Giant proteins in a giant cell: Molecular basis of ultrafast Ca ²⁺ -dependent cell contraction

Abstract

The giant single-celled eukaryote, Spirostomum , exhibits one of the fastest movements in the biological world. This ultrafast contraction is dependent on Ca 2+ rather than ATP and therefore differs to the actin-myosin system in muscle. We obtained the high-quality genome of Spirostomum minus from which we identified the key molecular components of its contractile apparatus, including two major Ca 2+ binding proteins (Spasmin 1 and 2) and two giant proteins (GSBP1 and GSBP2), which act as the backbone and allow for the binding of hundreds of spasmins. The evidence suggests that the GSBP-spasmin protein complex is the functional unit of the mesh-like contractile fibrillar system, which, coupled with various other subcellular structures, provides the mechanism for repetitive ultrafast cell contraction and extension. These findings improve our understanding of the Ca 2+ -dependent ultrafast movement and provide a blueprint for future biomimicry, design, and construction of this kind of micromachine.

Application of RNA interference and protein localization to investigate housekeeping and developmentally regulated genes in the emerging model protozoan Paramecium caudatum

“…Here, we aimed to expand this list by adapting common gene manipulation techniques for use in Paramecium caudatum , to provide researchers with the tools required for further study of gene function in this organism. We adapted the method of gene knockdown by RNA interference first, since this method has become one of the most widely used tools for loss-of-function studies in ciliates and has been well-established in several species including Paramecium tetraurelia and Paramecium bursaria 33 , 53 , 54 , Euplotes 55 , Oxytricha 50 , Stylonychia 56 , Stentor 57 , 58 and Spirostomum 59 . We also adapted the method of microinjection as a means to transform cells with fusion constructs for protein localization studies.…”

Topological damping in an ultrafast giant cell

“…An actin-like protein is found only localized around the macronucleus and along the membranelles, not throughout the cytoplasm ( 67 ). Microtubules are only localized in the longitudinal grooves of the cortex ( 9 , 68 ). Other major structural proteins, such as centrins, spasmins, GSBP1, and GSBP2, are the major players in calcium-triggered ultrafast contraction in Spirostomum but are all localized in the cortex as well ( 68 ).…”

“…Microtubules are only localized in the longitudinal grooves of the cortex ( 9 , 68 ). Other major structural proteins, such as centrins, spasmins, GSBP1, and GSBP2, are the major players in calcium-triggered ultrafast contraction in Spirostomum but are all localized in the cortex as well ( 68 ). None of these are entangled with the ER or vacuolar meshwork in Spirostomum .…”

A unified model for the dynamics of ATP-independent ultrafast contraction

“…Moreover, we examine a model extension in which motion encounters resistance from auxiliary elastic elements with unchanged rest lengths during Ca 2+ binding. This representation approximates, for instance, the microtubule sheath under the cortex of Spirostomum ( 11 , 36 ) or the elastic stalk supporting the thin myoneme fiber in Vorticella ( 49 ). We show that a linear change of variables effectively maps the model with auxiliary elastic elements to the model presented here.…”

“…However, the fastest motions currently known to occur in the biological world (measured as the proportional change in length per unit time) are due not to actomyosin assemblies but to lesser-known protein assemblies called myonemes ( 2 – 4 ). We use “myonemes” in the broadest sense ( 5 ), encompassing terms used for specific organisms such as “spasmonemes” and “M-bands.” Myonemes are common in ciliated protists: For example, they are found in the wine glass-shaped genus Vorticella ( 6 , 7 ), the cigar-shaped genus Spirostomum ( 8 – 11 ), the trumpet-shaped genus Stentor ( 12 , 13 ), the tear drop-shaped genus Lacrymaria ( 14 ), and the chandelier-shaped genus Zoothamnium ( 15 , 16 ). Similar Ca 2+ -responsive supramolecular assemblies are proposed to occur in a range of other single-celled organisms ( 17 – 24 ), and possibly even plants ( 25 – 27 ).…”