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Profile Publications (35)
Name: Dr. Raymond E Stephens
Position: Professor of Physiology & Biophysics, Emeritus
Research Description:
The macromolecular assemblage of structural and enzymatic proteins in fully assembled, fully functional cilia can undergo rapid turnover at an incorporation rate approaching that of total organelle regeneration. Paradoxically, this is true of the tubulin within the stable, fixed-length outer doublet microtubules of the 9+2 axoneme, even when tubulin synthesis and normal polymerization are blocked. Whereas microtubule-associated proteins can exchange by simple displacement from the tubule surface, tubulin itself must either 1) incorporate at microtubule tips and leave near the base since length is constant or 2) exchange at the junctions or seams between and within doublet microtubules where tubulin dimer lattice mismatches exist. Furthermore, precursor tubulin is associated with the ciliary membrane compartment, possibly complexed with lipids, while most other 9+2 building blocks are conveyed in direct association with the axoneme by intraflagellar transport (IFT).

It is well established that mature cilia contain abundant chaperones and that microtubules can accommodate a variable number of protofilaments along their lengths, accompanied by consequent lattice defects. Based on this, we have proposed two closely related hypotheses for tubulin exchange: 1) Chaperones may promote tubulin incorporation at the distal tip and concurrent removal at or toward the base; 2) Lattice defects may permit direct incorporation and passive propagation of tubulin, “treadmilling” it from the tip of the cilium, where incorporation is known to occur, toward more proximal regions, where the steric excess of tubulin must exit. These two concepts are not mutually exclusive and may be cooperative.

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