Hat a 47-amino acid peptide consisting of dynamin B presequence residues 28?4 and residues 103?12 (R-like recognition sequence) can serve as an efficient mitochondrial targeting sequence in D. discoideum. Residues 28?4 are rich in positively charged, hydroxylated, and hydrophobic residues and have a high potential to form an amphipathic a-helix. These features are shared with other signals targeting Met-Enkephalin biological activity proteins to the mitochondrial matrix [3]. But unlike other presequence, the dynamin B presequence contains a 25033180 central asparagine-rich region. The presence of poly-asparagine repeats is quite common in D. discoideum proteins, but their function is not well understood [48]. In the context of this work, we suggest that the asparagine-rich region serves simply as a spacer between the minimal targeting sequence and potential protease cleavage sites and is not critical for targeting and processing. Additionaly, our results indicate that the 27 N-terminal residues of the presequence are not required for mitochondrial targeting. As all our constructs contain potential MPP and MIP cleavage sites, we checked whether the fusion proteins undergo normal post-translational processing. Processing was not observed for nontargeted constructs NTS DN2, NTS DN3, and NTS DI3 (Fig. 3J). However, processing was observed for all constructs that are targeted to mitochondria. Thus, mitochondrial targeting appears to be a precondition for proteolytic removal of the NTS. Targeted constructs NTS DN1, NTS DC, NTS DI1, and NTS DI2 are proteolytically modified, although not with the same efficiency as the EYFP construct ZK 36374 custom synthesis carrying the complete NTS. This appears to be linked to differences in the expression levels of the individual proteins. The presence of a third band in the lanes for NTS DC and NTS DI1 suggests that processing involves an MPP-mediated cleavage step followed by an MIP-dependent cleavage step (Fig. 3K).Clustering of Lysine Residues Plays an Important Role in Mitochondrial TargetingClustering of positive residues within the targeting sequence on one side of an amphipathic helix has been shown to be critical for specific recognition by the mitochondrial protein import machinery [49]. A helical wheel projection and an ab initio model of the tertiary structure of the region formed by residues 28?4 show that five of the seven lysine residues contained in the region are predicted to cluster on one side of a helix. Lysine residues 29, 40, 47, 58 and 61 lie on the same face of the a-helix, while lysine 38 and 41 are on the opposite face (Fig. 4A). Further support for the notion that the efficient translocation requires predominant clustering of positive charges on one side of the helix is provided by the behavior of the smallest construct NTS DI3. With a moreDictyostelium Mitochondrial Targeting SequenceFigure 5. Importance of the R-like recognition sequence for mitochondrial targeting and processing. (A) Live cell epiflorescence imaging indicates that NTS RS and NTS 105A are targeted to mitochondria, while NTS DI2 RS and NTS DI2 105A show less efficient mitochondrial targeting in D. discoideum. Scale bars, 10 mm. (B) Immuno-blot loaded with whole cell lysate from untransformed D. discoideum cells (AX2), cells producing EYFP, and EYFP-tagged constructs NTS, NTS RS, NTS 105A, NTS DI2, NTS DI2 RS, and NTS DI2 105A. Similar to NTS, NTS?DRS and NTS 105A undergo processing. Processing is greatly impaired in the case of NTS DI2 RS and NTS DI2 105A. doi:10.1371/journal.pone.Hat a 47-amino acid peptide consisting of dynamin B presequence residues 28?4 and residues 103?12 (R-like recognition sequence) can serve as an efficient mitochondrial targeting sequence in D. discoideum. Residues 28?4 are rich in positively charged, hydroxylated, and hydrophobic residues and have a high potential to form an amphipathic a-helix. These features are shared with other signals targeting proteins to the mitochondrial matrix [3]. But unlike other presequence, the dynamin B presequence contains a 25033180 central asparagine-rich region. The presence of poly-asparagine repeats is quite common in D. discoideum proteins, but their function is not well understood [48]. In the context of this work, we suggest that the asparagine-rich region serves simply as a spacer between the minimal targeting sequence and potential protease cleavage sites and is not critical for targeting and processing. Additionaly, our results indicate that the 27 N-terminal residues of the presequence are not required for mitochondrial targeting. As all our constructs contain potential MPP and MIP cleavage sites, we checked whether the fusion proteins undergo normal post-translational processing. Processing was not observed for nontargeted constructs NTS DN2, NTS DN3, and NTS DI3 (Fig. 3J). However, processing was observed for all constructs that are targeted to mitochondria. Thus, mitochondrial targeting appears to be a precondition for proteolytic removal of the NTS. Targeted constructs NTS DN1, NTS DC, NTS DI1, and NTS DI2 are proteolytically modified, although not with the same efficiency as the EYFP construct carrying the complete NTS. This appears to be linked to differences in the expression levels of the individual proteins. The presence of a third band in the lanes for NTS DC and NTS DI1 suggests that processing involves an MPP-mediated cleavage step followed by an MIP-dependent cleavage step (Fig. 3K).Clustering of Lysine Residues Plays an Important Role in Mitochondrial TargetingClustering of positive residues within the targeting sequence on one side of an amphipathic helix has been shown to be critical for specific recognition by the mitochondrial protein import machinery [49]. A helical wheel projection and an ab initio model of the tertiary structure of the region formed by residues 28?4 show that five of the seven lysine residues contained in the region are predicted to cluster on one side of a helix. Lysine residues 29, 40, 47, 58 and 61 lie on the same face of the a-helix, while lysine 38 and 41 are on the opposite face (Fig. 4A). Further support for the notion that the efficient translocation requires predominant clustering of positive charges on one side of the helix is provided by the behavior of the smallest construct NTS DI3. With a moreDictyostelium Mitochondrial Targeting SequenceFigure 5. Importance of the R-like recognition sequence for mitochondrial targeting and processing. (A) Live cell epiflorescence imaging indicates that NTS RS and NTS 105A are targeted to mitochondria, while NTS DI2 RS and NTS DI2 105A show less efficient mitochondrial targeting in D. discoideum. Scale bars, 10 mm. (B) Immuno-blot loaded with whole cell lysate from untransformed D. discoideum cells (AX2), cells producing EYFP, and EYFP-tagged constructs NTS, NTS RS, NTS 105A, NTS DI2, NTS DI2 RS, and NTS DI2 105A. Similar to NTS, NTS?DRS and NTS 105A undergo processing. Processing is greatly impaired in the case of NTS DI2 RS and NTS DI2 105A. doi:10.1371/journal.pone.
