Makorin 1 controls embryonic patterning by alleviating Bruno1-mediated repression of oskar translation

Abstract

Makorins are evolutionary conserved proteins that contain C 3 H-type zinc finger modules and a RING E3 ubiquitin ligase domain. In Drosophila, maternal Makorin 1 (Mkrn1) has been linked to embryonic patterning but the mechanism remained unsolved. Here, we show that Mkrn1 is essential for axis specification and pole plasm assembly by translational activation of oskar (osk). We demonstrate that Mkrn1 interacts with poly(A) binding protein (pAbp) and binds specifically to osk 3' UTR in a region adjacent to A-rich sequences. Using Drosophila S2R+ cultured cells we show that this binding site overlaps with a Bruno1 (Bru1) responsive element (BREs) that regulates osk translation. We observe increased association of the translational repressor Bru1 with osk mRNA upon depletion of Mkrn1, indicating that both proteins compete for osk binding. Consistently, reducing Bru1 dosage partially rescues viability and Osk protein level in ovaries from Mkrn1 females. We conclude that Mkrn1 controls embryonic patterning and germ cell formation by specifically activating osk translation, most likely by competing with Bru1 to bind to osk 3' UTR.

MKRN2 Physically Interacts with GLE1 to Regulate mRNA Export and Zebrafish Retinal Development

“…For instance, the AKT2 gene product exhibited an increased accumulation of MKRN2 iCLIP reads within the 3 0 UTR relative to other transcript regions (Figure 5E). Hence, our iCLIP analysis indicates that MKRN2 binds to mRNAs primarily in the 3 0 UTR, consistent with recent iCLIP analyses of another Makorin family member, MKRN1, in human cells (Hildebrandt et al, 2019) and Drosophila (Dold et al, 2020).…”

“…Next, we specifically examined the interactions of the Makorin family of E3-RBPs. Previous studies have identified physical binding between many Makorin family members and poly(A) binding proteins (PABPs) (Cassar et al, 2015;Dold et al, 2020;Hildebrandt et al, 2017Hildebrandt et al, , 2019Miroci et al, 2012;Yellapragada et al, 2019), which are involved in diverse RNA-associated activities (Mangus et al, 2003). Consistent with this previous work, we similarly observed high-confidence interactions between all human Makorin family members and PABPs (Figure S1B).…”

“…MKRN2 is one of at least 26 putative human E3-RBPs that have been shown or predicted to bind RNA in human cells (Hildebrandt et al, 2017). While some have been linked to RNA processing, such as RC3H1 in mRNA turnover (Leppek et al, 2013) and MKRN1 in the regulation of translation (Dold et al, 2020;Hildebrandt et al, 2019;Miroci et al, 2012), many E3-RBPs lack known RNA-associated interaction partners or functions in RNA processing.…”

Patterning the Drosophila embryo: A paradigm for RNA‐based developmental genetic regulation

“…One aspect of osk translational activation is inhibition of Bru1 (Kim et al, 2015), and one BRE-containing region in the distal part of the osk 3 0 UTR (BRE-C) functions in activation as well as in repression (Reveal et al, 2010). A recent study implicated the RNA binding protein Makorin-1 (Mkrn1) as a key activator of osk translation (Dold et al, 2020). Mkrn1 binds the osk 3 0 UTR at an A-rich region immediately adjacent to BRE-C, where it antagonizes Bru1 binding and promotes recruitment of poly(A) binding protein.…”

MKRN3-mediated ubiquitination of Poly(A)-binding proteins modulates the stability and translation of GNRH1 mRNA in mammalian puberty

“…During energy starvation, SIRT1-mediated deacetylation of PABPC1 was found to cause nuclear retention of both PABPC1 protein and mRNA ( 25 ). Most recently, PABPC1 was found to be ubiquitinated by RNA-bound MKRN1, which controlled embryonic patterning and germ cell formation ( 26 , 27 ). It is both important and interesting to ask: (i) what are the tissue-specific functions of PABPs?…”