Chlamydomonas outer arm dynein alters conformation in response to Ca2+

We have previously shown that Ca2+ directly activates ATP-sensitive microtubule binding by a Chlamydomonas outer arm dynein subparticle containing the beta and gamma heavy chains (HCs). The gamma HC-associated LC4 light chain is a member of the calmodulin family and binds 1-2 Ca2+ with K-Ca = 3 x 10(-5) M in vitro, suggesting it may act as a Ca2+ sensor for outer arm dynein. Here we investigate interactions between the LC4 light chain and gamma HC. Two IQ consensus motifs for binding calmodulin-like proteins are located within the stem domain of the gamma heavy chain. In vitro experiments indicate that LC4 undergoes a Ca2+-dependent interaction with the IQ motif domain while remaining tethered to the HC. LC4 also moves into close proximity of the intermediate chain IC1 in the presence of Ca2+. The sedimentation profile of the gamma HC subunit changed subtly upon Ca2+ addition, suggesting that the entire complex had become more compact, and electron microscopy of the isolated gamma subunit revealed a distinct alteration in conformation of the N-terminal stem in response to Ca2+ addition. We propose that Ca2+-dependent conformational. change of LC4 has a direct effect on the stem domain of the gamma HC, which eventually leads to alterations in mechanochemical interactions between microtubules and the motor domain(s) of the outer dynein arm.