COPII Sec23 proteins form isoform-specific ER exit sites with differential effects on polarized growth

Abstract

30COPII, a coat of proteins that form vesicles on the ER, mediates vesicle 31 traffic from the ER to the Golgi. In contrast to metazoans that have few genes 32 encoding each COPII component, plants have expanded these gene families 33 leading to the hypothesis that plant COPII has functionally diversified. Here, we 34 analyzed the gene families encoding for the Sec23/24 heterodimer in the moss 35 Physcomitrium (Physcomitrella) patens. In P. patens, Sec23 and Sec24 gene 36 families are each comprised of seven genes. We discovered that Sec23D is 37 functionally redundant with Sec23E and together they are essential for 38 protonemal development. Sec23D is critical for secretion to the Golgi and the 39 plasma membrane in tip growing protonemata. Sec23D preferentially associates 40 with Sec24C/D, also essential for protonemata. In contrast, the remaining five 41

SABRE populates ER domains essential for cell plate maturation and cell expansion influencing cell and tissue patterning

“…However, in Δsabre , both the actin and microtubule cytoskeletons were not significantly altered, suggesting that SABRE’s impact on cell expansion is independent of the cytoskeleton. Of note, a recent study demonstrated that protonemata lacking COPII function, which mediates ER to Golgi transport, exhibited aggregated ER and polarized growth defects (Chang et al, 2020), suggesting that SABRE’s influence on the ER might alter ER secretory function. In contrast to defects in COPII function, which generally reduces secretion, Δsabre defects appear to specifically affect a subset of secretory cargo, given that delivery of the plasma membrane protein SNAP-TM-mCherry was unaffected in Δsabre but affected in COPII mutants (Chang et al, 2020).…”

“…Of note, a recent study demonstrated that protonemata lacking COPII function, which mediates ER to Golgi transport, exhibited aggregated ER and polarized growth defects (Chang et al, 2020), suggesting that SABRE’s influence on the ER might alter ER secretory function. In contrast to defects in COPII function, which generally reduces secretion, Δsabre defects appear to specifically affect a subset of secretory cargo, given that delivery of the plasma membrane protein SNAP-TM-mCherry was unaffected in Δsabre but affected in COPII mutants (Chang et al, 2020). In a surprising connection, a study in Drosophila discovered hobbit, a protein that the authors report is conserved broadly across eukaryotes (Neuman and Bashirullah, 2018).…”

“…To introduce ER labeling, we used CRISPR mediated HDR, to swap lifeact-mRuby into mCherry-KDEL (Supplementary Table 2). All other fluorescent labeled lines are as described before: GFP-tubulin (Wu and Bezanilla, 2018), Lifeact-GFP (van Gisbergen et al, 2012), SP-GFP-KDEL (Chang et al, 2020), NLS-GFP-GUS/SNAP-TM-mCherry (van Gisbergen et al, 2018, p. 10), mCherry-tubulin (Burkart et al, 2015), Lifeact-mRuby (van Gisbergen et al, 2020).…”

“…,Lifeact- GFP (van Gisbergen et al, 2012), SP-GFP-KDEL(Chang et al, 2020), NLS-GFP-GUS/SNAP-TM-mCherry(van Gisbergen et al, 2018, p. 10), mCherry-tubulin(Burkart et al, 2015),Lifeact- mRuby (van Gisbergen et al, 2020).…”

SABRE populates ER domains essential for cell plate maturation and cell expansion influencing cell and tissue patterning

“…However, in ∆sabre , both the actin and microtubule cytoskeletons were not affected, suggesting that SABRE’s impact on cell expansion is independent of the cytoskeleton. Of note, a recent study demonstrated that protonemata with impaired COPII function, which mediates ER to Golgi transport, exhibited aggregated ER and polarized growth defects ( Chang et al, 2020 ), suggesting that SABRE’s influence on the ER might alter ER secretory function. In contrast to defects in COPII function, which generally reduces secretion, ∆sabre defects appear to specifically influence a subset of secretory cargo.…”