Functional conservation of germ plasm organizers Bucky ball in zebrafish and Drosophila Oskar

by Pritesh Krishnakumar
Doctoral thesis
Date of Examination:2017-12-13
Date of issue:2018-12-12
Advisor:Dr. Roland Dosch
Referee:Prof. Dr. Michael Kessel
Referee:Prof. Dr. Dirk Görlich
Persistent Address: http://dx.doi.org/10.53846/goediss-7188

 

 

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Description:Thesis: Functional conservation of germ plasm organizers Bucky ball in zebrafish and Drosophila Oskar

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Abstract

English

The reproductive cells of many animals are specified by a maternal determinant termed germ plasm. Cells inheriting germ plasm during embryogenesis will develop into primordial germ cells (PGCs), the precursors to eggs and sperm. Zebrafish specify their germ cells by deposition of germ plasm in the egg. To this end, only two proteins are known in the entire animal kingdom that program germ cell development in vivo: Oskar, which is specific to insects and Bucky ball (Buc), which our lab discovered in the zebrafish. Based on this unique activity, these two proteins are termed germ plasm organizers. In my study, I show that the two germ plasm organizers transform a somatic cell into PGCs in zebrafish. This result indicates that both proteins act through a conserved biochemical network to specify germ cell fate. A comparison of the amino acid sequence of Drosophila Oskar and zebrafish Bucky Ball, showed no conserved domains to explain their similar function. Buc is a novel protein, whose protein sequence shows no homology to other known functional domains and hence, its biochemical activity is currently unknown. My analysis of the biochemical interaction network of Bucky ball revealed that it interacts with similar germ plasm factors as Oskar. These results suggest that germ plasm organizers form a common core complex to specify germ cells. Nonetheless, Oskar and Bucky ball represent a deviation from the generally accepted sequence-structure-function paradigm. I therefore analyzed their biophysical properties and discovered that both encode intrinsically disordered proteins (IDPs). IDPs are known to quickly change their sequence during evolution, because their function is based on structural features. IDPs are known to assemble into hydrogels by liquid-liquid phase separation. Pharmacologically disrupting hydrogels fragmented Buc-complexes in zebrafish indicating their critical role during aggregation of germ plasm. These data suggest that Buc and Oskar exert their conserved role through similar biophysical and biochemical properties. It has been frequently postulated in the literature that unrelated protein sequences perform the same function. To my knowledge, my data represents the first example of two proteins which were previously classified as “novel” or “species-specific”, but are indeed functionally conserved across distant species.
Keywords: Bucky ball; zebrafish
 
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