The Drosophila anterior-posterior axis is polarized by asymmetric myosin activation

Vitaly Zimyanin,Hélène Doerflinger,Daniel St Johnston

doi:10.1016/j.cub.2021.11.024

Vitaly Zimyanin, Hélène Doerflinger + Show 1 more

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https://doi.org/10.1016/j.cub.2021.11.024

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Journal: Current Biology	Publication Date: Dec 1, 2021
Citations: 16	License type: cc-by

Affiliation: University of Cambridge

Abstract

SummaryThe Drosophila anterior-posterior axis is specified at mid-oogenesis when the Par-1 kinase is recruited to the posterior cortex of the oocyte, where it polarizes the microtubule cytoskeleton to define where the axis determinants, bicoid and oskar mRNAs, localize. This polarity is established in response to an unknown signal from the follicle cells, but how this occurs is unclear. Here we show that the myosin chaperone Unc-45 and non-muscle myosin II (MyoII) are required upstream of Par-1 in polarity establishment. Furthermore, the myosin regulatory light chain (MRLC) is di-phosphorylated at the oocyte posterior in response to the follicle cell signal, inducing longer pulses of myosin contractility at the posterior that may increase cortical tension. Overexpression of MRLC-T21A that cannot be di-phosphorylated or treatment with the myosin light-chain kinase inhibitor ML-7 abolishes Par-1 localization, indicating that the posterior of MRLC di-phosphorylation is essential for both polarity establishment and maintenance. Thus, asymmetric myosin activation polarizes the anterior-posterior axis by recruiting and maintaining Par-1 at the posterior cortex. This raises an intriguing parallel with anterior-posterior axis formation in C. elegans, where MyoII also acts upstream of the PAR proteins to establish polarity, but to localize the anterior PAR proteins rather than Par-1.

Highlights

In many organisms, the primary body axis is defined by the polarization of the egg or zygote, generating cellular asymmetries that lead to the localization and segregation of cytoplasmic determinants
Almost nothing is known about how this PAR protein asymmetry is established, except that the ubiquitin ligase Slimb is necessary for the posterior recruitment of Par-1.26 Here we report that polarity signaling induces the specific activation of non-muscle myosin II (MyoII) at the posterior of the oocyte and show that this acts upstream of Slimb in the recruitment of Par-1, making it the first sign of polarity establishment identified to date
To test whether myosin regulatory light chain (MRLC) di-phosphorylation depends on the polarizing signal from the follicle cells, we examined MRLC-2P

Summary

Introduction

The primary body axis is defined by the polarization of the egg or zygote, generating cellular asymmetries that lead to the localization and segregation of cytoplasmic determinants. Polarity establishment starts when Aurora A associated with the sperm centrosome inhibits myosin activity at the posterior cortex to trigger a contraction of cortical actomyosin toward the anterior.[2,3,4] The anterior polarity proteins PAR-3, PAR-6, and aPKC, which are initially localized all around the egg membrane, are carried to the anterior by this cortical flow, allowing the posterior polarity factors PAR-2, PAR-1, and Lgl to localize to the posterior cortex.[5,6,7] After this establishment phase, polarity is maintained by mutual antagonism between the anterior and posterior PAR proteins.[8,9,10] The localized PAR proteins control spindle orientation and the asymmetric localization of determinants to drive an asymmetric first cell division to produce a large anterior AB cell and a smaller posterior P cell

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