Figure 3.

Multiple alignment of lysozyme carboxyl termini. A screen of the SWISS-PROT database [15] for proteins that harbour PTS1 signals produced a set of lyosozymes, well characterized secreted enzymes that are not usually found in peroxisomes. Rather than occurring sporadically, a large fraction of the known sequences from this family was obtained using the PTS1 prediction tool [7]. Moreover, these hits could not be rationalized as false positives as they did not deviate from the PTS1 sequence motif [11-13]. The multiple alignment shows intact vertebrate lysozyme carboxy-terminal 20-mers (with accession number and species name) retrieved from the SWISS-PROT database. From a total of 76 entries, 23 have predicted PTS1s (score > 0; at the top, marked with '+'), seven are in the twilight zone (-10 < score < 0; in the middle, marked with '#') and 46 are not predicted (score < -10; at the bottom, marked with '-'). There appears to be an overlap between the PTS1 motif and sequence variability within the lysozyme family. For example, the absolutely conserved cysteine near the carboxyl terminus is needed for the formation of a disulfide bridge in the mature protein [21]. This cysteine also meets the requirement for a small residue at the antepenultimate position of the PTS1 sequence.