Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:1.5.1.3 (dihydrofolate reductase)
 5,819 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Targeting sequences on peroxisomal membrane proteins have not yet been identified. We have attempted to find such a sequence within PMP47, a protein of the methylotrophic yeast, Candida boidinii. This protein of 423 amino acids shows sequence similarity with proteins in the family of mitochondrial carrier proteins. As such, it is predicted to have six membrane-spanning domains. Protease susceptibility experiments are consistent with a six-membrane-spanning model for PMP47, although the topology for the peroxisomal protein is inverted compared with the mitochondrial carrier proteins. PMP47 contains two potential peroxisomal targeting sequences (PTS1), an internal SKL (residues 320-322) and a carboxy terminal AKE (residues 421-423). Using a heterologous in vivo sorting system, we show that efficient sorting occurs in the absence of both sequences. Analysis of PMP47-dihydrofolate reductase (DHFR) fusion proteins revealed that amino acids 1-199 of PMP47, which contain the first three putative membrane spans, do not contain the necessary targeting information, whereas a fusion with amino acids 1-267, which contains five spans, is fully competent for sorting to peroxisomes. Similarly, a DHFR fusion construct containing residues 268-423 did not target to peroxisomes while residues 203-420 appeared to sort to that organelle, albeit at lower efficiency than the 1-267 construct. However, DHFR constructs containing only amino acids 185-267 or 203-267 of PMP47 were not found to be associated with peroxisomes. We conclude that amino acids 199-267 are necessary for peroxisomal targeting, although additional sequences may be required for efficient sorting to, or retention by, the organelles.
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PMID:An internal region of the peroxisomal membrane protein PMP47 is essential for sorting to peroxisomes. 813 13

Most mitochondrial preproteins carry a cleavable N-terminal presequence that mediates targeting to mitochondria and translocation across the mitochondrial membranes. In this study, we characterized the presequence of the citrate carrier (CIC, tricarboxylate carrier) of rat liver mitochondria. The CIC presequence was found to be dispensable both for targeting to mitochondria and insertion into the inner membrane. Unlike the presequence of the related phosphate carrier, fusion of the CIC presequence to the cytosolic enzyme dihydrofolate reductase did not confer mitochondrial targeting, indicating that the CIC presequence does not act as a targeting signal. However, the presequence was required to keep the CIC in a soluble state. Mature CIC lacking the presequence was prone to aggregation. We conclude that mitochondrial presequences do not necessarily act as mediators of targeting. In the case of the CIC, the presequence appears to determine the folding state of the preprotein.
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PMID:Biogenesis of rat mitochondrial citrate carrier (CIC): the N-terminal presequence facilitates the solubility of the preprotein but does not act as a targeting signal. 1248 4

Metabolite carrier proteins of the mitochondrial inner membrane share homology in their transmembrane domains, which also carries their targeting information. In addition, some carriers have cleavable presequences which are not essential for targeting, but have some other function before import. The cytosolic chaperones Hsc70 (heat-shock cognate 70) and Hsp90 (heat-shock protein 90) complex with carrier precursors and interact specifically with the Tom (translocase of the mitochondrial outer membrane) 70 import receptor to promote import. We analysed how the presequences of the PiC (phosphate carrier) and CIC (citrate carrier) relate to the mechanisms of chaperone-mediated import. Deletion of the PiC presequence reduced the efficiency of import but, notably, not by causing aggregation. Instead, binding of the protein to Hsc70 was reduced, as well as the dependence on Hsc70 for import. Hsp90 binding and function in import was not greatly affected, but it could not entirely compensate for the lack of Hsc70 interaction. Deletion of the presequence from CIC was shown to cause its aggregation, but had little effect on the contribution to import of either Hsc70 or Hsp90. The presequence of PiC, but not that of CIC, conferred Hsc70 binding to dihydrofolate reductase fusion proteins. In comparison, OGC (oxoglutarate carrier) lacks a presequence and was more soluble, though it is still dependent on both Hsc70 and Hsp90. We propose that carrier presequences evolved to improve targeting competence by different mechanisms, depending on physical properties of the precursors in the cytosolic targeting environment.
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PMID:Mitochondrial carrier protein biogenesis: role of the chaperones Hsc70 and Hsp90. 1914 89