A Calmodulin-Sensitive lnteraction between Micmtubules and a Higher Plant Homolog of Elongation Factor-I a

The microtubules (MTs) of higher plant cells a= organized into arrays with essential functions in plant cell growth and differentiation; however, molecular mechanisms underlying the organization and regulation of these arrays remain largely unknown. We have approached this problem using tubulin affinity chromatography to isolate carrot proteins that interact with MTs. From these proteins, a 50-kD polypeptide was selectively purified by exploiting its Ca2+-dependent binding to calmodulin (CaM). This polypeptide was identified as a homolog of elongation factor-lu (EF-1a)-a highly conserved and ubiqultous protein translation factor. The carrot EF-la homolog bundles MTs in vitro, and moreover, this bundling is modulated by the addition of Ca2+ and CaM together (Ca2+/CaM). A direct binding between the EF-la homolog and MTs was demonstrated, providing nove1 evidence for such an interaction. Based on these findings, and others discussed herein, we propose that an EF-la homolog mediates the lateral association of MTs in plant celk by a Ca2+/CaM-sensi- tive mechanism.