name alone does not specify a universal stepping speed or duty ratio.

A myosin usually contains an actin-binding motor domain with an ATPase active site, a neck associated with light chains, and a tail involved in cargo association or assembly. The myosin lever arm amplifies small motor-domain rearrangements. The tail can connect the motor to a membrane, receptor, organelle or another myosin molecule. Some isoforms are autoinhibited until cargo binding, calcium-dependent regulation or phosphorylation favours an active conformation.

The mechanochemical cycle can be organized around actin affinity. In a strongly bound nucleotide-free state, ATP binding weakens actin attachment and promotes detachment. ATP hydrolysis prepares a detached or weakly bound ADP–phosphate state. Subsequent actin interaction and transitions around phosphate release stabilize strong binding and generate the myosin power stroke. ADP release and the next ATP-binding event complete the cycle. The exact ordering and rates of structural transitions and phosphate release depend on isoform and load; the cycle should not be reduced to a claim that ATP cleavage itself is the instantaneous mechanical stroke.

Directionality depends on motor geometry and the polarity of actin. Most characterized myosins move toward the barbed end, but Myosin VI moves toward the pointed end. Motor assays established this reversed direction directly, showing that “myosin” does not imply a single transport direction.^[ref] In a cortex with complex actin architecture, filament orientation must therefore be known before predicting net cargo movement.

The myosin duty ratio is the fraction of a cycle spent strongly bound. A single low-duty motor repeatedly loses its track, whereas an assembly of many such motors can maintain collective contact. Myosin II illustrates this strategy: bipolar filaments bind oppositely oriented actin arrays and generate sliding or tension. In muscle, ordered filament geometry converts sliding into contraction. In nonmuscle cells, related contractile networks contribute to the division furrow, junction remodeling, migration and cortical tension. If the network is anchored, the same sliding mechanism can produce an approximately isometric force rather than large displacement.

Cargo transport poses a different requirement. Dimeric Myosin V motors coordinate their heads so that one remains attached while the other advances. A high-duty coordinated cycle supports myosin processivity, enabling repeated steps before dissociation. Single-fluorophore localization followed one labeled head moving by roughly twice the centre-of-mass step, supporting alternating hand-over-hand motion rather than equal short advances of both heads.^[ref] The approximate 36 nm centre-of-mass repeat reflects the actin track and motor geometry; it is not a step size shared by all myosins.

Transport and tension holding overlap. Load can slow nucleotide release and prolong strong binding, allowing some motors to act as dynamic anchors. Myosin VI can participate in endocytic membrane organization or cargo movement, depending on its partners and assembly state. Class-I myosins couple membranes to cortical actin and can regulate membrane tension and local transport. Specialized myosins organize sensory structures or long-range organelle positioning. These roles depend as much on tail-associated adaptors and local actin arrangement as on the catalytic head.

Regulation matches activity to demand. Light-chain phosphorylation can promote nonmuscle myosin II activation and filament assembly. Calcium-sensitive light chains, including calmodulin in appropriate classes, alter motor regulation or lever-arm organization. Cargo-associated adaptors can relieve autoinhibition and assemble transport complexes. Motor number, opposing loads, actin turnover and competition with other transport systems further determine the outcome.

Useful experiments separate these contributions. Actin-gliding assays establish direction and movement, single-molecule tracking measures stepping and run length, and force measurements reveal load-dependent kinetics. ATPase assays alone establish chemical turnover but not productive movement. In cells, rescue with a motor-defective or cargo-binding-defective protein can distinguish catalytic force production from a structural scaffolding role. The unifying mechanism is a coordinated affinity-and-conformation cycle; the biological output can be motion, sustained tension or controlled coupling of membranes to the cytoskeleton.

We understand several calcium sensors and downstream responses, but do not yet possess a general, predictive account of how a particular calcium signature selects a particular plant response. The unresolved problem is the causal transformation from a multidimensional signal to an outcome in its cellular context. It is too strong to say that nothing is known about calcium signal decoding.

A calcium signature includes the amplitude, duration, frequency and spatial distribution of increases in free calcium ion concentration. The nucleus, cytosol and regions close to ion channels can experience different signals. A stimulus may therefore change several candidate information-bearing variables simultaneously. Two treatments producing different oscillation frequencies might also differ in pulse height, total calcium exposure or the number of responding cells. A correlation between frequency and gene expression does not establish that frequency is the variable actually decoded. Nor need each stimulus have one unique calcium signature: different stimuli can recruit overlapping machinery, while the same stimulus can produce different responses in different tissues.

There are plausible molecular decoders. Calmodulin changes its interactions with targets after calcium binding. A calcium-dependent protein kinase combines a calcium sensor with a catalytic domain, directly connecting calcium binding to target phosphorylation. Calcineurin B-like proteins recruit CBL-interacting protein kinases, providing combinations of sensor, kinase and subcellular localization. In symbiotic signaling, nuclear oscillations feed into CCaMK and downstream developmental regulation. These mechanisms explain how calcium can influence an output; identifying one does not automatically explain discrimination among different waveforms.

Calcium sensor kinetics provide one possible mechanism. For a deliberately simplified sensor with active fraction $a$, free calcium concentration $C(t)$ and positive rate constants, considerWhen calcium returns to a negligible value between pulses, $a$ decays approximately as $\exp (-k_{\mathrm{off}}t)$. Pulses separated by much less than $1/k_{\mathrm{off}}$ can therefore accumulate sensor activation; widely separated pulses allow recovery. This is an illustrative kinetic model, not a fitted description of every plant sensor. Real decoders may have several calcium-binding sites, autoinhibition, target-dependent affinity and persistent phosphorylation. Crucially, simple integration of closely spaced pulses does not by itself prove a dedicated frequency code independent of pulse amplitude and duration.

There is experimental evidence that waveform matters. Imposed guard-cell calcium patterns can produce different stomatal outcomes, including a distinction between immediate changes and a maintained response. This goes beyond merely observing that a closing stoma has calcium oscillations. Nevertheless, it establishes a response relationship in a particular experimental system; assigning that relationship to one endogenous decoder requires additional molecular evidence.^[ref] Likewise, an activated CCaMK can trigger spontaneous nodulation, bypassing upstream requirements. Such bypass experiments place kinase activity downstream of early symbiotic signaling, but do not identify which feature of the native calcium trace the wild-type kinase reads.^[ref]

Measurement presents a separate limitation. A calcium reporter has its own affinity, response time and dynamic range. Slow reporters can merge brief pulses; saturation can erase amplitude differences. Probe expression can buffer calcium, and population measurements average heterogeneous cells. A calcium signal close to a channel may activate a nearby sensor without creating a large whole-cell change. Comparing different reporters or compartments without calibration can therefore create apparent biological differences that partly reflect measurement.

Finally, decoding depends on state. Sensor abundance, targets, prior phosphorylation, hormones and parallel redox signalling all modify the response to a given calcium signature. Sensors can also regulate channels and calcium removal, feeding back on signal production. A mutant that changes both waveform and output has not cleanly separated encoding from decoding. A decisive study would manipulate relevant waveform features while controlling the others, measure sensor activation and physiological response in the same cells, and test whether selective loss and restoration of decoder function abolish and restore discrimination. The central gap is thus a quantitative causal map linking local calcium dynamics, decoder kinetics and cellular state, rather than a lack of candidate calcium-binding proteins.