Secreted and membrane-bound proteins are important for physiological processes and are potential drug targets as they are easily accessible in the extracellular space and provide a gateway to the intracellular environment. About 3,000 protein-coding genes are predicted members of the secretome; examples of secreted proteins are cytokines, coagulation factors, hormones, and growth factors. Important members of the membrane proteome, consisting of about 5,500 genes encoding predicted membrane-bound proteins, are ion channels of molecular transporters, enzymes, receptors, and anchors for other proteins.

A majority of the human genes encode several splice variants. Almost two thirds of the genes representing the secretome have variants with different predicted localizations. See Fig. 1 for the number of genes that are intracellular, membrane-spanning, secreted, or with isoforms belonging more than one of the three categories.

An analysis across the 32 tissues shows an enrichment of secreted (e.g. in the liver) or membrane-spanning proteins (e.g. in the brain) among the tissue-enriched proteins.

In contrast, many of the tissue-enriched proteins in the testis are soluble. In general, the secretome accounts for 10-20% of the transcripts in a certain tissue. Two tissues, the pancreas and the salivary gland, differ from the other organs in their transcript profiles. Both organs are specialized to secrete proteins; the pancreas into the duodenum and the salivary gland into the oral cavity. The specialized function of these two organs is clearly seen in their transcriptomes: the pancreas and salivary gland have more than 70% and 55% of the transcripts encoded by the secretome, respectively.

Read more about secreted and membrane proteins in the Human Protein Atlas.