The calmodulin-like proteins, CML13 and CML14 function as myosin light chains for the class XI myosins in Arabidopsis

Myosins are crucial motor proteins associated with the actin cytoskeleton in eukaryotic cells. Structurally, myosins form heteromeric complexes, with smaller light chains such as calmodulin (CaM) bound to isoleucine-glutamine (IQ) domains in the neck region. These interactions facilitate mechano-enzymatic activity. Recently, we reported that Arabidopsis CaM-like (CML) proteins CML13 and CML14 interact with the IQ domains of various proteins and function as myosin VIII light chains. Here, we demonstrate that CaM, CML13, and CML14 specifically bind to the neck region of all 13 Arabidopsis myosin XI isoforms, with some specificity among the CaM/CML-IQ domains. We observed distinct residue preferences within the Myo XI IQ domains for CML13, CML14, and CaM. Recombinant CaM, CML13, and CML14 exhibited calcium-insensitive binding to the IQ domains of myosin XIs. CaM, CML13, and CML14 co-localized to microtubules when co-expressed with MAP65-1-myosin fusion proteins containing the IQ domains of myosin XIs. In addition, in vitro actin motility assays demonstrated that CML13, CML14, and CaM function as myosin XI light chains. A cml13 T-DNA mutant exhibited a shortened primary root phenotype that was complemented by the wild-type CML13 and was similar to that observed in a triple myosin XI mutant (xi-1,2,k). Overall, our data indicate that Arabidopsis CML13 and CML14 are novel myosin XI light chains that likely participate in various myosin XI functions.