MY01, a class XIV myosin, affects developmentally-regulated elimination of the macronucleus during conjugation of Tetrahymena thermophila

Identification, modeling, and characterization studies of Tetrahymena thermophila myosin FERM domains suggests a conserved core fold but functional differences

Myosins (MYO) define a superfamily of motor proteins which facilitate movement along cytoskeletal actin filaments in an ATP-dependent manner. To date, over 30 classes of myosin have been defined that vary in their roles and distribution across different taxa. The multidomain tail of myosin is responsible for the observed functional differences in different myosin classes facilitating differential binding to different cargos. One domain found in this region, the FERM domain, is found in several diverse proteins and is involved in many biological functions ranging from cell adhesion and actin-driven cytoskeleton assembly to cell signaling. Recently, new classes of unconventional myosin have been identified in Tetrahymena thermophila. In this study, we have identified, modeled, and characterized eight FERM domains from the unconventional T. thermophila myosins as their complete functional MyTH4-FERM cassettes. Our results reveal notable sequence, structural, and electrostatic differences between T. thermophila and other characterized FERM domains. Specifically, T. thermophila FERM domains contain helical inserts or extensions, which contribute to significant differences in surface electrostatic profiles of T. thermophila myosin FERMs when compared to the conventional FERM domains. Analyses of the modeled domains reveal differences in key functional residues as well as phosphoinositide-binding signatures and affinities. The work presented here broadens the scope of our understanding of myosin classes and their inherent functions, and provides a platform for experimentalists to design rational experimental studies to test the functional roles for T. thermophila myosins.

Unique sequences and predicted functions of myosins in Tetrahymena thermophila

Myosins are eukaryotic actin-dependent molecular motors that play important roles in many cellular events. The function of each myosin is determined by a variety of functional domains in its tail region. In some major model organisms, the functions and properties of myosins have been investigated based on their amino acid sequences. However, in protists, myosins have been little studied beyond the level of genome sequences. We therefore investigated the mRNA expression levels and amino acid sequences of 13 myosin genes in the ciliate Tetrahymena thermophila. This study is an overview of myosins in T. thermophila, which has no typical myosins, such as class I, II, or V myosins. We showed that all 13 myosins were expressed in vegetative cells. Furthermore, these myosins could be divided into 3 subclasses based on four functional domains in their tail regions. Subclass 1 comprised of 8 myosins has both MyTH4 and FERM domains, and has a potential to function in vesicle transport or anchoring between membrane and actin filaments. Subclass 2 comprised of 4 myosins has RCC1 (regulator of chromosome condensation 1) domains, which are found only in some protists, and may have unconventional features. Subclass 3 is comprised of one myosin, which has a long coiled-coil domain like class II myosin. In addition, phylogenetic analysis on the basis of motor domains showed that T. thermophila myosins are separated into two clusters: one consists of subclasses 1 and 2, and the other consists of subclass 3.

A FERM domain in a class XIV myosin interacts with actin and tubulin and localizes to the cytoskeleton, phagosomes, and nucleus in Tetrahymena thermophila

Previous studies have shown that Myo1(myosin class XIV) localizes to the cytoskeleton and is involved in amitosis of the macronucleus and trafficking of phagosomes. Myo1 contains a FERM domain that could be a site for interaction between Myo1 and the cytoskeleton. Here, we explore the function of FERM by investigating its cytoskeleton binding partners and involvement in localization of Myo1. Alignment of Myo1 FERM with a talin actin-binding sequence, a MAP-2 tubulin-binding sequence, the radixin FERM dimerization motif, and the SV40 nuclear localization sequence (NLS) revealed putative actin- and tubulin-binding sequences, a putative FERM dimerization motif, and NLS-like sequences in both the N-terminal and C-terminal regions of Myo1 FERM. Alignment of Myo1 with an ERM C-terminal motif revealed a similar sequence in the Myo1 motor domain. GFP-FERM and two truncated FERM domains were separately expressed in Tetrahymena. GFP-FERM contained the entire Myo1 FERM. Truncated Myo1 FERM domains contained either the N-terminal or the C-terminal region of FERM and one putative sequence for actin-binding, one for tubulin-binding, a putative dimerization motif, and a NLS-like sequence. Actin antibody coprecipitated GFP-fusion polypeptides and tubulin from lysate of cells expressing GFP-fusions. Cosedimentation assays performed with either whole cell extracts or anti-actin immunoprecipitation pellets revealed that F-actin (independent of ATP) and microtubules cosedimented with GFP-fusion polypeptides. GFP-FERM localized to the cytoskeleton, phagosomes, and nucleus. Truncated GFP-FERM domains localized to phagosomes but not to the cytoskeleton or nucleus.