A signal peptide is a short targeting segment of a newly made polypeptide chain. A familiar ER signal sequence lies near the N-terminus and is removed during entry; Günter Blobel and Bernhard Dobberstein's 1975 experiments linked a larger immunoglobulin light chain precursor to a processed product.[ref]
Transfer of Proteins across Membranes I by Günter Blobel and Bernhard Dobberstein. Transfer of proteins across membranes. I, Introduction p. 836; Figures 5 and 14; Discussion pp. 845–847
Cleavage is a separate question from recognition. Not every targeting signal is a cleaved signal peptide: an internal signal anchor can remain part of a transmembrane protein. Conversely, loss of a segment does not by itself prove membrane crossing. Compare the independent size and accessibility readouts in the microsomal protein import experiment.
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