Fig. 2: MCA-IIs associate with the ER.

a Gene ontology term (GO) enrichment analysis of biological processes for proteins more abundant in 6 month-old seeds of mca-II-KOc compared with WT (log₂FC ≥ 1). The black dotted box highlights relevant overrepresented and overlapping GO terms. The GO term “fatty acid metabolic process” (GO:0006631) was confirmed by the greatly diminished levels of oleosin and enlarged lipid droplets in the mutant (Fig. 5b). Furthermore, the GO terms “protein folding” (GO:0006457) and “ER body organization” (GO:00080119) were enriched and included proteins such as HEAT SHOCK 70 kDa proteins (HSP70s) and protein disulfide-isomerases (PDI5, PDI6: 3,11- and 5.0-fold, respectively), BiP 1 and 2 (3.4- and 2.6-fold, respectively). In the GO term “protein folding”, the FKBP15-2 (FK506- AND RAPAMYCIN-BINDING PROTEIN 15 KD-2, immunophilin protein) was not found in WT, it is involved in ER stress sensing and accelerates protein folding. Furthermore, in the same GO term, the proteasomal subunits (GO, protein folding) such as PAG1 (20 S proteasome alpha subunit G-1: 1.3-fold) and PBA1 (20 S proteasome subunit beta 1: not found in WT) were verified using α-PAG1 and α-PBA1 antibodies in Supplementary Fig. 11g, confirming high increase of PAG1 and a smaller one for PBA1. In the term “ER body organization”, PYK10 (BGLU23), BGLU18, and BGLU25 (highlighted), β-glucosidases are enriched ≥ 100-fold in mca-II-KOc. The GO term “gluconeogenesis” (GO:0006094) confirms the relevance of this analysis, as MCA-IIs were previously linked to this process⁴⁹. FDR, false discovery rate. b Confocal images of root embryonic cells from Arabidopsis seedlings co-expressing MCA-II-apro:MCA-II-a-GFP (upper) or MCA-II-fpro:MCA-II-f-GFP (lower; elongated cells from roots also shown were due to lower levels of GFP signal the ER is better visualized) with 35Spro:HDEL-CFP or from MCA-II-bpro:MCA-II-b without HDEL-CFP (middle). The yellow arrowhead in MCA-II-apro:MCA-II-a-GFP case denotes an ER sheet at which MCA-II-a is not localizing. Scale bars, 5 μm. The intensity plot profile of the GFP/CFP signal corresponds to intensities calculated at an ER sheet for MCa-II-a-GFP/CFP-HDEL (the region used for the plot profiling is shown in the “merged”). AUs, arbitrary units. Source data are provided as a Source Data file.