A Role for Macro-ER-Phagy in ER Quality Control. (2015)
Fig 6

Immunoblot analysis of ER-resident proteins with and without overexpression of GFP-Snc1-PEM.

A. A diagram showing two groups of ER-resident proteins: those that become macro-ER-phagy cargos (in green), and those that do not (in blue or grey). Like overexpressed Snc1-PEM or Snq2, the ER-resident membrane proteins Sec61 (translocon subunit) and Hmg1 (sterol biogenesis) are transported to the vacuole via macro-ER-phagy. In contrast, the ER-to-Golgi exit regulators Sec12 and Sec13 and the ER lumen chaperone Kar2 are not co-transported to the lysosome through macro-ER-phagy. Four or six different strains were used for this analysis: WT (YPT1), pep4∆, and pep4 prb1∆, ypt1-1, ypt1-1 pep4∆, and ypt1-1 pep4∆ prb1∆. In each strain, one ER-resident protein was tagged at its C-terminus (except for Kar2). (The pep4∆ strains were not used in all experiments because they do not show the full defect). The level of ER-resident proteins was determined by immunoblot analysis in cells that either do not (B-D) or do overexpress GFP-Snc1-PEM (F-H). B-C. Sec61-3xHA (B) and Sec13-3xHA (C) expressing cells (2 independent un-transformed colonies) were tested by immunoblot analysis (using anti-HA antibodies). Shown from top to bottom: strain genotype, HA-tagged protein, G6PDH (loading control), quantification of HA-tagged protein expressed as average fold of WT (p-value = 0.025 for Sec61). D. Summary of immunoblot analyses of ER-resident proteins level in cells that do not overexpress GFP-Snc1-PEM: WT (PEP4 PRB1) vs pep4∆ prb1∆ (left), ypt1-1 PEP4 PRB1 vs ypt1-1 pep4∆ prb1∆ (right). Immunoblots and quantification are shown in Fig 6B–6C and S6A–S6C Fig. In YPT1 cells, the levels of Sec61 and Hmg1 (green) are slightly increased (14 and 38%, respectively) in strains defective in vacuolar proteolysis; the levels of Sec12, Sec13 and Kar2 are not increased (blue and grey). The levels of Sec61 and Hmg1 (green) are increased by 2-2.5 fold in ypt1-1 mutant cells when compared to WT cells, regardless if they are defective in vacuolar proteolysis or not. In contrast, the levels of Sec12, Sec13 and Kar2 are only slightly increased (blue and grey). E. The level of the ER-resident protein Sec61 is similar whether GFP-Snc1-PEM is overexpressed or not. Wild-type cells were transformed with a 2μ plasmid, either empty or for overexpression of GFP-Snc1-GEM (2 independent transformants). Shown from top to bottom: plasmid, Sec61, G6PDH (loading control), GFP-Snc1-PEM, and quantification of Sec61 expressed as average fold of WT with empty plasmid. F-G. Sec61-3xHA (F) and Sec13-3xHA (G) expressing cells were transformed with a 2μ plasmid for overexpression of GFP-Snc1-PEM. Cell lysates were tested by immunoblot analysis (using ant-HA and anti-GFP antibodies). Shown from top to bottom: strain genotype, the specific ER-resident protein tested, quantification of the ER-resident protein bands expressed as average fold of WT, GFP-Snc1-PEM, quantification of the GFP-Snc1-PEM bands expressed as average fold of WT, and G6PDH (loading control). H. Summary of immunoblot analysis of ER-resident proteins level in cells overexpressing GFP-Snc1-PEM: WT (PEP4 PRB1) vs pep4∆ prb1∆ (left), ypt1-1 PEP4 PRB1 vs ypt1-1 pep4∆ prb1∆ (right). Immunoblots and quantification are shown in Fig 6F and 6G and S6F–S6H Fig. In YPT1 cells, like Snc1-PEM (black), the levels of Hmg1 and Sec61 (green) are increased >10 fold in strains defective in vacuolar proteolysis. In contrast, the levels of Sec12, Sec13 and Kar2 are not changed (blue and grey). Like Snc1-PEM (black), the protein levels of Hmg1 and Sec61 (green) are increased ~15 fold in ypt1-1 mutant cells when compared to WT cells, regardless if they are defective in vacuolar proteolysis or not. In contrast, the levels of Sec12, Sec13 and Kar2 are only slightly increased (blue and grey). +/- and error bars represent STDEV. Results in this figure represent at least two independent experiments.