A class of cell surface receptors recognised by its pharmacological profile. Sigma receptors were originally considered to be opioid receptors because they bind certain synthetic opioids. However they also interact with a variety of other psychoactive drugs, and their endogenous ligand is not known (although they can react to certain endogenous steroids). Sigma receptors are found in the immune, endocrine, and nervous systems, and in some peripheral tissues.
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... not all of these types of transport are regulated by specific transport receptors that recognize signal sequences within in proteins (that is to ... (see Engelsma et al Embo '04) Lastly your assumptions about the sigma factor are correct; it potentiates prokaryotic transcription together ...
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