The radial spoke is a multi-unit

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

structure found in the

axoneme In molecular biology, an axoneme, also called an axial filament, is the microtubule-based cytoskeletal structure that forms the core of a cilium or flagellum. Cilia and flagella are found on many cells, organisms, and microorganisms, to pr ...

s of

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

and

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

. Although experiments have determined the importance of the radial spoke in the proper function of these

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s, its structure and mode of action remain poorly understood.

Cellular location and structure

Radial spokes are T-shaped structures present inside the axoneme. Each spoke consists of a "head" and a "stalk," while each of these sub-structures is itself made up of many protein subunits. In all, the radial spoke is known to contain at least seventeen proteins, five in the head and twelve in the stalk. The spoke stalk binds to the A-tubule of each

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

outer doublet, and the spoke head faces in towards the center of the axoneme (see illustration at right).

Function

The radial spoke is known to play a role in the mechanical movement of the flagellum/cilium. For example,

mutant In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It i ...

organisms lacking properly functioning radial spokes have flagella and cilia that are immotile. Radial spokes also influence the cilium "waveform"; that is, the exact bending pattern the cilium repeats. How the radial spoke carries out this function is poorly understood. Radial spokes are believed to interact with both the central pair microtubules and the

dynein Dyneins are a family of cytoskeletal motor proteins (though they are actually protein complexes) that move along microtubules in cells. They convert the chemical energy stored in ATP to mechanical work. Dynein transports various cellular cargo ...

arms, perhaps in a way that maintains the rhythmic activation of the dynein motors. For example, one of the radial spoke subunits, RSP3, is an anchor protein predicted to hold another protein called

protein kinase A In cell biology, protein kinase A (PKA) is a family of serine-threonine kinases whose activity is dependent on cellular levels of cyclic AMP (cAMP). PKA is also known as cAMP-dependent protein kinase (). PKA has several functions in the cell, in ...

(PKA). PKA would theoretically then be able to activate/inactivate the adjacent dynein arms via its

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

activity.

Proteins

Human axonemal radial spoke subunits: RSPH1, RSPH3, RSPH4A, RSPH6A, RSPH9, RSPH10B, RSPH10B2, RSPH14.

References

{{Authority control Cell movement Molecular biology Proteins