Stathmin family protein SCG10 differentially regulates the plus and minus end dynamics of microtubules at steady state in vitro: implications for its role in neurite outgrowth.

SCG10 (superior cervical ganglia neural-specific 10 protein) is a neuron specific member of the stathmin family of microtubule regulatory proteins that like stathmin can bind to soluble tubulin and depolymerize microtubules. The direct actions of SCG10 on microtubules themselves and on their dynamics have not been investigated previously. Here, we analyzed the effects of SCG10 on the dynamic instability behavior of microtubules in vitro, both at steady state and early during microtubule polymerization. In contrast to stathmin, whose major action on dynamics is to destabilize microtubules by increasing the switching frequency from growth to shortening (the catastrophe frequency) at microtubule ends, SCG10 stabilized the plus ends both at steady state and early during polymerization by increasing the rate and extent of growth. For example, early during polymerization at high initial tubulin concentrations (20 microM), a low molar ratio of SCG10 to tubulin of 1:30 increased the growth rate by approximately 50%. In contrast to its effects at plus ends, SCG10 destabilized minus ends by increasing the shortening rate, the length shortened during shortening events, and the catastrophe frequency. Consistent with its ability to modulate microtubule dynamics at steady state, SCG10 bound to purified microtubules along their lengths. The dual activity of SCG10 at opposite microtubule ends may be important for its role in regulating growth cone microtubule dynamics. SCG10's ability to promote plus end growth may facilitate microtubule extension into filopodia, and its ability to destabilize minus ends could provide soluble tubulin for net plus end elongation.