ChlamydomonasOuter Arm Dynein Alters Conformation in Response to Ca2+

Abstract

We have previously shown that Ca2+directly activates ATP-sensitive microtubule binding by a Chlamydomonas outer arm dynein subparticle containing the β and γ heavy chains (HCs). The γ HC–associated LC4 light chain is a member of the calmodulin family and binds 1-2 Ca2+with KCa= 3 × 10−5M in vitro, suggesting it may act as a Ca2+sensor for outer arm dynein. Here we investigate interactions between the LC4 light chain and γ HC. Two IQ consensus motifs for binding calmodulin-like proteins are located within the stem domain of the γ 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 γ HC subunit changed subtly upon Ca2+addition, suggesting that the entire complex had become more compact, and electron microscopy of the isolated γ 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 γ HC, which eventually leads to alterations in mechanochemical interactions between microtubules and the motor domain(s) of the outer dynein arm.