Additional article facts, The outstanding-studied systems of cell protein satisfactory manipulate (QC) are the ones of soluble, cytoplasmic proteins. In yeast, degradation of most plasma membrane proteins requires the ubiquitin ligase Rsp5. Membrane Protein Quality Articles. Membrane proteins may be alpha - helices or beta - strands. Proteins can span the membrane with alpha helices. Orm1 and Orm2 are conserved endoplasmic reticulum membrane proteins regulating lipid homeostasis and protein high-quality control.