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Cell Motility and the Cytoskeleton 39:195–200 (1998)
Views and Reviews
Yeast Myosin II: A New Subclass of
Unconventional Conventional Myosins?
Karen M. May, Thein Z. Win, and Jeremy S. Hyams*
Department of Biology, University College, London, UK
Myosin II is the founder member of a large and structurally diverse clan of
actin-based motor proteins. The native myosin II molecule is a hexamer consisting
of two heavy chains, two essential light chains (ELC), and two regulatory light
chains (RLC). For convenience, the myosin IIs are often subdivided into four
subclasses: vertebrate skeletal and cardiac muscle myosin II form one subclass,
vertebrate smooth muscle and nonmuscle myosin II a second, invertebrate muscle
a third, and protozoan myosin II a fourth [Sellers and Goodson, 1995]. Different
mechanisms of regulation may exist between myosins within a single subclass yet
all myosin IIs share a common three-domain structure; the N-terminus of the heavy
chain forms two globular heads that contain the ATP- and actin-binding sites and
the a-helical neck region that is stabilised by the binding of the two classes of light
chains, whilst the C-terminus forms an extended coiled-coil tail that can consist of
anywhere between 700 and 1,200 amino acids. In nonmuscle cells, myosin II has at
least two well-defined functions, cell locomotion and cytokinesis. Yeast cells do
not locomote, and their mechanism of cytokinesis involves the deposition of a
cross-wall or septum. However, in the fission yeast, Schizosaccharomyces pombe,
deposition of the septum is anticipated by the appearance of a contractile
actomyosin ring [Marks and Hyams, 1985; May et al., 1997; Kitayama et al., 1997]
and actin is also present at the bud neck during cytokinesis in the budding yeast,
Saccharomyces cerevisiae [Kilmartin and Adams, 1984]. Here we report a
phylogenetic analysis of the N-terminal head domains of the myosin IIs from both
yeasts, a structural analysis of the tail domains of these proteins and we speculate
as to the nature of the light chains that regulate their function. On the basis of these
findings, we propose that the yeast myosin IIs constitute a divergent fifth class of
‘‘unconventional’’ conventional myosins. Cell Motil. Cytoskeleton 39:195–200,
1998. r 1998 Wiley-Liss, Inc.
INTRODUCTION
Recent years have seen a rapid expansion of the
myosin superfamily. Based on sequence homology within
the head or motor domain, at least 13 myosin classes are
now recognised [Mooseker and Cheney, 1995; Cope et
al., 1996]. Since members of individual classes are also
related by tail structure, it has been proposed that heads
and tails coevolved to perform a specific function [Cope
et al., 1996]. Most cells express at least two myosin
classes but only in the budding yeast, Saccharomyces
r 1998 Wiley-Liss, Inc.
cerevisiae, which expresses three, do we have a complete
catalogue of all the myosins expressed within a single
organism [Brown, 1997]. Whilst the cellular role of the
budding yeast myosin Is in the polarisation of the actin
cytoskeleton and in endocytosis [Goodson et al., 1996;
*Correspondence to: Jeremy S. Hyams, Department of Biology,
University College, Gower Street, London WC1E 6BT, UK
Received 4 December 1997; accepted 9 December 1997
May et al.
Figure 1.
196
Yeast Myosin II
Goodson and Spudich, 1995; Geli and Riezman, 1996]
and the myosin Vs [Johnston et al., 1991; Haarer et al.,
1994; Govindan et al., 1995] in a post-Golgi step in the
secretory pathway are relatively well established, myosin
II, encoded by the gene MYO1, has received surprisingly
little attention. Cells deleted for MYO1 are viable but
have defective cell wall organization at the mother-bud
neck and are consequently defective for cell division, or
at least cell separation [Watts et al., 1987; Rodriguez and
Paterson, 1990]. Analysis of the head domain of MYO1
places budding yeast on an early branch of the evolutionary history of myosin II with other simple eukaryotes
such as Acanthamoeba and Dictyostelium as its closest
relatives [Cheney et al., 1993; Mooseker and Cheney,
1995; Cope et al., 1996]. However, unusually, and in
contrast to the protozoan myosin IIs, the MYO1 tail
contains nine proline residues that might be expected to
affect its ability to form the typical extended coiled-coil
structure [Watts et al., 1987].
THEREBY HANGS A TAIL
Recently, we [May et al., 1997] and Kitayama et al.
[1997], independently isolated a myosin II from the
fission yeast, Schizosaccharomyces pombe, which we
designated myo21 [May et al., 1996]. myo21 is an
essential gene encoding a protein of 1526 amino acids.
Phylogenetic analysis identifies the closest relatives of
myo21 as MYO1 [1851 amino acids] and a second fission
yeast myosin II gene identified by the Sanger Centre
sequencing project, which we have designated myo221
(2104 amino acids; Fig. 1). Like MYO1, the tails of
both myo21 and myo221 contain numerous proline
residues, nine in the case of myo21 and 27 in myo221.
Analysis using the PairCoil program of Berger et al.
[1995] predicts that the tails of all three yeast myosin IIs
consist of numerous short segments of coiled-coil [May
et al., 1997] (Fig. 2a–c), rather than a single extended
structure, as seen in both the protozoan (Fig. 2d)
and vertebrate (Fig. 2e) myosin IIs. Thus, both the amino
Fig. 1. Phylogenetic analysis of the myosin superfamily. An unrooted
phylogenetic tree showing the major myosin classes. The four previously defined myosin II subclasses are within the large shaded
area (MII). myo21, myo221, and MYO1 (small shaded area) clearly
fall within the myosin IIs but are most similar to each other. The 42
myosin head sequences were aligned using the CLUSTALW software.
The tree was constructed using the NEIGHBOR joining method
from the PHYLIP suite of programs (Felsenstein, 1989). For accession numbers, see Cope et al. [1996]; GenBank accession numbers
for myo21 and myo221 are U75357 and Z98762 respectively. To
estimate the degree of confidence in branching, the tree was bootstrapped 100 times.
197
acid sequence of the head domains and the unique and
unusual structure of their tails appear to place these
yeast proteins in a distinct subclass of myosin II.
Despite this, the fission yeast protein at least appears to
function as a bona fide actin-based motor, most notably
as a component of the contractile cytokinetic actomyosin ring [May et al., 1997; Kitayama et al., 1997].
Cells deleted for myo221 are defective for cytokinesis
(T.Z. Win and J.S. Hyams, unpublished results), although
the precise function of this protein remains to be determined.
A LITTLE LIGHT ON LIGHT CHAINS
What do we know about the regulation of the yeast
myosin IIs? Both the MYO1 and myo21 sequences
contain IQ motifs that define the binding sites for both
essential and regulatory light chains [Houdusse and
Cohen, 1995; Watts et al., 1987; May et al., 1997]. In both
Drosophila [Karess et al., 1991] and Dictyostelium [Pollenz et al., 1992], mutations in the genes encoding either
light chain result in a defect in cytokinesis. Exhaustive
genetic screens for cytokinesis mutants in fission yeast
[Nurse et al., 1976; Chang et al., 1996] have identified
only a single light chain candidate. cdc41 encodes an EF
hand protein that shares features of both ELCs and RLCs,
but cannot be assigned unambiguously to either class
[McCollum et al., 1995]. As with the Dictyostelium and
Drosophila light chain mutants [see above], cdc42 mutants are defective for cytokinesis [Nurse et al., 1976] due
to a failure to assemble a functional actin ring [Marks et
al., 1987; McCollum et al., 1995]. The Cdc4 protein
colocalises with myosin II in the contractile ring [McCollum et al., 1995]. A search of the budding yeast genome
also failed to identify conventional ELC or RLC genes
(K.M. May and J.S. Hyams, unpublished results) [Wang
et al., 1997], although it did reveal an open reading frame
sharing 42% identity with cdc41 (Fig. 3), designated
MLC1 (R.C. Stephens and T.N. Davis, 1997, unpublished
observations, cited in the Saccharomyces Genetic Database). Like cdc41, MLC1 encodes an EF-hand protein
with four putative Ca21-binding loops and, also like
cdc41, it is an essential gene. Phylogenetic analysis
places both proteins together, with the protozoan ELCs as
the closest relative. However, the bootstrap values for the
branching of the yeast light chains are too low to ascribe
these proteins to any of the three classes of EF hand
protein with any confidence (Fig. 4). A detailed characterisation of MLC1 is now essential to clarify whether the
yeast myosin IIs are not only structurally distinct from
other members of this myosin class but are also regulated
by a unique class of light chains. The recent demonstra-
198
May et al.
Fig. 2. Coiled-coil predictions for myosin II sequences. The ability of
S. pombe myo21 (A), myo221 (B), and S cerevisiae MYO1 (C) to form
coiled-coils was predicted using the PairCoil program of Berger et al.
(1995). The horizontal axis shows amino acid number from the
N-terminus. Unlike Acanthamoeba (D) and human embryonic skeletal
(ESk) muscle (E) myosin IIs, the tails of which (roughly from amino
acid 800 to the C-terminus) consist of an almost unbroken stretch of
coiled-coil, the yeast myosin IIs form only short segments of coiledcoil. The breaks coincide with the positions of the proline residues in
the yeast and Acanthamoeba sequences.
Yeast Myosin II
199
UNITED BY DIVISION
Fig. 3. Alignment of the putative yeast myosin light chains. Both
Cdc41 and Mlc1 have four potential Ca21-binding loops (doubleheaded arrow), although, unlike calmodulin, three of these (I, II, and
IV) can be inferred not to bind Ca21.
tion that calmodulin colocalises with myosin II to the
cleavage plane during cytokinesis in fission yeast may
also prove to be relevant to these speculations [Moser et
al., 1997].
The sequencing of myosin genes has already revealed considerable diversity amongst the myosin superfamily [Mooseker and Cheney, 1995; Cope et al., 1996].
Although still relatively superficially characterised, the
yeast myosin IIs appear to offer a new twist in the tale.
For the present, the biological significance of the unique
structure of the yeast myosin II tails and their unusual
light chains remains unknown. In fission yeast, an
‘‘unconventional’’ conventional myosin can participate in
a cytokinetic mechanism that, superficially at least,
resembles the equivalent process in animal cells. The fact
that actomyosin-based cytokinesis appeared early in the
evolution of eukaryotic cells may suggest that at least
some of the fission yeast genes that regulate this process
[Fankhauser and Simanis, 1994] may show a similar
degree of conservation.
Fig. 4. Phylogenetic analysis of the putative yeast myosin light chains. An unrooted phylogenetic tree showing the
three families of EF-hand calcium modulated proteins: essential light chain (ELC), regulatory light chain (RLC),
and calmodulin. Cdc4 and Mlc1 have features of both essential and regulatory light chains.
200
May et al.
NOTE ADDED IN PROOF
The gene referred to here as myo221 has been
reported as myp21 by Bezanilla et al. (Mol. Biol. Cell
8:2693–2705) and as myo31 by Motegi et al. (FEBS
Letts., in press).
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