Proteins

Protein must be delivered to the proper destination in the cell to function properly. Signal sequences within the protein itself act like "zip codes" to ensure correct delivery. The synthesis of secreted proteins like insulin and of proteins that will be integral to the plasma membrane occurs at a ribo-some tethered to the endoplasmic reticulum, which is a system of membranes that transport materials within cells. The peptides formed there are then translocated into the lumen, or channel, of the endoplasmic reticulum, where they will be formed into a polypeptide chain. This translocation occurs because of a specific signal sequence that is formed by the first twenty or so amino acids in the protein. The core of this sequence consists of ten to fifteen amino acids that have hydrophobic side chains such as alanine, leucine, valine, isoleucine, and phenylalanine, which are usually cleaved from the protein later on. The nascent polypeptide chain is guided along this path by a signal receptor protein.

Proteins targeted for internal cellular functions are synthesized on ribosomal assemblages that float free in the cytoplasm. Such proteins also have their signal sequences. Proteins destined for the cell's nucleus have a specific nuclear signal sequence consisting of a small series of basic amino acids such as arginine and lysine bounded by proline. This nuclear signaling sequence can be located anywhere in the protein's sequence as long as it projects outward from the three-dimensional tertiary structure. Signal sequences for proteins targeted to be part of organelles such as the mitochondria Protein structure components. Alpha helices and beta sheets are linked by less-structured loop regions to form domains. The domains combine to eventually form fully functional proteins. Adapted from Robinson, 2001. and chloroplasts are anywhere from twenty to seventy amino acids long and are mostly hydrophilic. This charged nature allows easy travel through the hydrophilic cytoplasm to the organelle.