Figure 2

Membrane protein biogenesis in eukaryotes. In eukaryotic cells, membrane protein biogenesis occurs in a cotranslational way. Proteins residing in membranes of ER and Golgi apparatus or in plasma membrane use the secretory pathway. Like in prokaryotes, SRP recognizes polypeptides protruding from the ribosome complex, thereby transiently attenuating translation. As soon as the SRP-ribosome complex interacts with the SRP receptor and docks to the Sec61 translocon pore, translation resumes, BiP relocates, thereby opening the lumenal gate and the membrane protein enters the membrane by lateral diffusion through the Sec61 pore. Peroxisomal membrane proteins either use the Pex19/Pex3-mediated way (class I proteins, shown here) or are thought to reach the peroxisomal membrane via the ER (class II) [208]. Mitochondrial membrane proteins pass through the outer mitochondrial membrane (OMM) via the TOM complex (translocase of the outer membrane). While β-barrel proteins of the OMM are transported to the SAM and MDM complexes (sorting and assembly machinery), the TIM22 and TIM23 complexes (translocase of the inner mitochondrial membrane) are used to target proteins to the inner mitochondrial membrane [209].