A calmodulin-related light chain from fission yeast that functions with myosin-I and PI 4-kinase

Abstract

SummaryFission yeast myosin-I (Myo1p) not only associates with calmodulin, but also employs a second light chain called Cam2p. cam2 cells exhibit defects in cell polarity and growth consistent with a loss of Myo1p function. Loss of Cam2p leads to a reduction in Myo1p levels at endocytic patches and a 50% drop in the rates of Myo1p-driven actin filament motility. Thus, Cam2p plays a significant role in Myo1p function. However, further studies indicated the existence of an additional Cam2p-binding partner. Cam2p was still present at cortical patches in myo1 cells (or in myo1-IQ2 mutants, which lack an intact Cam2p-binding motif), whereas a cam2 null (cam2) suppressed cytokinesis defects of an essential light chain (ELC) mutant known to be impaired in binding to PI 4-kinase (Pik1p). Binding studies revealed that Cam2p and the ELC compete for Pik1p. Cortical localization of Cam2p in the myo1 background relied on its association with Pik1p, whereas overexpression studies indicated that Cam2p, in turn, contributes to Pik1p function. The fact that the Myo1p-associated defects of a cam2 mutant are more potent than those of a myo1-IQ2 mutant suggests that myosin light chains can contribute to actomyosin function both directly and indirectly (via phospholipid synthesis at sites of polarized growth).

Contractile Ring Stability in S. pombe Depends on F-BAR Protein Cdc15p and Bgs1p Transport from the Golgi Complex

“…This is a unifying hypothesis, given the well-established role of F-BAR proteins such as Cdc15p in clathrin-mediated endocytosis (Arasada and Pollard, 2011; Berro et al, 2010; Qualmann and Kelly, 2000; Tsujita et al, 2006). This hypothesis is consistent with previous evidence that Bgs1p is transported through the secretory pathway, such as Brefeldin A inhibiting the concentration of GFP-Bgs1p in the cleavage furrow (Liu et al, 2002) and genetic interactions between mutations in cytokinesis genes and genes required for membrane traffic through the Golgi apparatus (Park et al, 2009; Sammons et al, 2011; Walch-Solimena and Novick, 1999). Similarly, mutations in conserved genes required for vesicular transport in the Golgi apparatus (De Matteis and Luini, 2008; Jaulin et al, 2007; Valente et al, 2012) cause septation defects in fission yeast (Brazer et al, 2000; Mishra et al, 2005).…”

Regulation and control of myosin-I by the motor and light chain-binding domains

“…The LCBD appears to play a role in the subcellular localization of Myo1c in some cell types [73], indicating that light chains may have a role in myosin-I targeting. Myosin-I isoforms from lower eukaryotes co-purify with calmodulin-like proteins (e.g., [74, 75]) rather than calmodulin. Additionally, vertebrate Myo1c has been shown to bind calcium-binding protein 1 (CaBP1) and calcium- and integrin-binding-protein-1 (CIB1) [76].…”

Human Myosin 1e tail but not motor domain replaces fission yeast Myo1 domains to support myosin-I function during endocytosis

“…Vertebrate Myo1e has a single IQ motif bound by calmodulin that inhibits myosin ATPase activity in the presence of Ca 2+ (24). S. pombe Myo1 has two IQ motifs, IQ1 bound by calmodulin Cam1 and IQ2 bound by the calmodulin-like light chain Cam2 (8,25). The IQ motif of HsMyo1e shares greater amino acid sequence similarity with the first IQ motif of SpMyo1 (38%) than with the second IQ motif (23%) ( Fig.…”

“…Cam2 binding to the lever arm of SpMyo1 has been found to maximize myosin motility in vitro (25). In the absence of Cam2, SpMyo1 still localizes to actin patches, but in the absence of SpMyo1 or when Cam2-IQ2 motif binding is otherwise disrupted, Cam2 moves to larger stationary puncta at the poles as well as mobile cytosolic puncta (25). To determine whether the chimeric proteins were capable of recruiting Cam2, we transformed the mGFP-tagged myosin constructs into a myo1Δ strain expressing Cam2 tagged with mCherry ( Fig.…”

“…Myo1 contains an additional region at the C-terminus of the tail, called the central-acidic domain, which can directly bind the Arp2/3 complex to initiate branched actin nucleation (7,(11)(12)(13)(14). While Myo1e contains a single light-chain binding motif that binds calmodulin (24), S. pombe Myo1 contains two IQ motifs, which bind two distinct light-chain proteins, calmodulin Cam1 and calmodulin-like Cam2 (8,25). Lastly, the motor domains of yeast and mammalian class I myosins differ at the TEDS site, a conserved amino acid residue located on a surface loop within the actin-binding site (26).…”