Newly synthesized polypeptides and their complexes which are intentionally moved to the extracellular environment by otherwise intact cells.
Protein secretion by eukaryotic cells involves exocytosis but with prokaryotic cells instead relies upon various protein secretion systems. Proteins that are secreted can be digestive enzymes, blood proteins, members of more immobile extracellular matrices, etc..
To the extent that these secreted proteins are able to functionally venture beyond the boundaries of an organisms then they can be viewed as a form of extended phenotype. Exoenzymes secreted by bacteria and fungi are a good example of latter.
In a few cases proteins are "secreted" via the intentional destruction of a cell, such a by cell lysis. This is the case, for example, with the exotoxin, Shiga toxin, which leaves cells upon cell lysis, which in many instances (i.e., E. coli O157:H7) is bacteriophage mediated.
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