H4 Protein targeting

Both in prokaryotes and eukaryotes, newly synthesized proteins must be delivered to a specific subcellular location or exported from the cell for correct activity. This phenomenon is called protein targeting. 

Secretory proteins have an N-terminal signal peptide which targets the protein to be synthesized on the rough endoplasmic reticulum (RER). During synthesis it is translocated through the RER membrane into the lumen. Vesicles then bud off from the RER and carry the protein to the Golgi complex, where it becomes glycosylated. Other vesicles then carry it to the plasma membrane. Fusion of these transport vesicles with the plasma membrane then releases the protein to the cell exterior. 

Proteins destined for the ER have an N-terminal signal peptide, are synthesized on the RER, are translocated into the RER lumen and transported by vesicles to the Golgi. Once there, a C-terminal amino acid sequence (KDEL) is recognized by a Golgi receptor protein that causes other vesicles to return the protein to the ER. 

Lysosomal proteins are targeted to the lysosomes via the addition of a mannose 6-phosphate signal that is added in the ds-compartment of the Golgi and is recognized by a receptor protein in the frans-compartment of the Golgi. The protein is then transported by specialized vesicles to a late endosome that later matures into a lysosome. The mannose 6-phosphate receptor recycles back to the Golgi for re-use. 

Most mitochondria and chloroplast proteins are made on free cytosolic ribosomes, released into the cytosol and then taken up into the organelle. Uptake into the mitochondrial matrix requires a matrix-targeting sequence and occurs at sites where the outer and inner mitochondrial membranes come into contact. The process is mediated by hsp70 and hsp60 proteins and requires both ATP hydrolysis and an electrochemical gradient across the inner mitochondrial membrane. Targeting of proteins to other compartments of mitochondria or chloroplasts requires two signals. 

---

Translation in prokaryotes (H2) Translation in eukaryotes (H3) Protein targeting (H4)... 

The TAFs fulfill numerous functions (Review Burley and Boeder, 1996 Struhl and Moqtaderi, 1998). On the one hand they are ascribed a structure promoting function. Some of the TAFs display a high degree of homology to the histones H2A, H3 and H4, and it is speculated that they help to create a nucleosome-like structure at the promotor. Furthermore, the TAFs are targets for protein-protein interactions with transcriptional activators. TAFs also posses enzymatic activity. TAF11250 has both a histone acetylase activity and a protein kinase activity. While the former presumable plays a role in the reorganization of the nucleosome, the latter can lead to phosphorylation of TFIIF. 

CpG islands of tumor-suppressor genes are hypermethylated in cancer cells. Therefore, they recmit multiple repressors that lead to a characteristic histone modification pattern the deacetylation of histones H3 and H4, the methylation of lysine 9 of histone H3, and the demethylation of lysine 4 of histone H3 (52). In addition, it has been shown that the polycomb protein EZH2 associates with DNMTs and that specific methylation of lysine 27 of histone H3 is required to establish DNA methylation in a subset of target genes (6). 

Disadvantages of both phage and mRNA display are the requirement for numerous rounds of selection and amplification, as well as the need to express and purify target proteins. Selectively infective phage (J3) and bacterial (H4, J2) and yeast (Y2) two-hybrid methods can overcome these obstacles because they are one-step screening assays with in vivo expressed target proteins. 

---