Detyrosinated (Glu) Microtubules Are Stabilized by an ATP-Sensitive Plus-End Cap

JCS 113:3907 (2000/10)
Greg Gundersen

Microtubule cap is an idea people talked about for a very long time but no one really identifies any so far. Greg's paper is the only one I could find trying to show there is a microtubule cap existed. By gentle detergent extraction experiments, stabilized microtubules (Glu-MTs) were proved cannot polymerize or shrink at their plus-end. However calcium-generated fragments of Glu-MTs incorporated free tubules, showing that Glu-MTs ends are capped. Glu-MTs are ten times more stable than regular dynamic MTs (Tyr-MTs). Studies showed that this post-translational modification is the result of MT stabilization but not the reason for it, that is, detyrosination does not directly cause MT stability. Greg showed these MTs in interphase but rarely addressed M or C phase. Detergent extraction combined with labeled tubulins is a good experiment. And calcium caused Glu-MT fragments are good control too.

Authors failed to demonstrate the constitution of this hypothesized MT cap but did suggest several end-associated proteins such as EB1, APC, p150 (Glued), and vinculin focal adhesions were not localized at Glu-MT ends. Authors thought ATP modifies Glu-MT cap which then permits MTs to depolymerize by subunit exchange. AMP-PNP blocks it. These properties of the ATPase regulating Glu-MT stability resemble those of kinesin; both prefer ATP, but can use other nucleotide triphosphates, and both are inhibited by AMP-PMP and vanadate, but not by kinase of other inhibitors. Therefore it is unlikely a kinase-involved mechanism. Authors tried kif5 and kif3a, but neither of them fit.

One interesting question is it what Greg thinks is true. There is a MT cap for Glu-MTs in interphase. How about M or C phase? Microtubules in M or C phase are more dynamic than interphase but somehow cells also need to stabilize a sub-set of microtubules at specific stage, such as spindles, or stabilized microtubules in furrow regions.