Function, localization and evolution of SOSEKI polar proteins

Maritza van Dop

Research output: Thesisinternal PhD, WU

Abstract

The evolution of multi-cellular plants went hand in hand with the establishment of a complex polarity system to guide development and survival. Within the cell, polarity cues need to be established, read and translated into sub-cellular processes. Yet, the exact mechanisms that translate polarity into sub-cellular processes remain elusive. In Chapter 1, we discuss polarity and several proteins that use polar information to guide their localization.  The Arabidopsis embryo is introduced as an excellent model for studying cell polarity.

In Chapter 2, we take a closer look at development of the Arabidopsis embryo. Hereby we focus specifically on how oriented divisions are generated by developmental regulators and the division machinery. Recent advancement in 3D imaging of the embryo revealed that cell division abides to a ‘smallest plane’ rule, and that auxin can prevent adherence to this rule.  Studying how auxin effectors are linked to cell division regulators and cell polarity may provide a greater understanding of oriented cell division in the embryo.

Using the Arabidopsis embryo as model for auxin-regulated development, we identify a novel family of polarly localized proteins in Chapter 3. Unlike previously published polar proteins, this new family shows a robust localization to specific cell edges, which coined the name SOSEKI (SOK, Japanese for cornerstone). SOK localization is guided by integration of plant-wide apico-basal and radial polarity. Pharmacological inhibition of pathways commonly used by polarly localized proteins showed that SOK is localized through a novel mechanism. Mis-expression of SOK1 caused oblique cell divisions and polar localization was required for this activity. We identified a highly conserved N-terminal domain that structurally resembles the DIX domain found in Wnt polarity signalling proteins in animals (Ehebauer & Arias, 2009; Schwarz-Romond et al., 2007). In animals, this domain shows autocatalytic polymerization. SOK1 DIX-LIKE can dimerize and is required for polar edge clustering and biological activity, which shows that the fundamental function of DIX is conserved. Taken together, this chapter revealed a  compass of polar axes that guides SOK polar edge localization. In addition, we showed that both plants and animals use the DIX domain in the context of polarity. 

SOK showed striking localization and behavior, but nothing was known about the function of this protein family. In Chapter 4, we studied SOK function by generating sok mutants. We found that small mutations near the N-terminal end of SOK1 sometimes caused fertility defects, but that larger deletions had no effect. The sok1 deletion mutant showed upregulation of the SOK4 gene, which suggests that there may be a compensation mechanism or feedback loop. The potential redundancy between SOK1 and SOK4 led to further investigation of SOK expression and localization throughout the plant. Based on our findings, SOK2 and 3 may be redundant in the leaf, while SOK2, 3 and 5 overlap in the gynoecium.

As SOK was a completely novel protein family with unknown origin, we aimed to learn more about its evolutionary history. Therefore we investigated the protein sequence, properties and polar localization throughout plant evolution in Chapter 5. We showed that SOK first arose in early land plants, and that they contain several conserved domains that separate SOKs in an ancestral and a more recently evolved type. To assess the conservation of polarity, we studied four SOKs in the moss Physcomitrella patens. One of these tested PpSOKs showed polar edge accumulation in the gametophore, which suggests that edge polarity of SOK proteins is conserved throughout evolution. Next we performed phylogenetic and functional analysis on the DIX domain, which is the most highly conserved domain of SOK. Our results revealed that DIX is present in land plants, animals and the SAR group, and that it is capable of polymerization in all these clades.

The molecular context of a protein can reveal how it functions within the cell and how it obtains its localization. To address these questions in Chapter 6, we combined biochemistry and cell biology and identified shared and unique interactors of SOK1, SOK2 and SOK3. At least one of these interactors was recruited to the polar SOK1 site in a DIX-LIKE-dependent manner. We extended the network of interaction partners and found that SOK1 interacts with a network of laterally-polar proteins. The secondary interactors revealed links with amongst others the cytoskeleton. Based on these findings, we propose that DIX-like-mediated polymerization creates a polar scaffold that recruits interactors for local tasks. Such tasks may be modification of the cytoskeleton during cell growth or mechanical stress.

To conclude this thesis, the context and implications of our results were discussed in Chapter 7. In this discussion, we also provide an outlook for the future and suggestions for application of our results in research and biotechnology.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Weijers, Dolf, Promotor
Award date7 Sep 2018
Place of PublicationWageningen
Publisher
Print ISBNs9789463434799
DOIs
Publication statusPublished - 2018

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