Here we show a new effect of Ca2+ on microtubule morphology: Call can cause smooth curving of microtubules in the presence Of microtubule-associated proteins (MAPs). In vitro, microtubules self-organize, forming complex dissipative structures. Such structures may be strongly affected by relatively weak external factors. A factor such as Ca2+ potentially influences spatiotemporal patterns of microtubule assembly, but the dynamics are unclear. We tested Ca2+ effects on microtuble formation. Using EM, microtubule length, curvature, and alignment and were measured in two systems: 2 mg/ml microtubule protein containing MAPs and 1mM EGTA with and without 1 mM Ca2+. The two systems were then tested using light scattering. In low Ca2+, a birefringent microtubular pattern is seen, increasing with polymerization. When 1 MM Ca2+ is added to the solution. anisotropic phase is prevented without microtubule disruption. This demonstrates an additional mechanism by which Ca2+ can alter the dynamics and morphology of microtules. Crown Copyright (C) 2009 Published by Elsevier Inc. All rights reserved.