Multiplying myosins

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Abstract
Myosins are a diverse family of actin-based molecular motors that appeared early in eukaryotic evolution. Just how early, and how diverse, has begun to become clear from work that appears in this issue of PNAS (1) and recent work from Nature (2). For most of its existence, the term “myosin” applied only to the actin-activated ATPase that forms the bipolar thick filaments of muscle and the cytokinetic furrow. This biochemical definition has given way to a bioinformatic one based on presence of a canonical 80-kDa motor domain related to that of other myosins. Although most myosins have the same general body plan (an N-terminal motor domain, a “neck” consisting of varying numbers of IQ motifs, and a C-terminal tail involved in protein–protein interactions), the nonmotor regions vary enormously. This structural diversity reflects functional diversity (3). Humans have at least 11 different classes of myosin, each with a clearly distinguishable motor domain and a unique constellation of other sequence motifs (3, 4) Other organisms have their own complement of myosins, some similar to those in humans, but many are unique or found in only small groups of species (1, 2, 4). How does one begin to make sense of such complexity? The now standard approach has been to perform a “phylogenetic analysis,” basically, to determine the evolutionary relationships between members of a gene family on the basis of amino acid sequence similarities (homology) between conserved regions (5, 6). The derived relationships are presented visually as a “phylogenetic tree” [see figure 1 of ref. 1]. The evolutionary history reflected in these trees contains a wealth of information. Most practically, it provides a logical structure for classifying and naming the proteins. However, these trees also contain functional information: orthologs (homologs separated by species divergence) are predicted to …

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