Calmodulin modulates the interaction between iqgap1 and cdc42

Calmodulin is a ubiquitous protein involved in diverse intracellular processes. Though accumulating evidence suggests that increased levels of calmodulin may participate in oncogenesis, the downstream effectors of such transformation are obscure. One candidate is the recently described 189 kDa protein, IQGAPI IQGAPI contains four calmodulin-binding IQ motifs and regions of significant sequence similarity to (he catalytic domain of Ras-GTPase activating proteins and the tumor suppressers, neurofibromin and Sari {J Biol Chem 1994, 269, 20517). We isolated from breast cell lines a protein that binds calmodulin both in the presence and absence of Ca'. Nanoelectrospray tandem mass spectrometry unequivocally identified this protein as IQGAPI. IQGAPI co-immunopreci pi rated with calmodulin from malignant breast cell lysates, and its direct binding tu calmodulin was confirmed by I;sl-calmodulin overlay. The function of IQGAPI is not known, but it binds Cdc42 (EMBO J 1996, 15. 2997), a Rho family GTPase involved in cell cycle progression and cytoskeleton assembly. To explore a potential functional role for its association with calmodulin, we analyzed the interaction ot IQGAPI withCdc42. Full length purified human IQGAPI associated specifically with the active GTP-bound GST-Cdc42, and preincubation with calmodulin prevented the binding. Ca:/caimodulin disrupted the IQGAPI-Cdc42 interaction in a dose-dependent manner, with effects observed at physiological Ca' concentrations. Similarly, the affinity of IQGAPI from breast cancer cell lysates for GST-Cdc42 was higher in the absence of Ca:. As Cdc42 participates in cell proliferation and cytoskeletal changes, increased cytosolic Ca Vcalmodulin may contribute to neoplastic transformation via regulation of the IQGAPI -Cdc42 association. We conclude that IQGAPI possibly provides a molecular link between Or Vcalmodulin signaling pathways and Cdc42 -mediated processes.