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The left diagram shows a steroid (lipid) hormone (1) entering a cell and (2) binding to a receptor protein in the nucleus, causing (3) mRNA synthesis which is the first step of protein synthesis. The right side shows protein hormones (1) binding with receptors which (2) begins a transduction pathway. The transduction pathway ends (3) with transcription factors being activated in the nucleus, and protein synthesis beginning. In both diagrams, a is the hormone, b is the cell membrane, c is the cytoplasm, and d is the nucleus.
Other modifications include the addition of sugar molecules to certain amino acids in the protein (glycosylation), or the addition of a phosphate group (phosphorylation) or a sulfate group (sulfation). Many proteins are modified by proteases that remove short peptides from either end of the protein. The shortened polypeptides then fold into an active protein. One of the most common of these cleavages is the removal of specific signal peptides. These peptides target proteins for transport to a particular cellular organelle in a process known as protein sorting. An example of this is the hormone insulin, which is made as preproinsulin. After removal of the 24-amino-acid signal peptide from preproinsulin to form proinsulin, the latter polypeptide is further processed in the endoplasmic reticulum. This produces the final hormone, insulin, which is released from the cell.