Actin, algae and answers in search of a question: a perspective on emerging systems in biology Fayth Tan We’re fascinated with extraordinary biology—no standard lab animal regenerates like axolotls, or tolerates extreme environments like tardigrades. After nearly a hundred years of standardizing a small group of model organisms for the laboratory— mice, flies, baker’s yeast, E. coli— recent advancements in cutting-edge techniques now enable the study of a vastly wider diversity of organisms. But the exceptional abilities of “emerging systems” offer more than new, weird biology to explore—they may also be a conduit for new insights into old problems. By returning to familiar problems with greater breadth, we might find that we’ve underestimated how much we don’t know. The broader biological context represented by emerging systems catalyzes re-evaluations of entrenched assumptions, even about biology we think of as fundamental and generalizable. And while technology is a great enabler, there are an abundance of questions in understudied biology that might yield new insights with simple or well- established methods. With a fresh perspective, even the most familiar can be made strange again. Well-studied aspects of biology tend to be those obviously vital for life, and the protein actin certainly fulfills that expectation. Actin is present in the cell cytoskeleton in all domains of life, and is central to an array of essential functions including cell motility, shape, and division. It forms and breaks down filaments according to a cell’s needs, possessing a versatile combination of tensile strength and calibrated sensitivity. Its structure and sequence are highly conserved, especially among eukaryotes, appearing to be the poster protein for broad generalizability. Eukaryotes like yeast have a single actin, while eukaryotes with more specialized tissues have multiple isoforms, though they rarely vary by more than a few amino acids. Thus, it’d be reasonable to conclude that actin is evolutionarily constrained due to its indispensability, and that eukaryotic actins in particular are structurally and functionally alike. It’s a textbook biological narrative—neat, logical, and entirely ready to be complicated in frustrating and exciting ways. 1 The complication arose from a seemingly nondescript source: a mutant strain of the single-celled green algae, Chlamydomonas. It was, rather unexpectedly, found alive and well—despite the fact that it didn’t express conventional actin. Usually, this absence kills a cell. But the mutant’s survival is owed to an unusual feature of Chlamydomonas biology: it has not one, but two forms of actin. The first, a conventional actin, has 89% sequence similarity to mammalian actin. The second is a highly divergent actin called NAP (or “novel actin-like protein”) has only 64% similarity. NAP is expressed at negligible levels in wild type cells, but abundantly expressed when conventional actin is absent, fulfilling most of the functions usually carried out by conventional actin. Chlamydomonas is one of the rare organisms that has a single copy of each gene for both a conventional and divergent actin. Its dissimilar actins allow the cell to undergo genetic and biochemical perturbations to each actin while keeping the cell alive, enabling it to survive experimentation that would kill a mammalian cell. With the ability to individually target and manipulate two very different actins separately, actin’s roles in distinct cellular functions could now be teased apart. Nearly 20 years after its discovery, the Chlamydomonas actin mutant was an unlikely conduit to new discovery. Chlamydomonas is a popular system to study cilia—organelles involved in motility and extracellular sensing. Despite actin’s versatility, ciliary growth was one area in which it wasn’t thought to be involved. That role was primarily attributed to another cytoskeletal component, microtubules. From their composition, assembly and transport, microtubule dynamics wholly dominated the study of ciliary biology. Between the genetic and chemical perturbations established since the 1960s, and more recent microscopic and proteomic techniques, cilia appeared to be thoroughly studied. Previous work in the early 90s even addressed the question of actin involvement—when treated with a chemical that depolymerized actin, that Chlamydomonas cilia shortened. At the time, the phenotype was attributed to an “unknown target”, as actin filaments were not definitively detected in the cilia. In light of Chlamydomonas’s unusual actins, however, this conclusion warranted re- examination. Through the painstaking development and refinement of visualization methods, researchers illuminated the elusive actin filaments that evaded detection twenty years ago. What followed was the deft synthesis of observations and techniques made decades ago, yielding a previously hidden actin-based network for ciliary assembly in Chlamydomonas. It added an entirely new dimension to the field: everything microtubules were doing, actin was involved in as well. 2 From protein synthesis to the transport and incorporation of molecules into the cilia, actin was involved in many key steps of ciliary formation and regulation. This included functions usually attributed to microtubules, such as transporting proteins in vesicles from the Golgi body, or new functions in the ciliary gate, which selectively regulates the entry of molecules into the cilia. These mechanistic insights made in Chlamydomonas strongly informed future work in ciliary biology more generally: recent images showed that mammalian cilia contain actin filaments amidst microtubule structures – a surprise even to people who had spent their entire careers studying cilia. Saliently, both the biology and techniques necessary for this discovery weren’t new. The novelty lay in the insight of how they might be leveraged together. When two variations of a protein in a single green algae represent possibilities that might fundamentally alter our assumptions in multiple areas of biology, it’s clear that our understanding is limited. Modern biology rests on assumptions from about a century of work in a coterie of model systems, while trillions of organisms have been inventing evolutionary solutions for millions of years. Weird biology can be exciting in itself, but it also can be a means to an end. Returning to well-trodden work— the apparent dead ends, the seemingly unrelated problems, the intractable questions— with a broader perspective has the potential to unearth unexpected solutions. We haven’t solved these problems yet, but perhaps another organism, hidden amidst the dizzying biological breadth of life, already has. 3
Enter the password to open this PDF file:
-
-
-
-
-
-
-
-
-
-
-
-