Naming a targeting signal

Abstract

A staggering variety of potential fates awaits a protein emerging from a ribosome. The presence (or absence) of certain amino acid sequences in a nascent protein constrains the set of metabolic fates that actually befall it by making some fates more probable than others. These sequences, known as targeting signals, can determine the spatial destination of a protein, its folding, posttranslational modifications, and metabolic stability. The understanding of targeting signals has grown enormously over the last fifteen years (reviewed by Blobel, 1980; Wold, 1981; Creighton, 1984; Walter and Lingappa, 1987; Randall et al., 1987; Pfeffer and Rothman, 1987; Burgess and Kelly, 1987; Verner and Schatz, 1988; Gierasch, 1989; Hart1 and Neupert, 1990). Unfortunately, these advances have not been accompanied by a clarification and simplification of terminology. For instance, signals that enable a protein to translocate across the plane of a membrane are called signal sequences, topogenic sequences, targeting sequences, transit sequences, presequences, translocation sequences, or leader sequences. Ad hoc, often semantically cumbersome terms are also used to denote other targeting signals. Yet most of the transformations that proteins undergo can be grouped into five distinct classes of transitions: * Translocation of a protein across the plane of a membrane separating two compartments. * Transfer of a protein between compartments without crossing the plane of a membrane (vesicle-mediated transport). * Hydrolysis of some or all of the peptide bonds in a protein. * Chemical modifications of a protein that do not involve hydrolysis of peptide bonds. * Conformational modifications of a protein, including its folding and oligomerization.

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