Likewise, replacement of three glycines with leucines in the Fis1 TMD relocalized Fis1 from the OMM to the ER ( Beilharz et al.

In the cases of both Cytb 5 and SLMAP, the TMD of the OMM TA variant has lower hydrophobicity, and substitution of moderately hydrophobic residues with the more hydrophobic and helix-promoting leucine relocalized the variant to the ER ( Byers et al. Proteins that harbor isoforms or splice variants that localize to either the ER or the OMM, such as cytochrome b 5 (Cytb 5) and sarcolemmal membraneвЂ“associated protein (SLMAP), provided models for better-controlled analyses.

The TMDs of OMM TAs also exhibit lower helical contents than do the TMDs of ER-destined TAs ( Rao et al. On average, TAs on the outer mitochondrial membrane (OMM) have shorter and less hydrophobic TMDs than do TAs that traffic to or through the ER ( Figure 1) ( Beilharz et al. Sequence comparisons and mutational analyses showed that the targeting information is encoded, in part, by a combination of physicochemical properties in the TMD ( Figure 1). The observation that Fission 1 (Fis1)-like proteins can localize to multiple organelles highlights the promiscuity that could result from such degenerate topogenic signals. The notion of C-terminal TMDs as targeting signals, however, raises challenging questions as to how specific organelle information is encoded. 2003, Mullen & Trelease 2000, Yagita et al. TARGETING SIGNALSД®arly microscopy studies of green fluorescent protein (GFP) fused to targeting sequences showed that C-terminal sequences encompassing the TMD are necessary for and sufficient to direct the proper localization of TAs to diverse organelles such as the ER, mitochondria, peroxisomes, and secretory membranes ( Egan et al. Here, we summarize recent advances on these fronts from cell biological, genetic, structural, and biochemical studies. These studies begin to elucidate the nature of the targeting signals that direct TAs to diverse cellular membranes the pathways, driving forces, and molecular mechanisms of their targeting and the interconnection of the protein-targeting pathway with cellular chaperones and quality control machineries. Studies of TA biogenesis have provided extensive information to help address how cellular machineries overcome multiple challenges during posttranslational membrane protein targeting.

Recent work has uncovered multiple pathways that posttranslationally target TAs to different cellular membranes and has begun to elucidate the molecular mechanisms underlying these processes. In support of this hypothesis, synaptobrevin 2 (Syb2), a tail-anchored SNARE protein, can be targeted to and inserted into the ER after release from the ribosome ( Kutay et al. Because the C-terminal TMDs of TAs are obscured by the ribosome during translation, it was predicted early on that these proteins would be targeted by posttranslational mechanisms ( Kutay et al. TAs compose 3вЂ“5% of the eukaryotic membrane proteome and mediate diverse cellular processes, including protein translocation across organelle membranes, vesicular transport, apoptosis, and protein quality control ( Chartron et al. A salient example is the class of tail-anchored proteins (TAs), which contain a single TMD near the C terminus. Nevertheless, numerous membrane proteins cannot use SRP and must be targeted via posttranslational pathways, the mechanisms of which are far less well understood. SRP recognizes TMDs near the N terminus of nascent proteins and delivers them to translocation machineries on the eukaryotic endoplasmic reticulum (ER) or the bacterial plasma membrane during translation this cotranslational mode of targeting effectively minimizes the aggregation of membrane proteins in the cytosol ( Akopian et al. The best-studied pathway for membrane protein targeting and insertion utilizes the signal recognition particle (SRP). Given the multiple membranes in a eukaryotic cell, how are newly synthesized membrane proteins recognized and sorted to the correct target membranes? What are the energy requirements in these pathways? How are the hydrophobic transmembrane domains (TMDs) on nascent proteins effectively protected from aggregation as they traverse aqueous cellular environments? How are the TMDs efficiently inserted into the phospholipid bilayer? Finally, have cells evolved correction mechanisms for cases in which targeting and insertion fail? The process of membrane protein biogenesis presents several fundamental challenges for the cell. The proper functioning of biological membranes requires all newly synthesized membrane proteins to be localized to and inserted into the appropriate membrane destinations. Membrane proteins account for в€ј35% of the proteins encoded by the genome.

Membrane pro llc