Fig. 1

Modes of pre-translational regulation of the inclusion of protein targeting motifs. Four general mechanisms can regulate the inclusion of protein targeting motifs at the pre-translational level. The use of alternative promoters and/or alternative start codons can lead to different N-terminal protein ends differing in the presence of a targeting signal (a). Similarly, different C-terminal ends, which can be caused by alternative translation termination (b), can result in differential motif inclusion. Differential splicing of the exon containing the targeting motif (in this case, a cassette exon), can cause alternative motif inclusion (c). Alternative splicing can also result in a coding frameshift affecting the presence of a targeting motif (d). While alternative splicing can directly affect motif inclusion causing the motif to be spliced in or out (c), it can also indirectly regulate motif inclusion by affecting translation initiation (a) or termination (b), or by changing the coding frame (d). Start and stop codons are indicated respectively with green and red arrowheads, targeting motifs are shown as pink rectangles, exons are represented as boxes and introns as lines