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According to the signal hypothesis, a signal sequence, once having initiated export of a growing protein chain across the rough endoplasmic reticulum, is cleaved from the mature protein at a specific site. It has long been known that some part of the cleavage specificity resides in the last residue of the signal sequence, which invariably is one with a small, uncharged side-chain, but no further specific patterns of amino acids near the point of cleavage have been discovered so far. In this paper, some such patterns, based on a sample of 78 eukaryotic signal sequences, are presented and discussed, and a first attempt at formulating rules for the prediction of cleavage sites is made. [ABSTRACT FROM AUTHOR]

We have developed a semi-empirical method to determine the subunit molecular weight of a protein. The method is a minor modification of the Edman degradation and is based on a simple chemical procedure to modify specifically the N-terminal NH2 group. Initially, all NH2 groups, including the ε-NH2 groups of lysine residues and the N-terminal α-NH2 group, are reacted with phenylisothiocyanate and the protein derivative is subjected to one step of the Edman degradation. The newly exposed N terminus is then reacted with radioactively labelled phenylisothiocyanate. A value for the subunit molecular weight can be obtained from an analysis of the incorporated radioactivity and the amount of the protein in the sample. The molecular weight of five different proteins have been determined by this method. Our method is particularly useful for proteins containing lipid or sugar components and also for relatively small peptides. The procedure described in this paper for the specific modification of the N terminus has been found to be a powerful tool for protein sequencing. [ABSTRACT FROM AUTHOR]